2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2019 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
53 namespace TB = Tablebases;
57 using namespace Search;
61 // Different node types, used as a template parameter
62 enum NodeType { NonPV, PV };
64 // Razor and futility margins
65 constexpr int RazorMargin = 661;
66 Value futility_margin(Depth d, bool improving) {
67 return Value((168 - 51 * improving) * d / ONE_PLY);
70 // Reductions lookup table, initialized at startup
71 int Reductions[MAX_MOVES]; // [depth or moveNumber]
73 Depth reduction(bool i, Depth d, int mn) {
74 int r = Reductions[d / ONE_PLY] * Reductions[mn];
75 return ((r + 520) / 1024 + (!i && r > 999)) * ONE_PLY;
78 constexpr int futility_move_count(bool improving, int depth) {
79 return (5 + depth * depth) * (1 + improving) / 2;
82 // History and stats update bonus, based on depth
83 int stat_bonus(Depth depth) {
84 int d = depth / ONE_PLY;
85 return d > 17 ? -8 : 22 * d * d + 151 * d - 140;
88 // Add a small random component to draw evaluations to avoid 3fold-blindness
89 Value value_draw(Depth depth, Thread* thisThread) {
90 return depth < 4 * ONE_PLY ? VALUE_DRAW
91 : VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
94 // Skill structure is used to implement strength limit
96 explicit Skill(int l) : level(l) {}
97 bool enabled() const { return level < 20; }
98 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
99 Move pick_best(size_t multiPV);
102 Move best = MOVE_NONE;
105 // Breadcrumbs are used to mark nodes as being searched by a given thread.
107 std::atomic<Thread*> thread;
108 std::atomic<Key> key;
110 std::array<Breadcrumb, 1024> breadcrumbs;
112 // ThreadHolding keeps track of which thread left breadcrumbs at the given node for potential reductions.
113 // A free node will be marked upon entering the moves loop, and unmarked upon leaving that loop, by the ctor/dtor of this struct.
114 struct ThreadHolding {
115 explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
116 location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
121 // see if another already marked this location, if not, mark it ourselves.
122 Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
125 (*location).thread.store(thisThread, std::memory_order_relaxed);
126 (*location).key.store(posKey, std::memory_order_relaxed);
129 else if ( tmp != thisThread
130 && (*location).key.load(std::memory_order_relaxed) == posKey)
136 if (owning) // free the marked location.
137 (*location).thread.store(nullptr, std::memory_order_relaxed);
140 bool marked() { return otherThread; }
143 Breadcrumb* location;
144 bool otherThread, owning;
147 template <NodeType NT>
148 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
150 template <NodeType NT>
151 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
153 Value value_to_tt(Value v, int ply);
154 Value value_from_tt(Value v, int ply);
155 void update_pv(Move* pv, Move move, Move* childPv);
156 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
157 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietCount, int bonus);
158 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCount, int bonus);
160 // perft() is our utility to verify move generation. All the leaf nodes up
161 // to the given depth are generated and counted, and the sum is returned.
163 uint64_t perft(Position& pos, Depth depth) {
166 uint64_t cnt, nodes = 0;
167 const bool leaf = (depth == 2 * ONE_PLY);
169 for (const auto& m : MoveList<LEGAL>(pos))
171 if (Root && depth <= ONE_PLY)
176 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
181 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
189 /// Search::init() is called at startup to initialize various lookup tables
191 void Search::init() {
193 for (int i = 1; i < MAX_MOVES; ++i)
194 Reductions[i] = int(23.4 * std::log(i));
198 /// Search::clear() resets search state to its initial value
200 void Search::clear() {
202 Threads.main()->wait_for_search_finished();
204 Time.availableNodes = 0;
207 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
211 /// MainThread::search() is started when the program receives the UCI 'go'
212 /// command. It searches from the root position and outputs the "bestmove".
214 void MainThread::search() {
218 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
219 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
223 Color us = rootPos.side_to_move();
224 Time.init(Limits, us, rootPos.game_ply());
227 if (rootMoves.empty())
229 rootMoves.emplace_back(MOVE_NONE);
230 sync_cout << "info depth 0 score "
231 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
236 for (Thread* th : Threads)
238 th->bestMoveChanges = 0;
240 th->start_searching();
243 Thread::search(); // Let's start searching!
246 // When we reach the maximum depth, we can arrive here without a raise of
247 // Threads.stop. However, if we are pondering or in an infinite search,
248 // the UCI protocol states that we shouldn't print the best move before the
249 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
250 // until the GUI sends one of those commands.
252 while (!Threads.stop && (ponder || Limits.infinite))
253 {} // Busy wait for a stop or a ponder reset
255 // Stop the threads if not already stopped (also raise the stop if
256 // "ponderhit" just reset Threads.ponder).
259 // Wait until all threads have finished
260 for (Thread* th : Threads)
262 th->wait_for_search_finished();
264 // When playing in 'nodes as time' mode, subtract the searched nodes from
265 // the available ones before exiting.
267 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
269 Thread* bestThread = this;
271 // Check if there are threads with a better score than main thread
272 if ( Options["MultiPV"] == 1
274 && !(Skill(Options["Skill Level"]).enabled() || Options["UCI_LimitStrength"])
275 && rootMoves[0].pv[0] != MOVE_NONE)
277 std::map<Move, int64_t> votes;
278 Value minScore = this->rootMoves[0].score;
280 // Find out minimum score
281 for (Thread* th: Threads)
282 minScore = std::min(minScore, th->rootMoves[0].score);
284 // Vote according to score and depth, and select the best thread
285 for (Thread* th : Threads)
287 votes[th->rootMoves[0].pv[0]] +=
288 (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
290 if (bestThread->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY)
292 // Make sure we pick the shortest mate
293 if (th->rootMoves[0].score > bestThread->rootMoves[0].score)
296 else if ( th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY
297 || votes[th->rootMoves[0].pv[0]] > votes[bestThread->rootMoves[0].pv[0]])
302 previousScore = bestThread->rootMoves[0].score;
304 // Send again PV info if we have a new best thread
305 if (bestThread != this)
306 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
308 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
310 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
311 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
313 std::cout << sync_endl;
317 /// Thread::search() is the main iterative deepening loop. It calls search()
318 /// repeatedly with increasing depth until the allocated thinking time has been
319 /// consumed, the user stops the search, or the maximum search depth is reached.
321 void Thread::search() {
323 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
324 // The former is needed to allow update_continuation_histories(ss-1, ...),
325 // which accesses its argument at ss-6, also near the root.
326 // The latter is needed for statScores and killer initialization.
327 Stack stack[MAX_PLY+10], *ss = stack+7;
329 Value bestValue, alpha, beta, delta;
330 Move lastBestMove = MOVE_NONE;
331 Depth lastBestMoveDepth = DEPTH_ZERO;
332 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
333 double timeReduction = 1, totBestMoveChanges = 0;
334 Color us = rootPos.side_to_move();
336 std::memset(ss-7, 0, 10 * sizeof(Stack));
337 for (int i = 7; i > 0; i--)
338 (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
341 bestValue = delta = alpha = -VALUE_INFINITE;
342 beta = VALUE_INFINITE;
344 size_t multiPV = Options["MultiPV"];
346 // Pick integer skill levels, but non-deterministically round up or down
347 // such that the average integer skill corresponds to the input floating point one.
348 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
349 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
350 // for match (TC 60+0.6) results spanning a wide range of k values.
352 double floatLevel = Options["UCI_LimitStrength"] ?
353 clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
354 double(Options["Skill Level"]);
355 int intLevel = int(floatLevel) +
356 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
357 Skill skill(intLevel);
359 // When playing with strength handicap enable MultiPV search that we will
360 // use behind the scenes to retrieve a set of possible moves.
362 multiPV = std::max(multiPV, (size_t)4);
364 multiPV = std::min(multiPV, rootMoves.size());
366 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
368 // In analysis mode, adjust contempt in accordance with user preference
369 if (Limits.infinite || Options["UCI_AnalyseMode"])
370 ct = Options["Analysis Contempt"] == "Off" ? 0
371 : Options["Analysis Contempt"] == "Both" ? ct
372 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
373 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
376 // Evaluation score is from the white point of view
377 contempt = (us == WHITE ? make_score(ct, ct / 2)
378 : -make_score(ct, ct / 2));
380 // Iterative deepening loop until requested to stop or the target depth is reached
381 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
383 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
385 // Age out PV variability metric
387 totBestMoveChanges /= 2;
389 // Save the last iteration's scores before first PV line is searched and
390 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
391 for (RootMove& rm : rootMoves)
392 rm.previousScore = rm.score;
397 // MultiPV loop. We perform a full root search for each PV line
398 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
403 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
404 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
408 // Reset UCI info selDepth for each depth and each PV line
411 // Reset aspiration window starting size
412 if (rootDepth >= 4 * ONE_PLY)
414 Value previousScore = rootMoves[pvIdx].previousScore;
416 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
417 beta = std::min(previousScore + delta, VALUE_INFINITE);
419 // Adjust contempt based on root move's previousScore (dynamic contempt)
420 int dct = ct + 86 * previousScore / (abs(previousScore) + 176);
422 contempt = (us == WHITE ? make_score(dct, dct / 2)
423 : -make_score(dct, dct / 2));
426 // Start with a small aspiration window and, in the case of a fail
427 // high/low, re-search with a bigger window until we don't fail
429 int failedHighCnt = 0;
432 Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
433 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
435 // Bring the best move to the front. It is critical that sorting
436 // is done with a stable algorithm because all the values but the
437 // first and eventually the new best one are set to -VALUE_INFINITE
438 // and we want to keep the same order for all the moves except the
439 // new PV that goes to the front. Note that in case of MultiPV
440 // search the already searched PV lines are preserved.
441 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
443 // If search has been stopped, we break immediately. Sorting is
444 // safe because RootMoves is still valid, although it refers to
445 // the previous iteration.
449 // When failing high/low give some update (without cluttering
450 // the UI) before a re-search.
453 && (bestValue <= alpha || bestValue >= beta)
454 && Time.elapsed() > 3000)
455 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
457 // In case of failing low/high increase aspiration window and
458 // re-search, otherwise exit the loop.
459 if (bestValue <= alpha)
461 beta = (alpha + beta) / 2;
462 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
466 mainThread->stopOnPonderhit = false;
468 else if (bestValue >= beta)
470 beta = std::min(bestValue + delta, VALUE_INFINITE);
476 delta += delta / 4 + 5;
478 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
481 // Sort the PV lines searched so far and update the GUI
482 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
485 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
486 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
490 completedDepth = rootDepth;
492 if (rootMoves[0].pv[0] != lastBestMove) {
493 lastBestMove = rootMoves[0].pv[0];
494 lastBestMoveDepth = rootDepth;
497 // Have we found a "mate in x"?
499 && bestValue >= VALUE_MATE_IN_MAX_PLY
500 && VALUE_MATE - bestValue <= 2 * Limits.mate)
506 // If skill level is enabled and time is up, pick a sub-optimal best move
507 if (skill.enabled() && skill.time_to_pick(rootDepth))
508 skill.pick_best(multiPV);
510 // Do we have time for the next iteration? Can we stop searching now?
511 if ( Limits.use_time_management()
513 && !mainThread->stopOnPonderhit)
515 double fallingEval = (354 + 10 * (mainThread->previousScore - bestValue)) / 692.0;
516 fallingEval = clamp(fallingEval, 0.5, 1.5);
518 // If the bestMove is stable over several iterations, reduce time accordingly
519 timeReduction = lastBestMoveDepth + 9 * ONE_PLY < completedDepth ? 1.97 : 0.98;
520 double reduction = (1.36 + mainThread->previousTimeReduction) / (2.29 * timeReduction);
522 // Use part of the gained time from a previous stable move for the current move
523 for (Thread* th : Threads)
525 totBestMoveChanges += th->bestMoveChanges;
526 th->bestMoveChanges = 0;
528 double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
530 // Stop the search if we have only one legal move, or if available time elapsed
531 if ( rootMoves.size() == 1
532 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
534 // If we are allowed to ponder do not stop the search now but
535 // keep pondering until the GUI sends "ponderhit" or "stop".
536 if (mainThread->ponder)
537 mainThread->stopOnPonderhit = true;
547 mainThread->previousTimeReduction = timeReduction;
549 // If skill level is enabled, swap best PV line with the sub-optimal one
551 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
552 skill.best ? skill.best : skill.pick_best(multiPV)));
558 // search<>() is the main search function for both PV and non-PV nodes
560 template <NodeType NT>
561 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
563 constexpr bool PvNode = NT == PV;
564 const bool rootNode = PvNode && ss->ply == 0;
566 // Check if we have an upcoming move which draws by repetition, or
567 // if the opponent had an alternative move earlier to this position.
568 if ( pos.rule50_count() >= 3
569 && alpha < VALUE_DRAW
571 && pos.has_game_cycle(ss->ply))
573 alpha = value_draw(depth, pos.this_thread());
578 // Dive into quiescence search when the depth reaches zero
580 return qsearch<NT>(pos, ss, alpha, beta);
582 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
583 assert(PvNode || (alpha == beta - 1));
584 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
585 assert(!(PvNode && cutNode));
586 assert(depth / ONE_PLY * ONE_PLY == depth);
588 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
592 Move ttMove, move, excludedMove, bestMove;
593 Depth extension, newDepth;
594 Value bestValue, value, ttValue, eval, maxValue;
595 bool ttHit, ttPv, inCheck, givesCheck, improving, doLMR;
596 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
598 int moveCount, captureCount, quietCount, singularLMR;
600 // Step 1. Initialize node
601 Thread* thisThread = pos.this_thread();
602 inCheck = pos.checkers();
603 Color us = pos.side_to_move();
604 moveCount = captureCount = quietCount = singularLMR = ss->moveCount = 0;
605 bestValue = -VALUE_INFINITE;
606 maxValue = VALUE_INFINITE;
608 // Check for the available remaining time
609 if (thisThread == Threads.main())
610 static_cast<MainThread*>(thisThread)->check_time();
612 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
613 if (PvNode && thisThread->selDepth < ss->ply + 1)
614 thisThread->selDepth = ss->ply + 1;
618 // Step 2. Check for aborted search and immediate draw
619 if ( Threads.stop.load(std::memory_order_relaxed)
620 || pos.is_draw(ss->ply)
621 || ss->ply >= MAX_PLY)
622 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
623 : value_draw(depth, pos.this_thread());
625 // Step 3. Mate distance pruning. Even if we mate at the next move our score
626 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
627 // a shorter mate was found upward in the tree then there is no need to search
628 // because we will never beat the current alpha. Same logic but with reversed
629 // signs applies also in the opposite condition of being mated instead of giving
630 // mate. In this case return a fail-high score.
631 alpha = std::max(mated_in(ss->ply), alpha);
632 beta = std::min(mate_in(ss->ply+1), beta);
637 assert(0 <= ss->ply && ss->ply < MAX_PLY);
639 (ss+1)->ply = ss->ply + 1;
640 (ss+1)->excludedMove = bestMove = MOVE_NONE;
641 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
642 Square prevSq = to_sq((ss-1)->currentMove);
644 // Initialize statScore to zero for the grandchildren of the current position.
645 // So statScore is shared between all grandchildren and only the first grandchild
646 // starts with statScore = 0. Later grandchildren start with the last calculated
647 // statScore of the previous grandchild. This influences the reduction rules in
648 // LMR which are based on the statScore of parent position.
650 (ss + 4)->statScore = 0;
652 (ss + 2)->statScore = 0;
654 // Step 4. Transposition table lookup. We don't want the score of a partial
655 // search to overwrite a previous full search TT value, so we use a different
656 // position key in case of an excluded move.
657 excludedMove = ss->excludedMove;
658 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
659 tte = TT.probe(posKey, ttHit);
660 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
661 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
662 : ttHit ? tte->move() : MOVE_NONE;
663 ttPv = PvNode || (ttHit && tte->is_pv());
665 // if position has been searched at higher depths and we are shuffling, return value_draw
666 if (pos.rule50_count() > 36
668 && depth < 3 * ONE_PLY
670 && tte->depth() > depth
671 && pos.count<PAWN>() > 0)
674 // At non-PV nodes we check for an early TT cutoff
677 && tte->depth() >= depth
678 && ttValue != VALUE_NONE // Possible in case of TT access race
679 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
680 : (tte->bound() & BOUND_UPPER)))
682 // If ttMove is quiet, update move sorting heuristics on TT hit
687 if (!pos.capture_or_promotion(ttMove))
688 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
690 // Extra penalty for early quiet moves of the previous ply
691 if ((ss-1)->moveCount <= 2 && !pos.captured_piece())
692 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
694 // Penalty for a quiet ttMove that fails low
695 else if (!pos.capture_or_promotion(ttMove))
697 int penalty = -stat_bonus(depth);
698 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
699 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
705 // Step 5. Tablebases probe
706 if (!rootNode && TB::Cardinality)
708 int piecesCount = pos.count<ALL_PIECES>();
710 if ( piecesCount <= TB::Cardinality
711 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
712 && pos.rule50_count() == 0
713 && !pos.can_castle(ANY_CASTLING))
716 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
718 // Force check of time on the next occasion
719 if (thisThread == Threads.main())
720 static_cast<MainThread*>(thisThread)->callsCnt = 0;
722 if (err != TB::ProbeState::FAIL)
724 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
726 int drawScore = TB::UseRule50 ? 1 : 0;
728 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
729 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
730 : VALUE_DRAW + 2 * wdl * drawScore;
732 Bound b = wdl < -drawScore ? BOUND_UPPER
733 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
735 if ( b == BOUND_EXACT
736 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
738 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
739 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
740 MOVE_NONE, VALUE_NONE);
747 if (b == BOUND_LOWER)
748 bestValue = value, alpha = std::max(alpha, bestValue);
756 // Step 6. Static evaluation of the position
759 ss->staticEval = eval = VALUE_NONE;
761 goto moves_loop; // Skip early pruning when in check
765 // Never assume anything about values stored in TT
766 ss->staticEval = eval = tte->eval();
767 if (eval == VALUE_NONE)
768 ss->staticEval = eval = evaluate(pos);
770 // Can ttValue be used as a better position evaluation?
771 if ( ttValue != VALUE_NONE
772 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
777 if ((ss-1)->currentMove != MOVE_NULL)
779 int bonus = -(ss-1)->statScore / 512;
781 ss->staticEval = eval = evaluate(pos) + bonus;
784 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Eval::Tempo;
786 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
789 // Step 7. Razoring (~2 Elo)
790 if ( !rootNode // The required rootNode PV handling is not available in qsearch
791 && depth < 2 * ONE_PLY
792 && eval <= alpha - RazorMargin)
793 return qsearch<NT>(pos, ss, alpha, beta);
795 improving = ss->staticEval >= (ss-2)->staticEval
796 || (ss-2)->staticEval == VALUE_NONE;
798 // Step 8. Futility pruning: child node (~30 Elo)
800 && depth < 7 * ONE_PLY
801 && eval - futility_margin(depth, improving) >= beta
802 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
805 // Step 9. Null move search with verification search (~40 Elo)
807 && (ss-1)->currentMove != MOVE_NULL
808 && (ss-1)->statScore < 22661
810 && ss->staticEval >= beta - 33 * depth / ONE_PLY + 299
812 && pos.non_pawn_material(us)
813 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
815 assert(eval - beta >= 0);
817 // Null move dynamic reduction based on depth and value
818 Depth R = ((835 + 70 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 185, 3)) * ONE_PLY;
820 ss->currentMove = MOVE_NULL;
821 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
823 pos.do_null_move(st);
825 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
827 pos.undo_null_move();
829 if (nullValue >= beta)
831 // Do not return unproven mate scores
832 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
835 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13 * ONE_PLY))
838 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
840 // Do verification search at high depths, with null move pruning disabled
841 // for us, until ply exceeds nmpMinPly.
842 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / (4 * ONE_PLY);
843 thisThread->nmpColor = us;
845 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
847 thisThread->nmpMinPly = 0;
854 // Step 10. ProbCut (~10 Elo)
855 // If we have a good enough capture and a reduced search returns a value
856 // much above beta, we can (almost) safely prune the previous move.
858 && depth >= 5 * ONE_PLY
859 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
861 Value raisedBeta = std::min(beta + 191 - 46 * improving, VALUE_INFINITE);
862 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
863 int probCutCount = 0;
865 while ( (move = mp.next_move()) != MOVE_NONE
866 && probCutCount < 2 + 2 * cutNode)
867 if (move != excludedMove && pos.legal(move))
871 ss->currentMove = move;
872 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
874 assert(depth >= 5 * ONE_PLY);
876 pos.do_move(move, st);
878 // Perform a preliminary qsearch to verify that the move holds
879 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
881 // If the qsearch held, perform the regular search
882 if (value >= raisedBeta)
883 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
887 if (value >= raisedBeta)
892 // Step 11. Internal iterative deepening (~2 Elo)
893 if (depth >= 7 * ONE_PLY && !ttMove)
895 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
897 tte = TT.probe(posKey, ttHit);
898 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
899 ttMove = ttHit ? tte->move() : MOVE_NONE;
902 moves_loop: // When in check, search starts from here
904 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
905 nullptr, (ss-4)->continuationHistory,
906 nullptr, (ss-6)->continuationHistory };
908 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
910 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
911 &thisThread->captureHistory,
916 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
917 moveCountPruning = false;
918 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
920 // Mark this node as being searched.
921 ThreadHolding th(thisThread, posKey, ss->ply);
923 // Step 12. Loop through all pseudo-legal moves until no moves remain
924 // or a beta cutoff occurs.
925 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
929 if (move == excludedMove)
932 // At root obey the "searchmoves" option and skip moves not listed in Root
933 // Move List. As a consequence any illegal move is also skipped. In MultiPV
934 // mode we also skip PV moves which have been already searched and those
935 // of lower "TB rank" if we are in a TB root position.
936 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
937 thisThread->rootMoves.begin() + thisThread->pvLast, move))
940 ss->moveCount = ++moveCount;
942 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
943 sync_cout << "info depth " << depth / ONE_PLY
944 << " currmove " << UCI::move(move, pos.is_chess960())
945 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
947 (ss+1)->pv = nullptr;
949 extension = DEPTH_ZERO;
950 captureOrPromotion = pos.capture_or_promotion(move);
951 movedPiece = pos.moved_piece(move);
952 givesCheck = pos.gives_check(move);
954 // Step 13. Extensions (~70 Elo)
956 // Singular extension search (~60 Elo). If all moves but one fail low on a
957 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
958 // then that move is singular and should be extended. To verify this we do
959 // a reduced search on all the other moves but the ttMove and if the
960 // result is lower than ttValue minus a margin then we will extend the ttMove.
961 if ( depth >= 6 * ONE_PLY
964 && !excludedMove // Avoid recursive singular search
965 /* && ttValue != VALUE_NONE Already implicit in the next condition */
966 && abs(ttValue) < VALUE_KNOWN_WIN
967 && (tte->bound() & BOUND_LOWER)
968 && tte->depth() >= depth - 3 * ONE_PLY
971 Value singularBeta = ttValue - 2 * depth / ONE_PLY;
972 Depth halfDepth = depth / (2 * ONE_PLY) * ONE_PLY; // ONE_PLY invariant
973 ss->excludedMove = move;
974 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, halfDepth, cutNode);
975 ss->excludedMove = MOVE_NONE;
977 if (value < singularBeta)
982 if (value < singularBeta - std::min(4 * depth / ONE_PLY, 36))
987 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
988 // search without the ttMove. So we assume this expected Cut-node is not singular,
989 // that multiple moves fail high, and we can prune the whole subtree by returning
991 else if ( eval >= beta
992 && singularBeta >= beta)
996 // Check extension (~2 Elo)
998 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
1001 // Shuffle extension
1002 else if(pos.rule50_count() > 14 && ss->ply > 14 && depth < 3 * ONE_PLY && PvNode)
1003 extension = ONE_PLY;
1005 // Castling extension
1006 else if (type_of(move) == CASTLING)
1007 extension = ONE_PLY;
1009 // Shuffle extension
1011 && pos.rule50_count() > 18
1012 && depth < 3 * ONE_PLY
1013 && ++thisThread->shuffleExts < thisThread->nodes.load(std::memory_order_relaxed) / 4) // To avoid too many extensions
1014 extension = ONE_PLY;
1016 // Passed pawn extension
1017 else if ( move == ss->killers[0]
1018 && pos.advanced_pawn_push(move)
1019 && pos.pawn_passed(us, to_sq(move)))
1020 extension = ONE_PLY;
1022 // Calculate new depth for this move
1023 newDepth = depth - ONE_PLY + extension;
1025 // Step 14. Pruning at shallow depth (~170 Elo)
1027 && pos.non_pawn_material(us)
1028 && bestValue > VALUE_MATED_IN_MAX_PLY)
1030 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1031 moveCountPruning = moveCount >= futility_move_count(improving, depth / ONE_PLY);
1033 if ( !captureOrPromotion
1035 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material(~us) > BishopValueMg))
1037 // Move count based pruning
1038 if (moveCountPruning)
1041 // Reduced depth of the next LMR search
1042 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), DEPTH_ZERO);
1043 lmrDepth /= ONE_PLY;
1045 // Countermoves based pruning (~20 Elo)
1046 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1047 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1048 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1051 // Futility pruning: parent node (~2 Elo)
1054 && ss->staticEval + 250 + 211 * lmrDepth <= alpha)
1057 // Prune moves with negative SEE (~10 Elo)
1058 if (!pos.see_ge(move, Value(-(31 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1061 else if ( (!givesCheck || !extension)
1062 && !pos.see_ge(move, Value(-199) * (depth / ONE_PLY))) // (~20 Elo)
1066 // Speculative prefetch as early as possible
1067 prefetch(TT.first_entry(pos.key_after(move)));
1069 // Check for legality just before making the move
1070 if (!rootNode && !pos.legal(move))
1072 ss->moveCount = --moveCount;
1076 // Update the current move (this must be done after singular extension search)
1077 ss->currentMove = move;
1078 ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
1080 // Step 15. Make the move
1081 pos.do_move(move, st, givesCheck);
1083 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1084 // re-searched at full depth.
1085 if ( depth >= 3 * ONE_PLY
1086 && moveCount > 1 + 3 * rootNode
1087 && ( !captureOrPromotion
1089 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha))
1091 Depth r = reduction(improving, depth, moveCount);
1093 // Reduction if other threads are searching this position.
1097 // Decrease reduction if position is or has been on the PV
1101 // Decrease reduction if opponent's move count is high (~10 Elo)
1102 if ((ss-1)->moveCount > 15)
1105 // Decrease reduction if move has been singularly extended
1106 r -= singularLMR * ONE_PLY;
1108 if (!captureOrPromotion)
1110 // Increase reduction if ttMove is a capture (~0 Elo)
1114 // Increase reduction for cut nodes (~5 Elo)
1118 // Decrease reduction for moves that escape a capture. Filter out
1119 // castling moves, because they are coded as "king captures rook" and
1120 // hence break make_move(). (~5 Elo)
1121 else if ( type_of(move) == NORMAL
1122 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1125 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1126 + (*contHist[0])[movedPiece][to_sq(move)]
1127 + (*contHist[1])[movedPiece][to_sq(move)]
1128 + (*contHist[3])[movedPiece][to_sq(move)]
1131 // Reset statScore to zero if negative and most stats shows >= 0
1132 if ( ss->statScore < 0
1133 && (*contHist[0])[movedPiece][to_sq(move)] >= 0
1134 && (*contHist[1])[movedPiece][to_sq(move)] >= 0
1135 && thisThread->mainHistory[us][from_to(move)] >= 0)
1138 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1139 if (ss->statScore >= -99 && (ss-1)->statScore < -116)
1142 else if ((ss-1)->statScore >= -117 && ss->statScore < -144)
1145 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1146 r -= ss->statScore / 16384 * ONE_PLY;
1149 Depth d = clamp(newDepth - r, ONE_PLY, newDepth);
1151 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1153 doFullDepthSearch = (value > alpha && d != newDepth), doLMR = true;
1156 doFullDepthSearch = !PvNode || moveCount > 1, doLMR = false;
1158 // Step 17. Full depth search when LMR is skipped or fails high
1159 if (doFullDepthSearch)
1161 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1163 if (doLMR && !captureOrPromotion)
1165 int bonus = value > alpha ? stat_bonus(newDepth)
1166 : -stat_bonus(newDepth);
1168 if (move == ss->killers[0])
1171 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1175 // For PV nodes only, do a full PV search on the first move or after a fail
1176 // high (in the latter case search only if value < beta), otherwise let the
1177 // parent node fail low with value <= alpha and try another move.
1178 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1181 (ss+1)->pv[0] = MOVE_NONE;
1183 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1186 // Step 18. Undo move
1187 pos.undo_move(move);
1189 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1191 // Step 19. Check for a new best move
1192 // Finished searching the move. If a stop occurred, the return value of
1193 // the search cannot be trusted, and we return immediately without
1194 // updating best move, PV and TT.
1195 if (Threads.stop.load(std::memory_order_relaxed))
1200 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1201 thisThread->rootMoves.end(), move);
1203 // PV move or new best move?
1204 if (moveCount == 1 || value > alpha)
1207 rm.selDepth = thisThread->selDepth;
1212 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1213 rm.pv.push_back(*m);
1215 // We record how often the best move has been changed in each
1216 // iteration. This information is used for time management: When
1217 // the best move changes frequently, we allocate some more time.
1219 ++thisThread->bestMoveChanges;
1222 // All other moves but the PV are set to the lowest value: this
1223 // is not a problem when sorting because the sort is stable and the
1224 // move position in the list is preserved - just the PV is pushed up.
1225 rm.score = -VALUE_INFINITE;
1228 if (value > bestValue)
1236 if (PvNode && !rootNode) // Update pv even in fail-high case
1237 update_pv(ss->pv, move, (ss+1)->pv);
1239 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1243 assert(value >= beta); // Fail high
1250 if (move != bestMove)
1252 if (captureOrPromotion && captureCount < 32)
1253 capturesSearched[captureCount++] = move;
1255 else if (!captureOrPromotion && quietCount < 64)
1256 quietsSearched[quietCount++] = move;
1260 // The following condition would detect a stop only after move loop has been
1261 // completed. But in this case bestValue is valid because we have fully
1262 // searched our subtree, and we can anyhow save the result in TT.
1268 // Step 20. Check for mate and stalemate
1269 // All legal moves have been searched and if there are no legal moves, it
1270 // must be a mate or a stalemate. If we are in a singular extension search then
1271 // return a fail low score.
1273 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1276 bestValue = excludedMove ? alpha
1277 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1280 // Quiet best move: update move sorting heuristics
1281 if (!pos.capture_or_promotion(bestMove))
1282 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1283 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1285 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1287 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1288 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1289 && !pos.captured_piece())
1290 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1293 // Bonus for prior countermove that caused the fail low
1294 else if ( (depth >= 3 * ONE_PLY || PvNode)
1295 && !pos.captured_piece())
1296 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1299 bestValue = std::min(bestValue, maxValue);
1302 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1303 bestValue >= beta ? BOUND_LOWER :
1304 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1305 depth, bestMove, ss->staticEval);
1307 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1313 // qsearch() is the quiescence search function, which is called by the main search
1314 // function with zero depth, or recursively with further decreasing depth per call.
1315 template <NodeType NT>
1316 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1318 constexpr bool PvNode = NT == PV;
1320 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1321 assert(PvNode || (alpha == beta - 1));
1322 assert(depth <= DEPTH_ZERO);
1323 assert(depth / ONE_PLY * ONE_PLY == depth);
1329 Move ttMove, move, bestMove;
1331 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1332 bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
1337 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1339 ss->pv[0] = MOVE_NONE;
1342 Thread* thisThread = pos.this_thread();
1343 (ss+1)->ply = ss->ply + 1;
1344 bestMove = MOVE_NONE;
1345 inCheck = pos.checkers();
1348 // Check for an immediate draw or maximum ply reached
1349 if ( pos.is_draw(ss->ply)
1350 || ss->ply >= MAX_PLY)
1351 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1353 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1355 // Decide whether or not to include checks: this fixes also the type of
1356 // TT entry depth that we are going to use. Note that in qsearch we use
1357 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1358 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1359 : DEPTH_QS_NO_CHECKS;
1360 // Transposition table lookup
1362 tte = TT.probe(posKey, ttHit);
1363 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1364 ttMove = ttHit ? tte->move() : MOVE_NONE;
1365 pvHit = ttHit && tte->is_pv();
1369 && tte->depth() >= ttDepth
1370 && ttValue != VALUE_NONE // Only in case of TT access race
1371 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1372 : (tte->bound() & BOUND_UPPER)))
1375 // Evaluate the position statically
1378 ss->staticEval = VALUE_NONE;
1379 bestValue = futilityBase = -VALUE_INFINITE;
1385 // Never assume anything about values stored in TT
1386 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1387 ss->staticEval = bestValue = evaluate(pos);
1389 // Can ttValue be used as a better position evaluation?
1390 if ( ttValue != VALUE_NONE
1391 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1392 bestValue = ttValue;
1395 ss->staticEval = bestValue =
1396 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1397 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1399 // Stand pat. Return immediately if static value is at least beta
1400 if (bestValue >= beta)
1403 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1404 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1409 if (PvNode && bestValue > alpha)
1412 futilityBase = bestValue + 153;
1415 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1416 nullptr, (ss-4)->continuationHistory,
1417 nullptr, (ss-6)->continuationHistory };
1419 // Initialize a MovePicker object for the current position, and prepare
1420 // to search the moves. Because the depth is <= 0 here, only captures,
1421 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1423 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1424 &thisThread->captureHistory,
1426 to_sq((ss-1)->currentMove));
1428 // Loop through the moves until no moves remain or a beta cutoff occurs
1429 while ((move = mp.next_move()) != MOVE_NONE)
1431 assert(is_ok(move));
1433 givesCheck = pos.gives_check(move);
1440 && futilityBase > -VALUE_KNOWN_WIN
1441 && !pos.advanced_pawn_push(move))
1443 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1445 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1447 if (futilityValue <= alpha)
1449 bestValue = std::max(bestValue, futilityValue);
1453 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1455 bestValue = std::max(bestValue, futilityBase);
1460 // Detect non-capture evasions that are candidates to be pruned
1461 evasionPrunable = inCheck
1462 && (depth != DEPTH_ZERO || moveCount > 2)
1463 && bestValue > VALUE_MATED_IN_MAX_PLY
1464 && !pos.capture(move);
1466 // Don't search moves with negative SEE values
1467 if ( (!inCheck || evasionPrunable)
1468 && (!givesCheck || !(pos.blockers_for_king(~pos.side_to_move()) & from_sq(move)))
1469 && !pos.see_ge(move))
1472 // Speculative prefetch as early as possible
1473 prefetch(TT.first_entry(pos.key_after(move)));
1475 // Check for legality just before making the move
1476 if (!pos.legal(move))
1482 ss->currentMove = move;
1483 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
1485 // Make and search the move
1486 pos.do_move(move, st, givesCheck);
1487 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1488 pos.undo_move(move);
1490 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1492 // Check for a new best move
1493 if (value > bestValue)
1501 if (PvNode) // Update pv even in fail-high case
1502 update_pv(ss->pv, move, (ss+1)->pv);
1504 if (PvNode && value < beta) // Update alpha here!
1512 // All legal moves have been searched. A special case: If we're in check
1513 // and no legal moves were found, it is checkmate.
1514 if (inCheck && bestValue == -VALUE_INFINITE)
1515 return mated_in(ss->ply); // Plies to mate from the root
1517 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1518 bestValue >= beta ? BOUND_LOWER :
1519 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1520 ttDepth, bestMove, ss->staticEval);
1522 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1528 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1529 // "plies to mate from the current position". Non-mate scores are unchanged.
1530 // The function is called before storing a value in the transposition table.
1532 Value value_to_tt(Value v, int ply) {
1534 assert(v != VALUE_NONE);
1536 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1537 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1541 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1542 // from the transposition table (which refers to the plies to mate/be mated
1543 // from current position) to "plies to mate/be mated from the root".
1545 Value value_from_tt(Value v, int ply) {
1547 return v == VALUE_NONE ? VALUE_NONE
1548 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1549 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1553 // update_pv() adds current move and appends child pv[]
1555 void update_pv(Move* pv, Move move, Move* childPv) {
1557 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1563 // update_continuation_histories() updates histories of the move pairs formed
1564 // by moves at ply -1, -2, and -4 with current move.
1566 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1568 for (int i : {1, 2, 4, 6})
1569 if (is_ok((ss-i)->currentMove))
1570 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1574 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1576 void update_capture_stats(const Position& pos, Move move,
1577 Move* captures, int captureCount, int bonus) {
1579 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1580 Piece moved_piece = pos.moved_piece(move);
1581 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1583 if (pos.capture_or_promotion(move))
1584 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1586 // Decrease all the other played capture moves
1587 for (int i = 0; i < captureCount; ++i)
1589 moved_piece = pos.moved_piece(captures[i]);
1590 captured = type_of(pos.piece_on(to_sq(captures[i])));
1591 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1596 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1598 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1599 Move* quiets, int quietCount, int bonus) {
1601 if (ss->killers[0] != move)
1603 ss->killers[1] = ss->killers[0];
1604 ss->killers[0] = move;
1607 Color us = pos.side_to_move();
1608 Thread* thisThread = pos.this_thread();
1609 thisThread->mainHistory[us][from_to(move)] << bonus;
1610 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1612 if (is_ok((ss-1)->currentMove))
1614 Square prevSq = to_sq((ss-1)->currentMove);
1615 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1618 // Decrease all the other played quiet moves
1619 for (int i = 0; i < quietCount; ++i)
1621 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1622 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1626 // When playing with strength handicap, choose best move among a set of RootMoves
1627 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1629 Move Skill::pick_best(size_t multiPV) {
1631 const RootMoves& rootMoves = Threads.main()->rootMoves;
1632 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1634 // RootMoves are already sorted by score in descending order
1635 Value topScore = rootMoves[0].score;
1636 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1637 int weakness = 120 - 2 * level;
1638 int maxScore = -VALUE_INFINITE;
1640 // Choose best move. For each move score we add two terms, both dependent on
1641 // weakness. One is deterministic and bigger for weaker levels, and one is
1642 // random. Then we choose the move with the resulting highest score.
1643 for (size_t i = 0; i < multiPV; ++i)
1645 // This is our magic formula
1646 int push = ( weakness * int(topScore - rootMoves[i].score)
1647 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1649 if (rootMoves[i].score + push >= maxScore)
1651 maxScore = rootMoves[i].score + push;
1652 best = rootMoves[i].pv[0];
1661 /// MainThread::check_time() is used to print debug info and, more importantly,
1662 /// to detect when we are out of available time and thus stop the search.
1664 void MainThread::check_time() {
1669 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1670 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1672 static TimePoint lastInfoTime = now();
1674 TimePoint elapsed = Time.elapsed();
1675 TimePoint tick = Limits.startTime + elapsed;
1677 if (tick - lastInfoTime >= 1000)
1679 lastInfoTime = tick;
1683 // We should not stop pondering until told so by the GUI
1687 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1688 || (Limits.movetime && elapsed >= Limits.movetime)
1689 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1690 Threads.stop = true;
1694 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1695 /// that all (if any) unsearched PV lines are sent using a previous search score.
1697 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1699 std::stringstream ss;
1700 TimePoint elapsed = Time.elapsed() + 1;
1701 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1702 size_t pvIdx = pos.this_thread()->pvIdx;
1703 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1704 uint64_t nodesSearched = Threads.nodes_searched();
1705 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1707 for (size_t i = 0; i < multiPV; ++i)
1709 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1711 if (depth == ONE_PLY && !updated)
1714 Depth d = updated ? depth : depth - ONE_PLY;
1715 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1717 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1718 v = tb ? rootMoves[i].tbScore : v;
1720 if (ss.rdbuf()->in_avail()) // Not at first line
1724 << " depth " << d / ONE_PLY
1725 << " seldepth " << rootMoves[i].selDepth
1726 << " multipv " << i + 1
1727 << " score " << UCI::value(v);
1729 if (!tb && i == pvIdx)
1730 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1732 ss << " nodes " << nodesSearched
1733 << " nps " << nodesSearched * 1000 / elapsed;
1735 if (elapsed > 1000) // Earlier makes little sense
1736 ss << " hashfull " << TT.hashfull();
1738 ss << " tbhits " << tbHits
1739 << " time " << elapsed
1742 for (Move m : rootMoves[i].pv)
1743 ss << " " << UCI::move(m, pos.is_chess960());
1750 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1751 /// before exiting the search, for instance, in case we stop the search during a
1752 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1753 /// otherwise in case of 'ponder on' we have nothing to think on.
1755 bool RootMove::extract_ponder_from_tt(Position& pos) {
1760 assert(pv.size() == 1);
1762 if (pv[0] == MOVE_NONE)
1765 pos.do_move(pv[0], st);
1766 TTEntry* tte = TT.probe(pos.key(), ttHit);
1770 Move m = tte->move(); // Local copy to be SMP safe
1771 if (MoveList<LEGAL>(pos).contains(m))
1775 pos.undo_move(pv[0]);
1776 return pv.size() > 1;
1779 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1782 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1783 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1784 Cardinality = int(Options["SyzygyProbeLimit"]);
1785 bool dtz_available = true;
1787 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1788 // ProbeDepth == DEPTH_ZERO
1789 if (Cardinality > MaxCardinality)
1791 Cardinality = MaxCardinality;
1792 ProbeDepth = DEPTH_ZERO;
1795 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1797 // Rank moves using DTZ tables
1798 RootInTB = root_probe(pos, rootMoves);
1802 // DTZ tables are missing; try to rank moves using WDL tables
1803 dtz_available = false;
1804 RootInTB = root_probe_wdl(pos, rootMoves);
1810 // Sort moves according to TB rank
1811 std::sort(rootMoves.begin(), rootMoves.end(),
1812 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1814 // Probe during search only if DTZ is not available and we are winning
1815 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1820 // Clean up if root_probe() and root_probe_wdl() have failed
1821 for (auto& m : rootMoves)