2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2019 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
53 namespace TB = Tablebases;
57 using namespace Search;
61 // Different node types, used as a template parameter
62 enum NodeType { NonPV, PV };
64 // Razor and futility margins
65 constexpr int RazorMargin = 600;
66 Value futility_margin(Depth d, bool improving) {
67 return Value((175 - 50 * improving) * d / ONE_PLY);
70 // Reductions lookup table, initialized at startup
71 int Reductions[MAX_MOVES]; // [depth or moveNumber]
73 Depth reduction(bool i, Depth d, int mn) {
74 int r = Reductions[d / ONE_PLY] * Reductions[mn];
75 return ((r + 512) / 1024 + (!i && r > 1024)) * ONE_PLY;
78 constexpr int futility_move_count(bool improving, int depth) {
79 return (5 + depth * depth) * (1 + improving) / 2;
82 // History and stats update bonus, based on depth
83 int stat_bonus(Depth depth) {
84 int d = depth / ONE_PLY;
85 return d > 17 ? 0 : 29 * d * d + 138 * d - 134;
88 // Add a small random component to draw evaluations to avoid 3fold-blindness
89 Value value_draw(Depth depth, Thread* thisThread) {
90 return depth < 4 * ONE_PLY ? VALUE_DRAW
91 : VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
94 // Skill structure is used to implement strength limit
96 explicit Skill(int l) : level(l) {}
97 bool enabled() const { return level < 20; }
98 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
99 Move pick_best(size_t multiPV);
102 Move best = MOVE_NONE;
105 // 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(22.9 * 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()
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 and reset votes for moves which can be voted
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 (votes[th->rootMoves[0].pv[0]] > votes[bestThread->rootMoves[0].pv[0]])
295 previousScore = bestThread->rootMoves[0].score;
297 // Send again PV info if we have a new best thread
298 if (bestThread != this)
299 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
301 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
303 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
304 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
306 std::cout << sync_endl;
310 /// Thread::search() is the main iterative deepening loop. It calls search()
311 /// repeatedly with increasing depth until the allocated thinking time has been
312 /// consumed, the user stops the search, or the maximum search depth is reached.
314 void Thread::search() {
316 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
317 // The former is needed to allow update_continuation_histories(ss-1, ...),
318 // which accesses its argument at ss-6, also near the root.
319 // The latter is needed for statScores and killer initialization.
320 Stack stack[MAX_PLY+10], *ss = stack+7;
322 Value bestValue, alpha, beta, delta;
323 Move lastBestMove = MOVE_NONE;
324 Depth lastBestMoveDepth = DEPTH_ZERO;
325 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
326 double timeReduction = 1, totBestMoveChanges = 0;
327 Color us = rootPos.side_to_move();
329 std::memset(ss-7, 0, 10 * sizeof(Stack));
330 for (int i = 7; i > 0; i--)
331 (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
334 bestValue = delta = alpha = -VALUE_INFINITE;
335 beta = VALUE_INFINITE;
337 multiPV = Options["MultiPV"];
338 // Pick integer skill levels, but non-deterministically round up or down
339 // such that the average integer skill corresponds to the input floating point one.
341 int intLevel = int(Options["Skill Level"]) +
342 ((Options["Skill Level"] - int(Options["Skill Level"])) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
343 Skill skill(intLevel);
345 // When playing with strength handicap enable MultiPV search that we will
346 // use behind the scenes to retrieve a set of possible moves.
348 multiPV = std::max(multiPV, (size_t)4);
350 multiPV = std::min(multiPV, rootMoves.size());
352 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
354 // In analysis mode, adjust contempt in accordance with user preference
355 if (Limits.infinite || Options["UCI_AnalyseMode"])
356 ct = Options["Analysis Contempt"] == "Off" ? 0
357 : Options["Analysis Contempt"] == "Both" ? ct
358 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
359 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
362 // Evaluation score is from the white point of view
363 contempt = (us == WHITE ? make_score(ct, ct / 2)
364 : -make_score(ct, ct / 2));
366 // Iterative deepening loop until requested to stop or the target depth is reached
367 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
369 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
371 // Age out PV variability metric
373 totBestMoveChanges /= 2;
375 // Save the last iteration's scores before first PV line is searched and
376 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
377 for (RootMove& rm : rootMoves)
378 rm.previousScore = rm.score;
383 // MultiPV loop. We perform a full root search for each PV line
384 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
389 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
390 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
394 // Reset UCI info selDepth for each depth and each PV line
397 // Reset aspiration window starting size
398 if (rootDepth >= 5 * ONE_PLY)
400 Value previousScore = rootMoves[pvIdx].previousScore;
402 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
403 beta = std::min(previousScore + delta, VALUE_INFINITE);
405 // Adjust contempt based on root move's previousScore (dynamic contempt)
406 int dct = ct + 88 * previousScore / (abs(previousScore) + 200);
408 contempt = (us == WHITE ? make_score(dct, dct / 2)
409 : -make_score(dct, dct / 2));
412 // Start with a small aspiration window and, in the case of a fail
413 // high/low, re-search with a bigger window until we don't fail
415 int failedHighCnt = 0;
418 Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
419 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
421 // Bring the best move to the front. It is critical that sorting
422 // is done with a stable algorithm because all the values but the
423 // first and eventually the new best one are set to -VALUE_INFINITE
424 // and we want to keep the same order for all the moves except the
425 // new PV that goes to the front. Note that in case of MultiPV
426 // search the already searched PV lines are preserved.
427 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
429 // If search has been stopped, we break immediately. Sorting is
430 // safe because RootMoves is still valid, although it refers to
431 // the previous iteration.
435 // When failing high/low give some update (without cluttering
436 // the UI) before a re-search.
439 && (bestValue <= alpha || bestValue >= beta)
440 && Time.elapsed() > 3000)
441 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
443 // In case of failing low/high increase aspiration window and
444 // re-search, otherwise exit the loop.
445 if (bestValue <= alpha)
447 beta = (alpha + beta) / 2;
448 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
452 mainThread->stopOnPonderhit = false;
454 else if (bestValue >= beta)
456 beta = std::min(bestValue + delta, VALUE_INFINITE);
462 delta += delta / 4 + 5;
464 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
467 // Sort the PV lines searched so far and update the GUI
468 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
471 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
472 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
476 completedDepth = rootDepth;
478 if (rootMoves[0].pv[0] != lastBestMove) {
479 lastBestMove = rootMoves[0].pv[0];
480 lastBestMoveDepth = rootDepth;
483 // Have we found a "mate in x"?
485 && bestValue >= VALUE_MATE_IN_MAX_PLY
486 && VALUE_MATE - bestValue <= 2 * Limits.mate)
492 // If skill level is enabled and time is up, pick a sub-optimal best move
493 if (skill.enabled() && skill.time_to_pick(rootDepth))
494 skill.pick_best(multiPV);
496 // Do we have time for the next iteration? Can we stop searching now?
497 if ( Limits.use_time_management()
499 && !mainThread->stopOnPonderhit)
501 double fallingEval = (314 + 9 * (mainThread->previousScore - bestValue)) / 581.0;
502 fallingEval = clamp(fallingEval, 0.5, 1.5);
504 // If the bestMove is stable over several iterations, reduce time accordingly
505 timeReduction = lastBestMoveDepth + 10 * ONE_PLY < completedDepth ? 1.95 : 1.0;
506 double reduction = std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
508 // Use part of the gained time from a previous stable move for the current move
509 for (Thread* th : Threads)
511 totBestMoveChanges += th->bestMoveChanges;
512 th->bestMoveChanges = 0;
514 double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
516 // Stop the search if we have only one legal move, or if available time elapsed
517 if ( rootMoves.size() == 1
518 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
520 // If we are allowed to ponder do not stop the search now but
521 // keep pondering until the GUI sends "ponderhit" or "stop".
522 if (mainThread->ponder)
523 mainThread->stopOnPonderhit = true;
533 mainThread->previousTimeReduction = timeReduction;
535 // If skill level is enabled, swap best PV line with the sub-optimal one
537 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
538 skill.best ? skill.best : skill.pick_best(multiPV)));
544 // search<>() is the main search function for both PV and non-PV nodes
546 template <NodeType NT>
547 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
549 constexpr bool PvNode = NT == PV;
550 const bool rootNode = PvNode && ss->ply == 0;
552 // Check if we have an upcoming move which draws by repetition, or
553 // if the opponent had an alternative move earlier to this position.
554 if ( pos.rule50_count() >= 3
555 && alpha < VALUE_DRAW
557 && pos.has_game_cycle(ss->ply))
559 alpha = value_draw(depth, pos.this_thread());
564 // Dive into quiescence search when the depth reaches zero
566 return qsearch<NT>(pos, ss, alpha, beta);
568 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
569 assert(PvNode || (alpha == beta - 1));
570 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
571 assert(!(PvNode && cutNode));
572 assert(depth / ONE_PLY * ONE_PLY == depth);
574 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
578 Move ttMove, move, excludedMove, bestMove;
579 Depth extension, newDepth;
580 Value bestValue, value, ttValue, eval, maxValue;
581 bool ttHit, ttPv, inCheck, givesCheck, improving;
582 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
584 int moveCount, captureCount, quietCount, singularLMR;
586 // Step 1. Initialize node
587 Thread* thisThread = pos.this_thread();
588 inCheck = pos.checkers();
589 Color us = pos.side_to_move();
590 moveCount = captureCount = quietCount = singularLMR = ss->moveCount = 0;
591 bestValue = -VALUE_INFINITE;
592 maxValue = VALUE_INFINITE;
594 // Check for the available remaining time
595 if (thisThread == Threads.main())
596 static_cast<MainThread*>(thisThread)->check_time();
598 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
599 if (PvNode && thisThread->selDepth < ss->ply + 1)
600 thisThread->selDepth = ss->ply + 1;
604 // Step 2. Check for aborted search and immediate draw
605 if ( Threads.stop.load(std::memory_order_relaxed)
606 || pos.is_draw(ss->ply)
607 || ss->ply >= MAX_PLY)
608 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
609 : value_draw(depth, pos.this_thread());
611 // Step 3. Mate distance pruning. Even if we mate at the next move our score
612 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
613 // a shorter mate was found upward in the tree then there is no need to search
614 // because we will never beat the current alpha. Same logic but with reversed
615 // signs applies also in the opposite condition of being mated instead of giving
616 // mate. In this case return a fail-high score.
617 alpha = std::max(mated_in(ss->ply), alpha);
618 beta = std::min(mate_in(ss->ply+1), beta);
623 assert(0 <= ss->ply && ss->ply < MAX_PLY);
625 (ss+1)->ply = ss->ply + 1;
626 (ss+1)->excludedMove = bestMove = MOVE_NONE;
627 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
628 Square prevSq = to_sq((ss-1)->currentMove);
630 // Initialize statScore to zero for the grandchildren of the current position.
631 // So statScore is shared between all grandchildren and only the first grandchild
632 // starts with statScore = 0. Later grandchildren start with the last calculated
633 // statScore of the previous grandchild. This influences the reduction rules in
634 // LMR which are based on the statScore of parent position.
636 (ss + 4)->statScore = 0;
638 (ss + 2)->statScore = 0;
640 // Step 4. Transposition table lookup. We don't want the score of a partial
641 // search to overwrite a previous full search TT value, so we use a different
642 // position key in case of an excluded move.
643 excludedMove = ss->excludedMove;
644 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
645 tte = TT.probe(posKey, ttHit);
646 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
647 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
648 : ttHit ? tte->move() : MOVE_NONE;
649 ttPv = PvNode || (ttHit && tte->is_pv());
651 // At non-PV nodes we check for an early TT cutoff
654 && tte->depth() >= depth
655 && ttValue != VALUE_NONE // Possible in case of TT access race
656 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
657 : (tte->bound() & BOUND_UPPER)))
659 // If ttMove is quiet, update move sorting heuristics on TT hit
664 if (!pos.capture_or_promotion(ttMove))
665 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
667 // Extra penalty for early quiet moves of the previous ply
668 if ((ss-1)->moveCount <= 2 && !pos.captured_piece())
669 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
671 // Penalty for a quiet ttMove that fails low
672 else if (!pos.capture_or_promotion(ttMove))
674 int penalty = -stat_bonus(depth);
675 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
676 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
682 // Step 5. Tablebases probe
683 if (!rootNode && TB::Cardinality)
685 int piecesCount = pos.count<ALL_PIECES>();
687 if ( piecesCount <= TB::Cardinality
688 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
689 && pos.rule50_count() == 0
690 && !pos.can_castle(ANY_CASTLING))
693 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
695 // Force check of time on the next occasion
696 if (thisThread == Threads.main())
697 static_cast<MainThread*>(thisThread)->callsCnt = 0;
699 if (err != TB::ProbeState::FAIL)
701 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
703 int drawScore = TB::UseRule50 ? 1 : 0;
705 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
706 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
707 : VALUE_DRAW + 2 * wdl * drawScore;
709 Bound b = wdl < -drawScore ? BOUND_UPPER
710 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
712 if ( b == BOUND_EXACT
713 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
715 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
716 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
717 MOVE_NONE, VALUE_NONE);
724 if (b == BOUND_LOWER)
725 bestValue = value, alpha = std::max(alpha, bestValue);
733 // Step 6. Static evaluation of the position
736 ss->staticEval = eval = VALUE_NONE;
738 goto moves_loop; // Skip early pruning when in check
742 // Never assume anything about values stored in TT
743 ss->staticEval = eval = tte->eval();
744 if (eval == VALUE_NONE)
745 ss->staticEval = eval = evaluate(pos);
747 // Can ttValue be used as a better position evaluation?
748 if ( ttValue != VALUE_NONE
749 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
754 if ((ss-1)->currentMove != MOVE_NULL)
756 int bonus = -(ss-1)->statScore / 512;
758 ss->staticEval = eval = evaluate(pos) + bonus;
761 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Eval::Tempo;
763 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
766 // Step 7. Razoring (~2 Elo)
767 if ( !rootNode // The required rootNode PV handling is not available in qsearch
768 && depth < 2 * ONE_PLY
769 && eval <= alpha - RazorMargin)
770 return qsearch<NT>(pos, ss, alpha, beta);
772 improving = ss->staticEval >= (ss-2)->staticEval
773 || (ss-2)->staticEval == VALUE_NONE;
775 // Step 8. Futility pruning: child node (~30 Elo)
777 && depth < 7 * ONE_PLY
778 && eval - futility_margin(depth, improving) >= beta
779 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
782 // Step 9. Null move search with verification search (~40 Elo)
784 && (ss-1)->currentMove != MOVE_NULL
785 && (ss-1)->statScore < 23200
787 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
789 && pos.non_pawn_material(us)
790 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
792 assert(eval - beta >= 0);
794 // Null move dynamic reduction based on depth and value
795 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 200, 3)) * ONE_PLY;
797 ss->currentMove = MOVE_NULL;
798 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
800 pos.do_null_move(st);
802 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
804 pos.undo_null_move();
806 if (nullValue >= beta)
808 // Do not return unproven mate scores
809 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
812 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 12 * ONE_PLY))
815 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
817 // Do verification search at high depths, with null move pruning disabled
818 // for us, until ply exceeds nmpMinPly.
819 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / (4 * ONE_PLY);
820 thisThread->nmpColor = us;
822 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
824 thisThread->nmpMinPly = 0;
831 // Step 10. ProbCut (~10 Elo)
832 // If we have a good enough capture and a reduced search returns a value
833 // much above beta, we can (almost) safely prune the previous move.
835 && depth >= 5 * ONE_PLY
836 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
838 Value raisedBeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
839 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
840 int probCutCount = 0;
842 while ( (move = mp.next_move()) != MOVE_NONE
843 && probCutCount < 2 + 2 * cutNode)
844 if (move != excludedMove && pos.legal(move))
848 ss->currentMove = move;
849 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
851 assert(depth >= 5 * ONE_PLY);
853 pos.do_move(move, st);
855 // Perform a preliminary qsearch to verify that the move holds
856 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
858 // If the qsearch held, perform the regular search
859 if (value >= raisedBeta)
860 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
864 if (value >= raisedBeta)
869 // Step 11. Internal iterative deepening (~2 Elo)
870 if (depth >= 8 * ONE_PLY && !ttMove)
872 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
874 tte = TT.probe(posKey, ttHit);
875 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
876 ttMove = ttHit ? tte->move() : MOVE_NONE;
879 moves_loop: // When in check, search starts from here
881 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
882 nullptr, (ss-4)->continuationHistory,
883 nullptr, (ss-6)->continuationHistory };
885 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
887 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
888 &thisThread->captureHistory,
893 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
894 moveCountPruning = false;
895 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
897 // Mark this node as being searched.
898 ThreadHolding th(thisThread, posKey, ss->ply);
900 // Step 12. Loop through all pseudo-legal moves until no moves remain
901 // or a beta cutoff occurs.
902 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
906 if (move == excludedMove)
909 // At root obey the "searchmoves" option and skip moves not listed in Root
910 // Move List. As a consequence any illegal move is also skipped. In MultiPV
911 // mode we also skip PV moves which have been already searched and those
912 // of lower "TB rank" if we are in a TB root position.
913 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
914 thisThread->rootMoves.begin() + thisThread->pvLast, move))
917 ss->moveCount = ++moveCount;
919 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
920 sync_cout << "info depth " << depth / ONE_PLY
921 << " currmove " << UCI::move(move, pos.is_chess960())
922 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
924 // In MultiPV mode also skip moves which will be searched later as PV moves
925 if (rootNode && std::count(thisThread->rootMoves.begin() + thisThread->pvIdx + 1,
926 thisThread->rootMoves.begin() + thisThread->multiPV, move))
930 (ss+1)->pv = nullptr;
932 extension = DEPTH_ZERO;
933 captureOrPromotion = pos.capture_or_promotion(move);
934 movedPiece = pos.moved_piece(move);
935 givesCheck = pos.gives_check(move);
937 // Step 13. Extensions (~70 Elo)
939 // Singular extension search (~60 Elo). If all moves but one fail low on a
940 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
941 // then that move is singular and should be extended. To verify this we do
942 // a reduced search on all the other moves but the ttMove and if the
943 // result is lower than ttValue minus a margin then we will extend the ttMove.
944 if ( depth >= 8 * ONE_PLY
947 && !excludedMove // Avoid recursive singular search
948 /* && ttValue != VALUE_NONE Already implicit in the next condition */
949 && abs(ttValue) < VALUE_KNOWN_WIN
950 && (tte->bound() & BOUND_LOWER)
951 && tte->depth() >= depth - 3 * ONE_PLY
954 Value singularBeta = ttValue - 2 * depth / ONE_PLY;
955 Depth halfDepth = depth / (2 * ONE_PLY) * ONE_PLY; // ONE_PLY invariant
956 ss->excludedMove = move;
957 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, halfDepth, cutNode);
958 ss->excludedMove = MOVE_NONE;
960 if (value < singularBeta)
965 if (value < singularBeta - std::min(3 * depth / ONE_PLY, 39))
970 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
971 // search without the ttMove. So we assume this expected Cut-node is not singular,
972 // that multiple moves fail high, and we can prune the whole subtree by returning
974 else if ( eval >= beta
975 && singularBeta >= beta)
979 // Check extension (~2 Elo)
981 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
984 // Castling extension
985 else if (type_of(move) == CASTLING)
990 && pos.rule50_count() > 18
991 && depth < 3 * ONE_PLY
992 && ++thisThread->shuffleExts < thisThread->nodes.load(std::memory_order_relaxed) / 4) // To avoid too many extensions
995 // Passed pawn extension
996 else if ( move == ss->killers[0]
997 && pos.advanced_pawn_push(move)
998 && pos.pawn_passed(us, to_sq(move)))
1001 // Calculate new depth for this move
1002 newDepth = depth - ONE_PLY + extension;
1004 // Step 14. Pruning at shallow depth (~170 Elo)
1006 && pos.non_pawn_material(us)
1007 && bestValue > VALUE_MATED_IN_MAX_PLY)
1009 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1010 moveCountPruning = moveCount >= futility_move_count(improving, depth / ONE_PLY);
1012 if ( !captureOrPromotion
1014 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material(~us) > BishopValueMg))
1016 // Move count based pruning
1017 if (moveCountPruning)
1020 // Reduced depth of the next LMR search
1021 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), DEPTH_ZERO);
1022 lmrDepth /= ONE_PLY;
1024 // Countermoves based pruning (~20 Elo)
1025 if ( lmrDepth < 3 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1026 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1027 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1030 // Futility pruning: parent node (~2 Elo)
1033 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
1036 // Prune moves with negative SEE (~10 Elo)
1037 if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
1040 else if ( (!givesCheck || !extension)
1041 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY))) // (~20 Elo)
1045 // Speculative prefetch as early as possible
1046 prefetch(TT.first_entry(pos.key_after(move)));
1048 // Check for legality just before making the move
1049 if (!rootNode && !pos.legal(move))
1051 ss->moveCount = --moveCount;
1055 // Update the current move (this must be done after singular extension search)
1056 ss->currentMove = move;
1057 ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
1059 // Step 15. Make the move
1060 pos.do_move(move, st, givesCheck);
1062 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1063 // re-searched at full depth.
1064 if ( depth >= 3 * ONE_PLY
1065 && moveCount > 1 + 3 * rootNode
1066 && ( !captureOrPromotion
1068 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha))
1070 Depth r = reduction(improving, depth, moveCount);
1072 // Reduction if other threads are searching this position.
1076 // Decrease reduction if position is or has been on the PV
1080 // Decrease reduction if opponent's move count is high (~10 Elo)
1081 if ((ss-1)->moveCount > 15)
1084 // Decrease reduction if move has been singularly extended
1085 r -= singularLMR * ONE_PLY;
1087 if (!captureOrPromotion)
1089 // Increase reduction if ttMove is a capture (~0 Elo)
1093 // Increase reduction for cut nodes (~5 Elo)
1097 // Decrease reduction for moves that escape a capture. Filter out
1098 // castling moves, because they are coded as "king captures rook" and
1099 // hence break make_move(). (~5 Elo)
1100 else if ( type_of(move) == NORMAL
1101 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1104 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1105 + (*contHist[0])[movedPiece][to_sq(move)]
1106 + (*contHist[1])[movedPiece][to_sq(move)]
1107 + (*contHist[3])[movedPiece][to_sq(move)]
1110 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1111 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1114 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1117 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1118 r -= ss->statScore / 16384 * ONE_PLY;
1121 Depth d = clamp(newDepth - r, ONE_PLY, newDepth);
1123 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1125 doFullDepthSearch = (value > alpha && d != newDepth);
1128 doFullDepthSearch = !PvNode || moveCount > 1;
1130 // Step 17. Full depth search when LMR is skipped or fails high
1131 if (doFullDepthSearch)
1132 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1134 // For PV nodes only, do a full PV search on the first move or after a fail
1135 // high (in the latter case search only if value < beta), otherwise let the
1136 // parent node fail low with value <= alpha and try another move.
1137 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1140 (ss+1)->pv[0] = MOVE_NONE;
1142 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1145 // Step 18. Undo move
1146 pos.undo_move(move);
1148 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1150 // Step 19. Check for a new best move
1151 // Finished searching the move. If a stop occurred, the return value of
1152 // the search cannot be trusted, and we return immediately without
1153 // updating best move, PV and TT.
1154 if (Threads.stop.load(std::memory_order_relaxed))
1159 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1160 thisThread->rootMoves.end(), move);
1162 // PV move or new best move?
1163 if (moveCount == 1 || value > alpha)
1166 rm.selDepth = thisThread->selDepth;
1171 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1172 rm.pv.push_back(*m);
1174 // We record how often the best move has been changed in each
1175 // iteration. This information is used for time management: When
1176 // the best move changes frequently, we allocate some more time.
1178 ++thisThread->bestMoveChanges;
1181 // All other moves but the PV are set to the lowest value: this
1182 // is not a problem when sorting because the sort is stable and the
1183 // move position in the list is preserved - just the PV is pushed up.
1184 rm.score = -VALUE_INFINITE;
1187 if (value > bestValue)
1195 if (PvNode && !rootNode) // Update pv even in fail-high case
1196 update_pv(ss->pv, move, (ss+1)->pv);
1198 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1202 assert(value >= beta); // Fail high
1209 if (move != bestMove)
1211 if (captureOrPromotion && captureCount < 32)
1212 capturesSearched[captureCount++] = move;
1214 else if (!captureOrPromotion && quietCount < 64)
1215 quietsSearched[quietCount++] = move;
1219 // The following condition would detect a stop only after move loop has been
1220 // completed. But in this case bestValue is valid because we have fully
1221 // searched our subtree, and we can anyhow save the result in TT.
1227 // Step 20. Check for mate and stalemate
1228 // All legal moves have been searched and if there are no legal moves, it
1229 // must be a mate or a stalemate. If we are in a singular extension search then
1230 // return a fail low score.
1232 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1235 bestValue = excludedMove ? alpha
1236 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1239 // Quiet best move: update move sorting heuristics
1240 if (!pos.capture_or_promotion(bestMove))
1241 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1242 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1244 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1246 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1247 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1248 && !pos.captured_piece())
1249 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1252 // Bonus for prior countermove that caused the fail low
1253 else if ( (depth >= 3 * ONE_PLY || PvNode)
1254 && !pos.captured_piece())
1255 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1258 bestValue = std::min(bestValue, maxValue);
1261 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1262 bestValue >= beta ? BOUND_LOWER :
1263 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1264 depth, bestMove, ss->staticEval);
1266 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1272 // qsearch() is the quiescence search function, which is called by the main search
1273 // function with zero depth, or recursively with further decreasing depth per call.
1274 template <NodeType NT>
1275 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1277 constexpr bool PvNode = NT == PV;
1279 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1280 assert(PvNode || (alpha == beta - 1));
1281 assert(depth <= DEPTH_ZERO);
1282 assert(depth / ONE_PLY * ONE_PLY == depth);
1288 Move ttMove, move, bestMove;
1290 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1291 bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
1296 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1298 ss->pv[0] = MOVE_NONE;
1301 Thread* thisThread = pos.this_thread();
1302 (ss+1)->ply = ss->ply + 1;
1303 bestMove = MOVE_NONE;
1304 inCheck = pos.checkers();
1307 // Check for an immediate draw or maximum ply reached
1308 if ( pos.is_draw(ss->ply)
1309 || ss->ply >= MAX_PLY)
1310 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1312 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1314 // Decide whether or not to include checks: this fixes also the type of
1315 // TT entry depth that we are going to use. Note that in qsearch we use
1316 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1317 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1318 : DEPTH_QS_NO_CHECKS;
1319 // Transposition table lookup
1321 tte = TT.probe(posKey, ttHit);
1322 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1323 ttMove = ttHit ? tte->move() : MOVE_NONE;
1324 pvHit = ttHit && tte->is_pv();
1328 && tte->depth() >= ttDepth
1329 && ttValue != VALUE_NONE // Only in case of TT access race
1330 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1331 : (tte->bound() & BOUND_UPPER)))
1334 // Evaluate the position statically
1337 ss->staticEval = VALUE_NONE;
1338 bestValue = futilityBase = -VALUE_INFINITE;
1344 // Never assume anything about values stored in TT
1345 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1346 ss->staticEval = bestValue = evaluate(pos);
1348 // Can ttValue be used as a better position evaluation?
1349 if ( ttValue != VALUE_NONE
1350 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1351 bestValue = ttValue;
1354 ss->staticEval = bestValue =
1355 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1356 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1358 // Stand pat. Return immediately if static value is at least beta
1359 if (bestValue >= beta)
1362 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1363 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1368 if (PvNode && bestValue > alpha)
1371 futilityBase = bestValue + 128;
1374 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1375 nullptr, (ss-4)->continuationHistory,
1376 nullptr, (ss-6)->continuationHistory };
1378 // Initialize a MovePicker object for the current position, and prepare
1379 // to search the moves. Because the depth is <= 0 here, only captures,
1380 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1382 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1383 &thisThread->captureHistory,
1385 to_sq((ss-1)->currentMove));
1387 // Loop through the moves until no moves remain or a beta cutoff occurs
1388 while ((move = mp.next_move()) != MOVE_NONE)
1390 assert(is_ok(move));
1392 givesCheck = pos.gives_check(move);
1399 && futilityBase > -VALUE_KNOWN_WIN
1400 && !pos.advanced_pawn_push(move))
1402 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1404 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1406 if (futilityValue <= alpha)
1408 bestValue = std::max(bestValue, futilityValue);
1412 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1414 bestValue = std::max(bestValue, futilityBase);
1419 // Detect non-capture evasions that are candidates to be pruned
1420 evasionPrunable = inCheck
1421 && (depth != DEPTH_ZERO || moveCount > 2)
1422 && bestValue > VALUE_MATED_IN_MAX_PLY
1423 && !pos.capture(move);
1425 // Don't search moves with negative SEE values
1426 if ( (!inCheck || evasionPrunable)
1427 && (!givesCheck || !(pos.blockers_for_king(~pos.side_to_move()) & from_sq(move)))
1428 && !pos.see_ge(move))
1431 // Speculative prefetch as early as possible
1432 prefetch(TT.first_entry(pos.key_after(move)));
1434 // Check for legality just before making the move
1435 if (!pos.legal(move))
1441 ss->currentMove = move;
1442 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
1444 // Make and search the move
1445 pos.do_move(move, st, givesCheck);
1446 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1447 pos.undo_move(move);
1449 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1451 // Check for a new best move
1452 if (value > bestValue)
1460 if (PvNode) // Update pv even in fail-high case
1461 update_pv(ss->pv, move, (ss+1)->pv);
1463 if (PvNode && value < beta) // Update alpha here!
1471 // All legal moves have been searched. A special case: If we're in check
1472 // and no legal moves were found, it is checkmate.
1473 if (inCheck && bestValue == -VALUE_INFINITE)
1474 return mated_in(ss->ply); // Plies to mate from the root
1476 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1477 bestValue >= beta ? BOUND_LOWER :
1478 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1479 ttDepth, bestMove, ss->staticEval);
1481 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1487 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1488 // "plies to mate from the current position". Non-mate scores are unchanged.
1489 // The function is called before storing a value in the transposition table.
1491 Value value_to_tt(Value v, int ply) {
1493 assert(v != VALUE_NONE);
1495 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1496 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1500 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1501 // from the transposition table (which refers to the plies to mate/be mated
1502 // from current position) to "plies to mate/be mated from the root".
1504 Value value_from_tt(Value v, int ply) {
1506 return v == VALUE_NONE ? VALUE_NONE
1507 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1508 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1512 // update_pv() adds current move and appends child pv[]
1514 void update_pv(Move* pv, Move move, Move* childPv) {
1516 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1522 // update_continuation_histories() updates histories of the move pairs formed
1523 // by moves at ply -1, -2, and -4 with current move.
1525 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1527 for (int i : {1, 2, 4, 6})
1528 if (is_ok((ss-i)->currentMove))
1529 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1533 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1535 void update_capture_stats(const Position& pos, Move move,
1536 Move* captures, int captureCount, int bonus) {
1538 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1539 Piece moved_piece = pos.moved_piece(move);
1540 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1542 if (pos.capture_or_promotion(move))
1543 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1545 // Decrease all the other played capture moves
1546 for (int i = 0; i < captureCount; ++i)
1548 moved_piece = pos.moved_piece(captures[i]);
1549 captured = type_of(pos.piece_on(to_sq(captures[i])));
1550 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1555 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1557 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1558 Move* quiets, int quietCount, int bonus) {
1560 if (ss->killers[0] != move)
1562 ss->killers[1] = ss->killers[0];
1563 ss->killers[0] = move;
1566 Color us = pos.side_to_move();
1567 Thread* thisThread = pos.this_thread();
1568 thisThread->mainHistory[us][from_to(move)] << bonus;
1569 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1571 if (is_ok((ss-1)->currentMove))
1573 Square prevSq = to_sq((ss-1)->currentMove);
1574 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1577 // Decrease all the other played quiet moves
1578 for (int i = 0; i < quietCount; ++i)
1580 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1581 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1585 // When playing with strength handicap, choose best move among a set of RootMoves
1586 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1588 Move Skill::pick_best(size_t multiPV) {
1590 const RootMoves& rootMoves = Threads.main()->rootMoves;
1591 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1593 // RootMoves are already sorted by score in descending order
1594 Value topScore = rootMoves[0].score;
1595 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1596 int weakness = 120 - 2 * level;
1597 int maxScore = -VALUE_INFINITE;
1599 // Choose best move. For each move score we add two terms, both dependent on
1600 // weakness. One is deterministic and bigger for weaker levels, and one is
1601 // random. Then we choose the move with the resulting highest score.
1602 for (size_t i = 0; i < multiPV; ++i)
1604 // This is our magic formula
1605 int push = ( weakness * int(topScore - rootMoves[i].score)
1606 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1608 if (rootMoves[i].score + push >= maxScore)
1610 maxScore = rootMoves[i].score + push;
1611 best = rootMoves[i].pv[0];
1620 /// MainThread::check_time() is used to print debug info and, more importantly,
1621 /// to detect when we are out of available time and thus stop the search.
1623 void MainThread::check_time() {
1628 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1629 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1631 static TimePoint lastInfoTime = now();
1633 TimePoint elapsed = Time.elapsed();
1634 TimePoint tick = Limits.startTime + elapsed;
1636 if (tick - lastInfoTime >= 1000)
1638 lastInfoTime = tick;
1642 // We should not stop pondering until told so by the GUI
1646 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1647 || (Limits.movetime && elapsed >= Limits.movetime)
1648 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1649 Threads.stop = true;
1653 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1654 /// that all (if any) unsearched PV lines are sent using a previous search score.
1656 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1658 std::stringstream ss;
1659 TimePoint elapsed = Time.elapsed() + 1;
1660 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1661 size_t pvIdx = pos.this_thread()->pvIdx;
1662 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1663 uint64_t nodesSearched = Threads.nodes_searched();
1664 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1666 for (size_t i = 0; i < multiPV; ++i)
1668 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1670 if (depth == ONE_PLY && !updated)
1673 Depth d = updated ? depth : depth - ONE_PLY;
1674 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1676 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1677 v = tb ? rootMoves[i].tbScore : v;
1679 if (ss.rdbuf()->in_avail()) // Not at first line
1683 << " depth " << d / ONE_PLY
1684 << " seldepth " << rootMoves[i].selDepth
1685 << " multipv " << i + 1
1686 << " score " << UCI::value(v);
1688 if (!tb && i == pvIdx)
1689 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1691 ss << " nodes " << nodesSearched
1692 << " nps " << nodesSearched * 1000 / elapsed;
1694 if (elapsed > 1000) // Earlier makes little sense
1695 ss << " hashfull " << TT.hashfull();
1697 ss << " tbhits " << tbHits
1698 << " time " << elapsed
1701 for (Move m : rootMoves[i].pv)
1702 ss << " " << UCI::move(m, pos.is_chess960());
1709 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1710 /// before exiting the search, for instance, in case we stop the search during a
1711 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1712 /// otherwise in case of 'ponder on' we have nothing to think on.
1714 bool RootMove::extract_ponder_from_tt(Position& pos) {
1719 assert(pv.size() == 1);
1721 if (pv[0] == MOVE_NONE)
1724 pos.do_move(pv[0], st);
1725 TTEntry* tte = TT.probe(pos.key(), ttHit);
1729 Move m = tte->move(); // Local copy to be SMP safe
1730 if (MoveList<LEGAL>(pos).contains(m))
1734 pos.undo_move(pv[0]);
1735 return pv.size() > 1;
1738 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1741 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1742 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1743 Cardinality = int(Options["SyzygyProbeLimit"]);
1744 bool dtz_available = true;
1746 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1747 // ProbeDepth == DEPTH_ZERO
1748 if (Cardinality > MaxCardinality)
1750 Cardinality = MaxCardinality;
1751 ProbeDepth = DEPTH_ZERO;
1754 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1756 // Rank moves using DTZ tables
1757 RootInTB = root_probe(pos, rootMoves);
1761 // DTZ tables are missing; try to rank moves using WDL tables
1762 dtz_available = false;
1763 RootInTB = root_probe_wdl(pos, rootMoves);
1769 // Sort moves according to TB rank
1770 std::sort(rootMoves.begin(), rootMoves.end(),
1771 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1773 // Probe during search only if DTZ is not available and we are winning
1774 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1779 // Clean up if root_probe() and root_probe_wdl() have failed
1780 for (auto& m : rootMoves)