2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2023 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(158 * (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 + 1460 - int(delta) * 1024 / int(rootDelta)) / 1024 + (!i && r > 937);
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(350 * d - 400, 1650);
87 // Add a small random component to draw evaluations to avoid 3-fold blindness
88 Value value_draw(const Thread* thisThread) {
89 return VALUE_DRAW - 1 + Value(thisThread->nodes & 0x2);
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) {
100 double e = double(uci_elo - 1320) / (3190 - 1320);
101 level = std::clamp((((37.2473 * e - 40.8525) * e + 22.2943) * e - 0.311438), 0.0, 19.0);
104 level = double(skill_level);
106 bool enabled() const { return level < 20.0; }
107 bool time_to_pick(Depth depth) const { return depth == 1 + int(level); }
108 Move pick_best(size_t multiPV);
111 Move best = MOVE_NONE;
114 template <NodeType nodeType>
115 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
117 template <NodeType nodeType>
118 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
120 Value value_to_tt(Value v, int ply);
121 Value value_from_tt(Value v, int ply, int r50c);
122 void update_pv(Move* pv, Move move, const Move* childPv);
123 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
124 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus);
125 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
126 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
128 // perft() is our utility to verify move generation. All the leaf nodes up
129 // to the given depth are generated and counted, and the sum is returned.
131 uint64_t perft(Position& pos, Depth depth) {
134 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
136 uint64_t cnt, nodes = 0;
137 const bool leaf = (depth == 2);
139 for (const auto& m : MoveList<LEGAL>(pos))
141 if (Root && depth <= 1)
146 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);
151 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
159 /// Search::init() is called at startup to initialize various lookup tables
161 void Search::init() {
163 for (int i = 1; i < MAX_MOVES; ++i)
164 Reductions[i] = int((20.26 + std::log(Threads.size()) / 2) * std::log(i));
168 /// Search::clear() resets search state to its initial value
170 void Search::clear() {
172 Threads.main()->wait_for_search_finished();
174 Time.availableNodes = 0;
177 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
181 /// MainThread::search() is started when the program receives the UCI 'go'
182 /// command. It searches from the root position and outputs the "bestmove".
184 void MainThread::search() {
188 nodes = perft<true>(rootPos, Limits.perft);
189 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
193 Color us = rootPos.side_to_move();
194 Time.init(Limits, us, rootPos.game_ply());
197 Eval::NNUE::verify();
199 if (rootMoves.empty())
201 rootMoves.emplace_back(MOVE_NONE);
202 sync_cout << "info depth 0 score "
203 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
208 Threads.start_searching(); // start non-main threads
209 Thread::search(); // main thread start searching
212 // When we reach the maximum depth, we can arrive here without a raise of
213 // Threads.stop. However, if we are pondering or in an infinite search,
214 // the UCI protocol states that we shouldn't print the best move before the
215 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
216 // until the GUI sends one of those commands.
218 while (!Threads.stop && (ponder || Limits.infinite))
219 {} // Busy wait for a stop or a ponder reset
221 // Stop the threads if not already stopped (also raise the stop if
222 // "ponderhit" just reset Threads.ponder).
225 // Wait until all threads have finished
226 Threads.wait_for_search_finished();
228 // When playing in 'nodes as time' mode, subtract the searched nodes from
229 // the available ones before exiting.
231 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
233 Thread* bestThread = this;
234 Skill skill = Skill(Options["Skill Level"], Options["UCI_LimitStrength"] ? int(Options["UCI_Elo"]) : 0);
236 if ( int(Options["MultiPV"]) == 1
239 && rootMoves[0].pv[0] != MOVE_NONE)
240 bestThread = Threads.get_best_thread();
242 bestPreviousScore = bestThread->rootMoves[0].score;
243 bestPreviousAverageScore = bestThread->rootMoves[0].averageScore;
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) << 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)
281 (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
282 (ss-i)->staticEval = VALUE_NONE;
285 for (int i = 0; i <= MAX_PLY + 2; ++i)
290 bestValue = delta = alpha = -VALUE_INFINITE;
291 beta = VALUE_INFINITE;
295 if (mainThread->bestPreviousScore == VALUE_INFINITE)
296 for (int i = 0; i < 4; ++i)
297 mainThread->iterValue[i] = VALUE_ZERO;
299 for (int i = 0; i < 4; ++i)
300 mainThread->iterValue[i] = mainThread->bestPreviousScore;
303 size_t multiPV = size_t(Options["MultiPV"]);
304 Skill skill(Options["Skill Level"], Options["UCI_LimitStrength"] ? int(Options["UCI_Elo"]) : 0);
306 // When playing with strength handicap enable MultiPV search that we will
307 // use behind the scenes to retrieve a set of possible moves.
309 multiPV = std::max(multiPV, (size_t)4);
311 multiPV = std::min(multiPV, rootMoves.size());
313 complexityAverage.set(153, 1);
315 optimism[us] = optimism[~us] = VALUE_ZERO;
317 int searchAgainCounter = 0;
319 // Iterative deepening loop until requested to stop or the target depth is reached
320 while ( ++rootDepth < MAX_PLY
322 && !(Limits.depth && mainThread && rootDepth > Limits.depth))
324 // Age out PV variability metric
326 totBestMoveChanges /= 2;
328 // Save the last iteration's scores before first PV line is searched and
329 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
330 for (RootMove& rm : rootMoves)
331 rm.previousScore = rm.score;
336 if (!Threads.increaseDepth)
337 searchAgainCounter++;
339 // MultiPV loop. We perform a full root search for each PV line
340 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
345 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
346 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
350 // Reset UCI info selDepth for each depth and each PV line
353 // Reset aspiration window starting size
356 Value prev = rootMoves[pvIdx].averageScore;
357 delta = Value(10) + int(prev) * prev / 15400;
358 alpha = std::max(prev - delta,-VALUE_INFINITE);
359 beta = std::min(prev + delta, VALUE_INFINITE);
361 // Adjust optimism based on root move's previousScore
362 int opt = 116 * prev / (std::abs(prev) + 170);
363 optimism[ us] = Value(opt);
364 optimism[~us] = -optimism[us];
367 // Start with a small aspiration window and, in the case of a fail
368 // high/low, re-search with a bigger window until we don't fail
370 int failedHighCnt = 0;
373 // Adjust the effective depth searched, but ensuring at least one effective increment for every
374 // four searchAgain steps (see issue #2717).
375 Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - 3 * (searchAgainCounter + 1) / 4);
376 bestValue = Stockfish::search<Root>(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) << 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 + 2;
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) << 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 // Use part of the gained time from a previous stable move for the current move
454 for (Thread* th : Threads)
456 totBestMoveChanges += th->bestMoveChanges;
457 th->bestMoveChanges = 0;
460 // Do we have time for the next iteration? Can we stop searching now?
461 if ( Limits.use_time_management()
463 && !mainThread->stopOnPonderhit)
465 double fallingEval = (71 + 12 * (mainThread->bestPreviousAverageScore - bestValue)
466 + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 656.7;
467 fallingEval = std::clamp(fallingEval, 0.5, 1.5);
469 // If the bestMove is stable over several iterations, reduce time accordingly
470 timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.37 : 0.65;
471 double reduction = (1.4 + mainThread->previousTimeReduction) / (2.15 * timeReduction);
472 double bestMoveInstability = 1 + 1.7 * totBestMoveChanges / Threads.size();
473 int complexity = mainThread->complexityAverage.value();
474 double complexPosition = std::min(1.0 + (complexity - 261) / 1738.7, 1.5);
476 double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability * complexPosition;
478 // Cap used time in case of a single legal move for a better viewer experience in tournaments
479 // yielding correct scores and sufficiently fast moves.
480 if (rootMoves.size() == 1)
481 totalTime = std::min(500.0, totalTime);
483 // Stop the search if we have exceeded the totalTime
484 if (Time.elapsed() > totalTime)
486 // If we are allowed to ponder do not stop the search now but
487 // keep pondering until the GUI sends "ponderhit" or "stop".
488 if (mainThread->ponder)
489 mainThread->stopOnPonderhit = true;
493 else if ( !mainThread->ponder
494 && Time.elapsed() > totalTime * 0.53)
495 Threads.increaseDepth = false;
497 Threads.increaseDepth = true;
500 mainThread->iterValue[iterIdx] = bestValue;
501 iterIdx = (iterIdx + 1) & 3;
507 mainThread->previousTimeReduction = timeReduction;
509 // If skill level is enabled, swap best PV line with the sub-optimal one
511 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
512 skill.best ? skill.best : skill.pick_best(multiPV)));
518 // search<>() is the main search function for both PV and non-PV nodes
520 template <NodeType nodeType>
521 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
523 constexpr bool PvNode = nodeType != NonPV;
524 constexpr bool rootNode = nodeType == Root;
526 // Check if we have an upcoming move which draws by repetition, or
527 // if the opponent had an alternative move earlier to this position.
529 && pos.rule50_count() >= 3
530 && alpha < VALUE_DRAW
531 && pos.has_game_cycle(ss->ply))
533 alpha = value_draw(pos.this_thread());
538 // Dive into quiescence search when the depth reaches zero
540 return qsearch<PvNode ? PV : NonPV>(pos, ss, alpha, beta);
542 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
543 assert(PvNode || (alpha == beta - 1));
544 assert(0 < depth && depth < MAX_PLY);
545 assert(!(PvNode && cutNode));
547 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
549 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
553 Move ttMove, move, excludedMove, bestMove;
554 Depth extension, newDepth;
555 Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
556 bool givesCheck, improving, priorCapture, singularQuietLMR;
557 bool capture, moveCountPruning, ttCapture;
559 int moveCount, captureCount, quietCount, improvement, complexity;
561 // Step 1. Initialize node
562 Thread* thisThread = pos.this_thread();
563 ss->inCheck = pos.checkers();
564 priorCapture = pos.captured_piece();
565 Color us = pos.side_to_move();
566 moveCount = captureCount = quietCount = ss->moveCount = 0;
567 bestValue = -VALUE_INFINITE;
568 maxValue = VALUE_INFINITE;
570 // Check for the available remaining time
571 if (thisThread == Threads.main())
572 static_cast<MainThread*>(thisThread)->check_time();
574 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
575 if (PvNode && thisThread->selDepth < ss->ply + 1)
576 thisThread->selDepth = ss->ply + 1;
580 // Step 2. Check for aborted search and immediate draw
581 if ( Threads.stop.load(std::memory_order_relaxed)
582 || pos.is_draw(ss->ply)
583 || ss->ply >= MAX_PLY)
584 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
585 : value_draw(pos.this_thread());
587 // Step 3. Mate distance pruning. Even if we mate at the next move our score
588 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
589 // a shorter mate was found upward in the tree then there is no need to search
590 // because we will never beat the current alpha. Same logic but with reversed
591 // signs applies also in the opposite condition of being mated instead of giving
592 // mate. In this case return a fail-high score.
593 alpha = std::max(mated_in(ss->ply), alpha);
594 beta = std::min(mate_in(ss->ply+1), beta);
599 thisThread->rootDelta = beta - alpha;
601 assert(0 <= ss->ply && ss->ply < MAX_PLY);
603 (ss+1)->ttPv = false;
604 (ss+1)->excludedMove = bestMove = MOVE_NONE;
605 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
606 (ss+2)->cutoffCnt = 0;
607 ss->doubleExtensions = (ss-1)->doubleExtensions;
608 Square prevSq = to_sq((ss-1)->currentMove);
610 // Initialize statScore to zero for the grandchildren of the current position.
611 // So statScore is shared between all grandchildren and only the first grandchild
612 // starts with statScore = 0. Later grandchildren start with the last calculated
613 // statScore of the previous grandchild. This influences the reduction rules in
614 // LMR which are based on the statScore of parent position.
616 (ss+2)->statScore = 0;
618 // Step 4. Transposition table lookup.
619 excludedMove = ss->excludedMove;
621 tte = TT.probe(posKey, ss->ttHit);
622 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
623 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
624 : ss->ttHit ? tte->move() : MOVE_NONE;
625 ttCapture = ttMove && pos.capture(ttMove);
627 // At this point, if excluded, skip straight to step 6, static eval. However,
628 // to save indentation, we list the condition in all code between here and there.
630 ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
632 // At non-PV nodes we check for an early TT cutoff
636 && tte->depth() > depth - (tte->bound() == BOUND_EXACT)
637 && ttValue != VALUE_NONE // Possible in case of TT access race
638 && (tte->bound() & (ttValue >= beta ? BOUND_LOWER : BOUND_UPPER)))
640 // If ttMove is quiet, update move sorting heuristics on TT hit (~2 Elo)
645 // Bonus for a quiet ttMove that fails high (~2 Elo)
647 update_quiet_stats(pos, ss, ttMove, stat_bonus(depth));
649 // Extra penalty for early quiet moves of the previous ply (~0 Elo on STC, ~2 Elo on LTC)
650 if ((ss-1)->moveCount <= 2 && !priorCapture)
651 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
653 // Penalty for a quiet ttMove that fails low (~1 Elo)
656 int penalty = -stat_bonus(depth);
657 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
658 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
662 // Partial workaround for the graph history interaction problem
663 // For high rule50 counts don't produce transposition table cutoffs.
664 if (pos.rule50_count() < 90)
668 // Step 5. Tablebases probe
669 if (!rootNode && !excludedMove && TB::Cardinality)
671 int piecesCount = pos.count<ALL_PIECES>();
673 if ( piecesCount <= TB::Cardinality
674 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
675 && pos.rule50_count() == 0
676 && !pos.can_castle(ANY_CASTLING))
679 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
681 // Force check of time on the next occasion
682 if (thisThread == Threads.main())
683 static_cast<MainThread*>(thisThread)->callsCnt = 0;
685 if (err != TB::ProbeState::FAIL)
687 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
689 int drawScore = TB::UseRule50 ? 1 : 0;
691 // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
692 value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
693 : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
694 : VALUE_DRAW + 2 * wdl * drawScore;
696 Bound b = wdl < -drawScore ? BOUND_UPPER
697 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
699 if ( b == BOUND_EXACT
700 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
702 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
703 std::min(MAX_PLY - 1, depth + 6),
704 MOVE_NONE, VALUE_NONE);
711 if (b == BOUND_LOWER)
712 bestValue = value, alpha = std::max(alpha, bestValue);
720 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
722 // Step 6. Static evaluation of the position
725 // Skip early pruning when in check
726 ss->staticEval = eval = VALUE_NONE;
732 else if (excludedMove) {
733 // excludeMove implies that we had a ttHit on the containing non-excluded search with ss->staticEval filled from TT
734 // However static evals from the TT aren't good enough (-13 elo), presumably due to changing optimism context
735 // Recalculate value with current optimism (without updating thread avgComplexity)
736 ss->staticEval = eval = evaluate(pos, &complexity);
740 // Never assume anything about values stored in TT
741 ss->staticEval = eval = tte->eval();
742 if (eval == VALUE_NONE)
743 ss->staticEval = eval = evaluate(pos, &complexity);
744 else // Fall back to (semi)classical complexity for TT hits, the NNUE complexity is lost
745 complexity = abs(ss->staticEval - pos.psq_eg_stm());
747 // ttValue can be used as a better position evaluation (~7 Elo)
748 if ( ttValue != VALUE_NONE
749 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
754 ss->staticEval = eval = evaluate(pos, &complexity);
755 // Save static evaluation into transposition table
756 tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
758 thisThread->complexityAverage.update(complexity);
760 // Use static evaluation difference to improve quiet move ordering (~4 Elo)
761 if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
763 int bonus = std::clamp(-19 * int((ss-1)->staticEval + ss->staticEval), -1940, 1940);
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
774 improving = improvement > 0;
776 // Step 7. Razoring (~1 Elo).
777 // If eval is really low check with qsearch if it can exceed alpha, if it can't,
778 // return a fail low.
779 if (eval < alpha - 394 - 255 * depth * depth)
781 value = qsearch<NonPV>(pos, ss, alpha - 1, alpha);
786 // Step 8. Futility pruning: child node (~40 Elo).
787 // The depth condition is important for mate finding.
790 && eval - futility_margin(depth, improving) - (ss-1)->statScore / 304 >= beta
792 && eval < 28580) // larger than VALUE_KNOWN_WIN, but smaller than TB wins
795 // Step 9. Null move search with verification search (~35 Elo)
797 && (ss-1)->currentMove != MOVE_NULL
798 && (ss-1)->statScore < 18200
800 && eval >= ss->staticEval
801 && ss->staticEval >= beta - 20 * depth - improvement / 14 + 235 + complexity / 24
803 && pos.non_pawn_material(us)
804 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
806 assert(eval - beta >= 0);
808 // Null move dynamic reduction based on depth, eval and complexity of position
809 Depth R = std::min(int(eval - beta) / 165, 6) + depth / 3 + 4 - (complexity > 800);
811 ss->currentMove = MOVE_NULL;
812 ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
814 pos.do_null_move(st);
816 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
818 pos.undo_null_move();
820 if (nullValue >= beta)
822 // Do not return unproven mate or TB scores
823 if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
826 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
829 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
831 // Do verification search at high depths, with null move pruning disabled
832 // for us, until ply exceeds nmpMinPly.
833 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
834 thisThread->nmpColor = us;
836 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
838 thisThread->nmpMinPly = 0;
845 probCutBeta = beta + 180 - 54 * improving;
847 // Step 10. ProbCut (~10 Elo)
848 // If we have a good enough capture and a reduced search returns a value
849 // much above beta, we can (almost) safely prune the previous move.
852 && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
853 // if value from transposition table is lower than probCutBeta, don't attempt probCut
854 // there and in further interactions with transposition table cutoff depth is set to depth - 3
855 // because probCut search has depth set to depth - 4 but we also do a move before it
856 // so effective depth is equal to depth - 3
858 && tte->depth() >= depth - 3
859 && ttValue != VALUE_NONE
860 && ttValue < probCutBeta))
862 assert(probCutBeta < VALUE_INFINITE);
864 MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &captureHistory);
866 while ((move = mp.next_move()) != MOVE_NONE)
867 if (move != excludedMove && pos.legal(move))
869 assert(pos.capture(move) || promotion_type(move) == QUEEN);
871 ss->currentMove = move;
872 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
874 [pos.moved_piece(move)]
877 pos.do_move(move, st);
879 // Perform a preliminary qsearch to verify that the move holds
880 value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
882 // If the qsearch held, perform the regular search
883 if (value >= probCutBeta)
884 value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
888 if (value >= probCutBeta)
890 // Save ProbCut data into transposition table
891 tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, BOUND_LOWER, depth - 3, move, ss->staticEval);
897 // Step 11. If the position is not in TT, decrease depth by 3.
898 // Use qsearch if depth is equal or below zero (~9 Elo)
904 return qsearch<PV>(pos, ss, alpha, beta);
911 moves_loop: // When in check, search starts here
913 // Step 12. A small Probcut idea, when we are in check (~4 Elo)
914 probCutBeta = beta + 402;
919 && (tte->bound() & BOUND_LOWER)
920 && tte->depth() >= depth - 3
921 && ttValue >= probCutBeta
922 && abs(ttValue) <= VALUE_KNOWN_WIN
923 && abs(beta) <= VALUE_KNOWN_WIN
928 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
929 nullptr , (ss-4)->continuationHistory,
930 nullptr , (ss-6)->continuationHistory };
932 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
934 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
941 moveCountPruning = singularQuietLMR = false;
943 // Indicate PvNodes that will probably fail low if the node was searched
944 // at a depth equal or greater than the current depth, and the result of this search was a fail low.
945 bool likelyFailLow = PvNode
947 && (tte->bound() & BOUND_UPPER)
948 && tte->depth() >= depth;
950 // Step 13. Loop through all pseudo-legal moves until no moves remain
951 // or a beta cutoff occurs.
952 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
956 if (move == excludedMove)
959 // At root obey the "searchmoves" option and skip moves not listed in Root
960 // Move List. As a consequence any illegal move is also skipped. In MultiPV
961 // mode we also skip PV moves which have been already searched and those
962 // of lower "TB rank" if we are in a TB root position.
963 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
964 thisThread->rootMoves.begin() + thisThread->pvLast, move))
967 // Check for legality
968 if (!rootNode && !pos.legal(move))
971 ss->moveCount = ++moveCount;
973 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
974 sync_cout << "info depth " << depth
975 << " currmove " << UCI::move(move, pos.is_chess960())
976 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
978 (ss+1)->pv = nullptr;
981 capture = pos.capture(move);
982 movedPiece = pos.moved_piece(move);
983 givesCheck = pos.gives_check(move);
985 // Calculate new depth for this move
986 newDepth = depth - 1;
988 Value delta = beta - alpha;
990 Depth r = reduction(improving, depth, moveCount, delta, thisThread->rootDelta);
992 // Step 14. Pruning at shallow depth (~120 Elo). Depth conditions are important for mate finding.
994 && pos.non_pawn_material(us)
995 && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
997 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold (~8 Elo)
998 moveCountPruning = moveCount >= futility_move_count(improving, depth);
1000 // Reduced depth of the next LMR search
1001 int lmrDepth = std::max(newDepth - r, 0);
1006 // Futility pruning for captures (~2 Elo)
1011 && ss->staticEval + 185 + 203 * lmrDepth + PieceValue[EG][pos.piece_on(to_sq(move))]
1012 + captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] / 6 < alpha)
1015 // SEE based pruning (~11 Elo)
1016 if (!pos.see_ge(move, Value(-220) * depth))
1021 int history = (*contHist[0])[movedPiece][to_sq(move)]
1022 + (*contHist[1])[movedPiece][to_sq(move)]
1023 + (*contHist[3])[movedPiece][to_sq(move)];
1025 // Continuation history based pruning (~2 Elo)
1027 && history < -4180 * (depth - 1))
1030 history += 2 * thisThread->mainHistory[us][from_to(move)];
1032 lmrDepth += history / 7208;
1033 lmrDepth = std::max(lmrDepth, -2);
1035 // Futility pruning: parent node (~13 Elo)
1038 && ss->staticEval + 103 + 136 * lmrDepth <= alpha)
1041 lmrDepth = std::max(lmrDepth, 0);
1043 // Prune moves with negative SEE (~4 Elo)
1044 if (!pos.see_ge(move, Value(-25 * lmrDepth * lmrDepth - 16 * lmrDepth)))
1049 // Step 15. Extensions (~100 Elo)
1050 // We take care to not overdo to avoid search getting stuck.
1051 if (ss->ply < thisThread->rootDepth * 2)
1053 // Singular extension search (~94 Elo). If all moves but one fail low on a
1054 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
1055 // then that move is singular and should be extended. To verify this we do
1056 // a reduced search on all the other moves but the ttMove and if the
1057 // result is lower than ttValue minus a margin, then we will extend the ttMove.
1059 && depth >= 4 - (thisThread->completedDepth > 22) + 2 * (PvNode && tte->is_pv())
1061 && !excludedMove // Avoid recursive singular search
1062 /* && ttValue != VALUE_NONE Already implicit in the next condition */
1063 && abs(ttValue) < VALUE_KNOWN_WIN
1064 && (tte->bound() & BOUND_LOWER)
1065 && tte->depth() >= depth - 3)
1067 Value singularBeta = ttValue - (3 + (ss->ttPv && !PvNode)) * depth;
1068 Depth singularDepth = (depth - 1) / 2;
1070 ss->excludedMove = move;
1071 // the search with excludedMove will update ss->staticEval
1072 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
1073 ss->excludedMove = MOVE_NONE;
1075 if (value < singularBeta)
1078 singularQuietLMR = !ttCapture;
1080 // Avoid search explosion by limiting the number of double extensions
1082 && value < singularBeta - 25
1083 && ss->doubleExtensions <= 10)
1086 depth += depth < 12;
1090 // Multi-cut pruning
1091 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
1092 // search without the ttMove. So we assume this expected Cut-node is not singular,
1093 // that multiple moves fail high, and we can prune the whole subtree by returning
1095 else if (singularBeta >= beta)
1096 return singularBeta;
1098 // If the eval of ttMove is greater than beta, we reduce it (negative extension)
1099 else if (ttValue >= beta)
1102 // If the eval of ttMove is less than alpha and value, we reduce it (negative extension)
1103 else if (ttValue <= alpha && ttValue <= value)
1107 // Check extensions (~1 Elo)
1108 else if ( givesCheck
1110 && abs(ss->staticEval) > 78)
1113 // Quiet ttMove extensions (~1 Elo)
1116 && move == ss->killers[0]
1117 && (*contHist[0])[movedPiece][to_sq(move)] >= 5600)
1121 // Add extension to new depth
1122 newDepth += extension;
1123 ss->doubleExtensions = (ss-1)->doubleExtensions + (extension == 2);
1125 // Speculative prefetch as early as possible
1126 prefetch(TT.first_entry(pos.key_after(move)));
1128 // Update the current move (this must be done after singular extension search)
1129 ss->currentMove = move;
1130 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1135 // Step 16. Make the move
1136 pos.do_move(move, st, givesCheck);
1138 // Decrease reduction if position is or has been on the PV
1139 // and node is not likely to fail low. (~3 Elo)
1144 // Decrease reduction if opponent's move count is high (~1 Elo)
1145 if ((ss-1)->moveCount > 7)
1148 // Increase reduction for cut nodes (~3 Elo)
1152 // Increase reduction if ttMove is a capture (~3 Elo)
1156 // Decrease reduction for PvNodes based on depth
1158 r -= 1 + 11 / (3 + depth);
1160 // Decrease reduction if ttMove has been singularly extended (~1 Elo)
1161 if (singularQuietLMR)
1164 // Decrease reduction if we move a threatened piece (~1 Elo)
1166 && (mp.threatenedPieces & from_sq(move)))
1169 // Increase reduction if next ply has a lot of fail high
1170 if ((ss+1)->cutoffCnt > 3)
1173 // Decrease reduction if move is a killer and we have a good history
1174 if (move == ss->killers[0]
1175 && (*contHist[0])[movedPiece][to_sq(move)] >= 3600)
1178 ss->statScore = 2 * thisThread->mainHistory[us][from_to(move)]
1179 + (*contHist[0])[movedPiece][to_sq(move)]
1180 + (*contHist[1])[movedPiece][to_sq(move)]
1181 + (*contHist[3])[movedPiece][to_sq(move)]
1184 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1185 r -= ss->statScore / (12800 + 4410 * (depth > 7 && depth < 19));
1187 // Step 17. Late moves reduction / extension (LMR, ~117 Elo)
1188 // We use various heuristics for the sons of a node after the first son has
1189 // been searched. In general we would like to reduce them, but there are many
1190 // cases where we extend a son if it has good chances to be "interesting".
1192 && moveCount > 1 + (PvNode && ss->ply <= 1)
1195 || (cutNode && (ss-1)->moveCount > 1)))
1197 // In general we want to cap the LMR depth search at newDepth, but when
1198 // reduction is negative, we allow this move a limited search extension
1199 // beyond the first move depth. This may lead to hidden double extensions.
1200 Depth d = std::clamp(newDepth - r, 1, newDepth + 1);
1202 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1204 // Do full depth search when reduced LMR search fails high
1205 if (value > alpha && d < newDepth)
1207 // Adjust full depth search based on LMR results - if result
1208 // was good enough search deeper, if it was bad enough search shallower
1209 const bool doDeeperSearch = value > (alpha + 66 + 11 * (newDepth - d));
1210 const bool doEvenDeeperSearch = value > alpha + 582 && ss->doubleExtensions <= 5;
1211 const bool doShallowerSearch = value < bestValue + newDepth;
1213 ss->doubleExtensions = ss->doubleExtensions + doEvenDeeperSearch;
1215 newDepth += doDeeperSearch - doShallowerSearch + doEvenDeeperSearch;
1218 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1220 int bonus = value > alpha ? stat_bonus(newDepth)
1221 : -stat_bonus(newDepth);
1223 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1227 // Step 18. Full depth search when LMR is skipped. If expected reduction is high, reduce its depth by 1.
1228 else if (!PvNode || moveCount > 1)
1230 // Increase reduction for cut nodes and not ttMove (~1 Elo)
1231 if (!ttMove && cutNode)
1234 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth - (r > 4), !cutNode);
1237 // For PV nodes only, do a full PV search on the first move or after a fail
1238 // high (in the latter case search only if value < beta), otherwise let the
1239 // parent node fail low with value <= alpha and try another move.
1240 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1243 (ss+1)->pv[0] = MOVE_NONE;
1245 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1248 // Step 19. Undo move
1249 pos.undo_move(move);
1251 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1253 // Step 20. Check for a new best move
1254 // Finished searching the move. If a stop occurred, the return value of
1255 // the search cannot be trusted, and we return immediately without
1256 // updating best move, PV and TT.
1257 if (Threads.stop.load(std::memory_order_relaxed))
1262 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1263 thisThread->rootMoves.end(), move);
1265 rm.averageScore = rm.averageScore != -VALUE_INFINITE ? (2 * value + rm.averageScore) / 3 : value;
1267 // PV move or new best move?
1268 if (moveCount == 1 || value > alpha)
1270 rm.score = rm.uciScore = value;
1271 rm.selDepth = thisThread->selDepth;
1272 rm.scoreLowerbound = rm.scoreUpperbound = false;
1274 if (value >= beta) {
1275 rm.scoreLowerbound = true;
1278 else if (value <= alpha) {
1279 rm.scoreUpperbound = true;
1280 rm.uciScore = alpha;
1286 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1287 rm.pv.push_back(*m);
1289 // We record how often the best move has been changed in each iteration.
1290 // This information is used for time management. In MultiPV mode,
1291 // we must take care to only do this for the first PV line.
1293 && !thisThread->pvIdx)
1294 ++thisThread->bestMoveChanges;
1297 // All other moves but the PV are set to the lowest value: this
1298 // is not a problem when sorting because the sort is stable and the
1299 // move position in the list is preserved - just the PV is pushed up.
1300 rm.score = -VALUE_INFINITE;
1303 if (value > bestValue)
1311 if (PvNode && !rootNode) // Update pv even in fail-high case
1312 update_pv(ss->pv, move, (ss+1)->pv);
1314 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1318 // Reduce other moves if we have found at least one score improvement
1321 && beta < VALUE_KNOWN_WIN
1322 && alpha > -VALUE_KNOWN_WIN)
1330 assert(value >= beta); // Fail high
1337 // If the move is worse than some previously searched move, remember it to update its stats later
1338 if (move != bestMove)
1340 if (capture && captureCount < 32)
1341 capturesSearched[captureCount++] = move;
1343 else if (!capture && quietCount < 64)
1344 quietsSearched[quietCount++] = move;
1348 // The following condition would detect a stop only after move loop has been
1349 // completed. But in this case bestValue is valid because we have fully
1350 // searched our subtree, and we can anyhow save the result in TT.
1356 // Step 21. Check for mate and stalemate
1357 // All legal moves have been searched and if there are no legal moves, it
1358 // must be a mate or a stalemate. If we are in a singular extension search then
1359 // return a fail low score.
1361 assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1364 bestValue = excludedMove ? alpha :
1365 ss->inCheck ? mated_in(ss->ply)
1368 // If there is a move which produces search value greater than alpha we update stats of searched moves
1370 update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
1371 quietsSearched, quietCount, capturesSearched, captureCount, depth);
1373 // Bonus for prior countermove that caused the fail low
1374 else if (!priorCapture)
1376 // Extra bonuses for PV/Cut nodes or bad fail lows
1377 int bonus = (depth > 4) + (PvNode || cutNode) + (bestValue < alpha - 88 * depth);
1378 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth) * bonus);
1382 bestValue = std::min(bestValue, maxValue);
1384 // If no good move is found and the previous position was ttPv, then the previous
1385 // opponent move is probably good and the new position is added to the search tree.
1386 if (bestValue <= alpha)
1387 ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
1389 // Write gathered information in transposition table
1390 if (!excludedMove && !(rootNode && thisThread->pvIdx))
1391 tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
1392 bestValue >= beta ? BOUND_LOWER :
1393 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1394 depth, bestMove, ss->staticEval);
1396 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1402 // qsearch() is the quiescence search function, which is called by the main search
1403 // function with zero depth, or recursively with further decreasing depth per call.
1405 template <NodeType nodeType>
1406 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1408 static_assert(nodeType != Root);
1409 constexpr bool PvNode = nodeType == PV;
1411 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1412 assert(PvNode || (alpha == beta - 1));
1417 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1421 Move ttMove, move, bestMove;
1423 Value bestValue, value, ttValue, futilityValue, futilityBase;
1424 bool pvHit, givesCheck, capture;
1427 // Step 1. Initialize node
1431 ss->pv[0] = MOVE_NONE;
1434 Thread* thisThread = pos.this_thread();
1435 bestMove = MOVE_NONE;
1436 ss->inCheck = pos.checkers();
1439 // Step 2. Check for an immediate draw or maximum ply reached
1440 if ( pos.is_draw(ss->ply)
1441 || ss->ply >= MAX_PLY)
1442 return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
1444 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1446 // Decide whether or not to include checks: this fixes also the type of
1447 // TT entry depth that we are going to use. Note that in qsearch we use
1448 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1449 ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1450 : DEPTH_QS_NO_CHECKS;
1451 // Step 3. Transposition table lookup
1453 tte = TT.probe(posKey, ss->ttHit);
1454 ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
1455 ttMove = ss->ttHit ? tte->move() : MOVE_NONE;
1456 pvHit = ss->ttHit && tte->is_pv();
1458 // At non-PV nodes we check for an early TT cutoff
1461 && tte->depth() >= ttDepth
1462 && ttValue != VALUE_NONE // Only in case of TT access race
1463 && (tte->bound() & (ttValue >= beta ? BOUND_LOWER : BOUND_UPPER)))
1466 // Step 4. Static evaluation of the position
1469 ss->staticEval = VALUE_NONE;
1470 bestValue = futilityBase = -VALUE_INFINITE;
1476 // Never assume anything about values stored in TT
1477 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1478 ss->staticEval = bestValue = evaluate(pos);
1480 // ttValue can be used as a better position evaluation (~13 Elo)
1481 if ( ttValue != VALUE_NONE
1482 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1483 bestValue = ttValue;
1486 // In case of null move search use previous static eval with a different sign
1487 ss->staticEval = bestValue =
1488 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1489 : -(ss-1)->staticEval;
1491 // Stand pat. Return immediately if static value is at least beta
1492 if (bestValue >= beta)
1494 // Save gathered info in transposition table
1496 tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
1497 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1502 if (PvNode && bestValue > alpha)
1505 futilityBase = bestValue + 158;
1508 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1509 nullptr , (ss-4)->continuationHistory,
1510 nullptr , (ss-6)->continuationHistory };
1512 // Initialize a MovePicker object for the current position, and prepare
1513 // to search the moves. Because the depth is <= 0 here, only captures,
1514 // queen promotions, and other checks (only if depth >= DEPTH_QS_CHECKS)
1515 // will be generated.
1516 Square prevSq = to_sq((ss-1)->currentMove);
1517 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1518 &thisThread->captureHistory,
1522 int quietCheckEvasions = 0;
1524 // Step 5. Loop through all pseudo-legal moves until no moves remain
1525 // or a beta cutoff occurs.
1526 while ((move = mp.next_move()) != MOVE_NONE)
1528 assert(is_ok(move));
1530 // Check for legality
1531 if (!pos.legal(move))
1534 givesCheck = pos.gives_check(move);
1535 capture = pos.capture(move);
1540 if (bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
1542 // Futility pruning and moveCount pruning (~10 Elo)
1544 && to_sq(move) != prevSq
1545 && futilityBase > -VALUE_KNOWN_WIN
1546 && type_of(move) != PROMOTION)
1551 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1553 if (futilityValue <= alpha)
1555 bestValue = std::max(bestValue, futilityValue);
1559 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1561 bestValue = std::max(bestValue, futilityBase);
1566 // We prune after 2nd quiet check evasion where being 'in check' is implicitly checked through the counter
1567 // and being a 'quiet' apart from being a tt move is assumed after an increment because captures are pushed ahead.
1568 if (quietCheckEvasions > 1)
1571 // Continuation history based pruning (~3 Elo)
1573 && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < 0
1574 && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < 0)
1577 // Do not search moves with bad enough SEE values (~5 Elo)
1578 if (!pos.see_ge(move, Value(-108)))
1583 // Speculative prefetch as early as possible
1584 prefetch(TT.first_entry(pos.key_after(move)));
1586 // Update the current move
1587 ss->currentMove = move;
1588 ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
1590 [pos.moved_piece(move)]
1593 quietCheckEvasions += !capture && ss->inCheck;
1595 // Step 7. Make and search the move
1596 pos.do_move(move, st, givesCheck);
1597 value = -qsearch<nodeType>(pos, ss+1, -beta, -alpha, depth - 1);
1598 pos.undo_move(move);
1600 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1602 // Step 8. Check for a new best move
1603 if (value > bestValue)
1611 if (PvNode) // Update pv even in fail-high case
1612 update_pv(ss->pv, move, (ss+1)->pv);
1614 if (PvNode && value < beta) // Update alpha here!
1622 // Step 9. Check for mate
1623 // All legal moves have been searched. A special case: if we're in check
1624 // and no legal moves were found, it is checkmate.
1625 if (ss->inCheck && bestValue == -VALUE_INFINITE)
1627 assert(!MoveList<LEGAL>(pos).size());
1629 return mated_in(ss->ply); // Plies to mate from the root
1632 // Save gathered info in transposition table
1633 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1634 bestValue >= beta ? BOUND_LOWER : BOUND_UPPER,
1635 ttDepth, bestMove, ss->staticEval);
1637 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1643 // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
1644 // "plies to mate from the current position". Standard scores are unchanged.
1645 // The function is called before storing a value in the transposition table.
1647 Value value_to_tt(Value v, int ply) {
1649 assert(v != VALUE_NONE);
1651 return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
1652 : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
1656 // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
1657 // from the transposition table (which refers to the plies to mate/be mated from
1658 // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
1659 // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
1660 // and the graph history interaction, we return an optimal TB score instead.
1662 Value value_from_tt(Value v, int ply, int r50c) {
1664 if (v == VALUE_NONE)
1667 if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
1669 if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
1670 return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
1675 if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
1677 if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
1678 return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
1687 // update_pv() adds current move and appends child pv[]
1689 void update_pv(Move* pv, Move move, const Move* childPv) {
1691 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1697 // update_all_stats() updates stats at the end of search() when a bestMove is found
1699 void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq,
1700 Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) {
1702 Color us = pos.side_to_move();
1703 Thread* thisThread = pos.this_thread();
1704 CapturePieceToHistory& captureHistory = thisThread->captureHistory;
1705 Piece moved_piece = pos.moved_piece(bestMove);
1706 PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
1707 int bonus1 = stat_bonus(depth + 1);
1709 if (!pos.capture(bestMove))
1711 int bonus2 = bestValue > beta + 146 ? bonus1 // larger bonus
1712 : stat_bonus(depth); // smaller bonus
1714 // Increase stats for the best move in case it was a quiet move
1715 update_quiet_stats(pos, ss, bestMove, bonus2);
1717 // Decrease stats for all non-best quiet moves
1718 for (int i = 0; i < quietCount; ++i)
1720 thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
1721 update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
1725 // Increase stats for the best move in case it was a capture move
1726 captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
1728 // Extra penalty for a quiet early move that was not a TT move or
1729 // main killer move in previous ply when it gets refuted.
1730 if ( ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
1731 && !pos.captured_piece())
1732 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
1734 // Decrease stats for all non-best capture moves
1735 for (int i = 0; i < captureCount; ++i)
1737 moved_piece = pos.moved_piece(capturesSearched[i]);
1738 captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
1739 captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
1744 // update_continuation_histories() updates histories of the move pairs formed
1745 // by moves at ply -1, -2, -4, and -6 with current move.
1747 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1749 for (int i : {1, 2, 4, 6})
1751 // Only update first 2 continuation histories if we are in check
1752 if (ss->inCheck && i > 2)
1754 if (is_ok((ss-i)->currentMove))
1755 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1760 // update_quiet_stats() updates move sorting heuristics
1762 void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus) {
1765 if (ss->killers[0] != move)
1767 ss->killers[1] = ss->killers[0];
1768 ss->killers[0] = move;
1771 Color us = pos.side_to_move();
1772 Thread* thisThread = pos.this_thread();
1773 thisThread->mainHistory[us][from_to(move)] << bonus;
1774 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1776 // Update countermove history
1777 if (is_ok((ss-1)->currentMove))
1779 Square prevSq = to_sq((ss-1)->currentMove);
1780 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1784 // When playing with strength handicap, choose best move among a set of RootMoves
1785 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1787 Move Skill::pick_best(size_t multiPV) {
1789 const RootMoves& rootMoves = Threads.main()->rootMoves;
1790 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1792 // RootMoves are already sorted by score in descending order
1793 Value topScore = rootMoves[0].score;
1794 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1795 int maxScore = -VALUE_INFINITE;
1796 double weakness = 120 - 2 * level;
1798 // Choose best move. For each move score we add two terms, both dependent on
1799 // weakness. One is deterministic and bigger for weaker levels, and one is
1800 // random. Then we choose the move with the resulting highest score.
1801 for (size_t i = 0; i < multiPV; ++i)
1803 // This is our magic formula
1804 int push = int(( weakness * int(topScore - rootMoves[i].score)
1805 + delta * (rng.rand<unsigned>() % int(weakness))) / 128);
1807 if (rootMoves[i].score + push >= maxScore)
1809 maxScore = rootMoves[i].score + push;
1810 best = rootMoves[i].pv[0];
1820 /// MainThread::check_time() is used to print debug info and, more importantly,
1821 /// to detect when we are out of available time and thus stop the search.
1823 void MainThread::check_time() {
1828 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1829 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1831 static TimePoint lastInfoTime = now();
1833 TimePoint elapsed = Time.elapsed();
1834 TimePoint tick = Limits.startTime + elapsed;
1836 if (tick - lastInfoTime >= 1000)
1838 lastInfoTime = tick;
1842 // We should not stop pondering until told so by the GUI
1846 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1847 || (Limits.movetime && elapsed >= Limits.movetime)
1848 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1849 Threads.stop = true;
1853 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1854 /// that all (if any) unsearched PV lines are sent using a previous search score.
1856 string UCI::pv(const Position& pos, Depth depth) {
1858 std::stringstream ss;
1859 TimePoint elapsed = Time.elapsed() + 1;
1860 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1861 size_t pvIdx = pos.this_thread()->pvIdx;
1862 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1863 uint64_t nodesSearched = Threads.nodes_searched();
1864 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1866 for (size_t i = 0; i < multiPV; ++i)
1868 bool updated = rootMoves[i].score != -VALUE_INFINITE;
1870 if (depth == 1 && !updated && i > 0)
1873 Depth d = updated ? depth : std::max(1, depth - 1);
1874 Value v = updated ? rootMoves[i].uciScore : rootMoves[i].previousScore;
1876 if (v == -VALUE_INFINITE)
1879 bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
1880 v = tb ? rootMoves[i].tbScore : v;
1882 if (ss.rdbuf()->in_avail()) // Not at first line
1887 << " seldepth " << rootMoves[i].selDepth
1888 << " multipv " << i + 1
1889 << " score " << UCI::value(v);
1891 if (Options["UCI_ShowWDL"])
1892 ss << UCI::wdl(v, pos.game_ply());
1894 if (i == pvIdx && !tb && updated) // tablebase- and previous-scores are exact
1895 ss << (rootMoves[i].scoreLowerbound ? " lowerbound" : (rootMoves[i].scoreUpperbound ? " upperbound" : ""));
1897 ss << " nodes " << nodesSearched
1898 << " nps " << nodesSearched * 1000 / elapsed
1899 << " hashfull " << TT.hashfull()
1900 << " tbhits " << tbHits
1901 << " time " << elapsed
1904 for (Move m : rootMoves[i].pv)
1905 ss << " " << UCI::move(m, pos.is_chess960());
1912 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1913 /// before exiting the search, for instance, in case we stop the search during a
1914 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1915 /// otherwise in case of 'ponder on' we have nothing to think on.
1917 bool RootMove::extract_ponder_from_tt(Position& pos) {
1920 ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
1924 assert(pv.size() == 1);
1926 if (pv[0] == MOVE_NONE)
1929 pos.do_move(pv[0], st);
1930 TTEntry* tte = TT.probe(pos.key(), ttHit);
1934 Move m = tte->move(); // Local copy to be SMP safe
1935 if (MoveList<LEGAL>(pos).contains(m))
1939 pos.undo_move(pv[0]);
1940 return pv.size() > 1;
1943 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1946 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1947 ProbeDepth = int(Options["SyzygyProbeDepth"]);
1948 Cardinality = int(Options["SyzygyProbeLimit"]);
1949 bool dtz_available = true;
1951 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1952 // ProbeDepth == DEPTH_ZERO
1953 if (Cardinality > MaxCardinality)
1955 Cardinality = MaxCardinality;
1959 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1961 // Rank moves using DTZ tables
1962 RootInTB = root_probe(pos, rootMoves);
1966 // DTZ tables are missing; try to rank moves using WDL tables
1967 dtz_available = false;
1968 RootInTB = root_probe_wdl(pos, rootMoves);
1974 // Sort moves according to TB rank
1975 std::stable_sort(rootMoves.begin(), rootMoves.end(),
1976 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1978 // Probe during search only if DTZ is not available and we are winning
1979 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1984 // Clean up if root_probe() and root_probe_wdl() have failed
1985 for (auto& m : rootMoves)
1990 } // namespace Stockfish