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) 2015-2017 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"
46 namespace Tablebases {
55 namespace TB = Tablebases;
59 using namespace Search;
63 // Different node types, used as a template parameter
64 enum NodeType { NonPV, PV };
66 // Razoring and futility margin based on depth
67 const int razor_margin[4] = { 483, 570, 603, 554 };
68 Value futility_margin(Depth d) { return Value(150 * d / ONE_PLY); }
70 // Futility and reductions lookup tables, initialized at startup
71 int FutilityMoveCounts[2][16]; // [improving][depth]
72 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
74 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
75 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
78 // History and stats update bonus, based on depth
79 Value stat_bonus(Depth depth) {
80 int d = depth / ONE_PLY ;
81 return Value(d * d + 2 * d - 2);
84 // Skill structure is used to implement strength limit
86 Skill(int l) : level(l) {}
87 bool enabled() const { return level < 20; }
88 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
89 Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
90 Move pick_best(size_t multiPV);
93 Move best = MOVE_NONE;
96 // EasyMoveManager structure is used to detect an 'easy move'. When the PV is
97 // stable across multiple search iterations, we can quickly return the best move.
98 struct EasyMoveManager {
103 pv[0] = pv[1] = pv[2] = MOVE_NONE;
106 Move get(Key key) const {
107 return expectedPosKey == key ? pv[2] : MOVE_NONE;
110 void update(Position& pos, const std::vector<Move>& newPv) {
112 assert(newPv.size() >= 3);
114 // Keep track of how many times in a row the 3rd ply remains stable
115 stableCnt = (newPv[2] == pv[2]) ? stableCnt + 1 : 0;
117 if (!std::equal(newPv.begin(), newPv.begin() + 3, pv))
119 std::copy(newPv.begin(), newPv.begin() + 3, pv);
122 pos.do_move(newPv[0], st[0]);
123 pos.do_move(newPv[1], st[1]);
124 expectedPosKey = pos.key();
125 pos.undo_move(newPv[1]);
126 pos.undo_move(newPv[0]);
135 EasyMoveManager EasyMove;
136 Value DrawValue[COLOR_NB];
138 template <NodeType NT>
139 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning);
141 template <NodeType NT, bool InCheck>
142 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
144 Value value_to_tt(Value v, int ply);
145 Value value_from_tt(Value v, int ply);
146 void update_pv(Move* pv, Move move, Move* childPv);
147 void update_cm_stats(Stack* ss, Piece pc, Square s, Value bonus);
148 void update_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, Value bonus);
154 /// Search::init() is called during startup to initialize various lookup tables
156 void Search::init() {
158 for (int imp = 0; imp <= 1; ++imp)
159 for (int d = 1; d < 64; ++d)
160 for (int mc = 1; mc < 64; ++mc)
162 double r = log(d) * log(mc) / 2;
164 Reductions[NonPV][imp][d][mc] = int(std::round(r));
165 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
167 // Increase reduction for non-PV nodes when eval is not improving
168 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
169 Reductions[NonPV][imp][d][mc]++;
172 for (int d = 0; d < 16; ++d)
174 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
175 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
180 /// Search::clear() resets search state to zero, to obtain reproducible results
182 void Search::clear() {
186 for (Thread* th : Threads)
188 th->counterMoves.clear();
190 th->counterMoveHistory.clear();
191 th->resetCalls = true;
194 Threads.main()->previousScore = VALUE_INFINITE;
198 /// Search::perft() is our utility to verify move generation. All the leaf nodes
199 /// up to the given depth are generated and counted, and the sum is returned.
201 uint64_t Search::perft(Position& pos, Depth depth) {
204 uint64_t cnt, nodes = 0;
205 const bool leaf = (depth == 2 * ONE_PLY);
207 for (const auto& m : MoveList<LEGAL>(pos))
209 if (Root && depth <= ONE_PLY)
214 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
219 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
224 template uint64_t Search::perft<true>(Position&, Depth);
227 /// MainThread::search() is called by the main thread when the program receives
228 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
230 void MainThread::search() {
232 Color us = rootPos.side_to_move();
233 Time.init(Limits, us, rootPos.game_ply());
235 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
236 DrawValue[ us] = VALUE_DRAW - Value(contempt);
237 DrawValue[~us] = VALUE_DRAW + Value(contempt);
239 if (rootMoves.empty())
241 rootMoves.push_back(RootMove(MOVE_NONE));
242 sync_cout << "info depth 0 score "
243 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
248 for (Thread* th : Threads)
250 th->start_searching();
252 Thread::search(); // Let's start searching!
255 // When playing in 'nodes as time' mode, subtract the searched nodes from
256 // the available ones before exiting.
258 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
260 // When we reach the maximum depth, we can arrive here without a raise of
261 // Signals.stop. However, if we are pondering or in an infinite search,
262 // the UCI protocol states that we shouldn't print the best move before the
263 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
264 // until the GUI sends one of those commands (which also raises Signals.stop).
265 if (!Signals.stop && (Limits.ponder || Limits.infinite))
267 Signals.stopOnPonderhit = true;
271 // Stop the threads if not already stopped
274 // Wait until all threads have finished
275 for (Thread* th : Threads)
277 th->wait_for_search_finished();
279 // Check if there are threads with a better score than main thread
280 Thread* bestThread = this;
281 if ( !this->easyMovePlayed
282 && Options["MultiPV"] == 1
284 && !Skill(Options["Skill Level"]).enabled()
285 && rootMoves[0].pv[0] != MOVE_NONE)
287 for (Thread* th : Threads)
289 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
290 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
292 if (scoreDiff > 0 && depthDiff >= 0)
297 previousScore = bestThread->rootMoves[0].score;
299 // Send new PV when needed
300 if (bestThread != this)
301 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
303 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
305 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
306 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
308 std::cout << sync_endl;
311 // Sizes and phases of the skip-blocks, used for distributing search depths across the threads.
312 static int skipsize[20] = {1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4};
313 static int phase [20] = {0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7};
315 // Thread::search() is the main iterative deepening loop. It calls search()
316 // repeatedly with increasing depth until the allocated thinking time has been
317 // consumed, the user stops the search, or the maximum search depth is reached.
319 void Thread::search() {
321 Stack stack[MAX_PLY+7], *ss = stack+4; // To allow referencing (ss-4) and (ss+2)
322 Value bestValue, alpha, beta, delta;
323 Move easyMove = MOVE_NONE;
324 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
326 std::memset(ss-4, 0, 7 * sizeof(Stack));
327 for(int i = -4; i < 0; i++)
328 (ss+i)->counterMoves = &this->counterMoveHistory[NO_PIECE][0]; // use as sentinel.
330 bestValue = delta = alpha = -VALUE_INFINITE;
331 beta = VALUE_INFINITE;
332 completedDepth = DEPTH_ZERO;
336 easyMove = EasyMove.get(rootPos.key());
338 mainThread->easyMovePlayed = mainThread->failedLow = false;
339 mainThread->bestMoveChanges = 0;
343 size_t multiPV = Options["MultiPV"];
344 Skill skill(Options["Skill Level"]);
346 // When playing with strength handicap enable MultiPV search that we will
347 // use behind the scenes to retrieve a set of possible moves.
349 multiPV = std::max(multiPV, (size_t)4);
351 multiPV = std::min(multiPV, rootMoves.size());
353 int hIdx = (idx - 1) % 20; // helper index, cycle after 20 threads
355 // Iterative deepening loop until requested to stop or the target depth is reached
356 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
358 && (!Limits.depth || Threads.main()->rootDepth / ONE_PLY <= Limits.depth))
360 // skip half of the plies in blocks depending on game ply and helper index.
361 if (idx && ((rootDepth / ONE_PLY + rootPos.game_ply() + phase[hIdx]) / skipsize[hIdx]) % 2)
364 // Age out PV variability metric
366 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
368 // Save the last iteration's scores before first PV line is searched and
369 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
370 for (RootMove& rm : rootMoves)
371 rm.previousScore = rm.score;
373 // MultiPV loop. We perform a full root search for each PV line
374 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
376 // Reset aspiration window starting size
377 if (rootDepth >= 5 * ONE_PLY)
380 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
381 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
384 // Start with a small aspiration window and, in the case of a fail
385 // high/low, re-search with a bigger window until we're not failing
389 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
391 // Bring the best move to the front. It is critical that sorting
392 // is done with a stable algorithm because all the values but the
393 // first and eventually the new best one are set to -VALUE_INFINITE
394 // and we want to keep the same order for all the moves except the
395 // new PV that goes to the front. Note that in case of MultiPV
396 // search the already searched PV lines are preserved.
397 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
399 // If search has been stopped, we break immediately. Sorting and
400 // writing PV back to TT is safe because RootMoves is still
401 // valid, although it refers to the previous iteration.
405 // When failing high/low give some update (without cluttering
406 // the UI) before a re-search.
409 && (bestValue <= alpha || bestValue >= beta)
410 && Time.elapsed() > 3000)
411 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
413 // In case of failing low/high increase aspiration window and
414 // re-search, otherwise exit the loop.
415 if (bestValue <= alpha)
417 beta = (alpha + beta) / 2;
418 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
422 mainThread->failedLow = true;
423 Signals.stopOnPonderhit = false;
426 else if (bestValue >= beta)
428 alpha = (alpha + beta) / 2;
429 beta = std::min(bestValue + delta, VALUE_INFINITE);
434 delta += delta / 4 + 5;
436 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
439 // Sort the PV lines searched so far and update the GUI
440 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
445 if (Signals.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000)
446 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
450 completedDepth = rootDepth;
455 // If skill level is enabled and time is up, pick a sub-optimal best move
456 if (skill.enabled() && skill.time_to_pick(rootDepth))
457 skill.pick_best(multiPV);
459 // Have we found a "mate in x"?
461 && bestValue >= VALUE_MATE_IN_MAX_PLY
462 && VALUE_MATE - bestValue <= 2 * Limits.mate)
465 // Do we have time for the next iteration? Can we stop searching now?
466 if (Limits.use_time_management())
468 if (!Signals.stop && !Signals.stopOnPonderhit)
470 // Stop the search if only one legal move is available, or if all
471 // of the available time has been used, or if we matched an easyMove
472 // from the previous search and just did a fast verification.
473 const int F[] = { mainThread->failedLow,
474 bestValue - mainThread->previousScore };
476 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
477 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
479 bool doEasyMove = rootMoves[0].pv[0] == easyMove
480 && mainThread->bestMoveChanges < 0.03
481 && Time.elapsed() > Time.optimum() * 5 / 44;
483 if ( rootMoves.size() == 1
484 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628
485 || (mainThread->easyMovePlayed = doEasyMove, doEasyMove))
487 // If we are allowed to ponder do not stop the search now but
488 // keep pondering until the GUI sends "ponderhit" or "stop".
490 Signals.stopOnPonderhit = true;
496 if (rootMoves[0].pv.size() >= 3)
497 EasyMove.update(rootPos, rootMoves[0].pv);
506 // Clear any candidate easy move that wasn't stable for the last search
507 // iterations; the second condition prevents consecutive fast moves.
508 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
511 // If skill level is enabled, swap best PV line with the sub-optimal one
513 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
514 rootMoves.end(), skill.best_move(multiPV)));
520 // search<>() is the main search function for both PV and non-PV nodes
522 template <NodeType NT>
523 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
525 const bool PvNode = NT == PV;
526 const bool rootNode = PvNode && (ss-1)->ply == 0;
528 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
529 assert(PvNode || (alpha == beta - 1));
530 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
531 assert(!(PvNode && cutNode));
532 assert(depth / ONE_PLY * ONE_PLY == depth);
534 Move pv[MAX_PLY+1], quietsSearched[64];
538 Move ttMove, move, excludedMove, bestMove;
539 Depth extension, newDepth;
540 Value bestValue, value, ttValue, eval;
541 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
542 bool captureOrPromotion, doFullDepthSearch, moveCountPruning;
544 int moveCount, quietCount;
546 // Step 1. Initialize node
547 Thread* thisThread = pos.this_thread();
548 inCheck = pos.checkers();
549 moveCount = quietCount = ss->moveCount = 0;
550 ss->history = VALUE_ZERO;
551 bestValue = -VALUE_INFINITE;
552 ss->ply = (ss-1)->ply + 1;
554 // Check for the available remaining time
555 if (thisThread->resetCalls.load(std::memory_order_relaxed))
557 thisThread->resetCalls = false;
558 // At low node count increase the checking rate to about 0.1% of nodes
559 // otherwise use a default value.
560 thisThread->callsCnt = Limits.nodes ? std::min((int64_t)4096, Limits.nodes / 1024)
564 if (--thisThread->callsCnt <= 0)
566 for (Thread* th : Threads)
567 th->resetCalls = true;
572 // Used to send selDepth info to GUI
573 if (PvNode && thisThread->maxPly < ss->ply)
574 thisThread->maxPly = ss->ply;
578 // Step 2. Check for aborted search and immediate draw
579 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
580 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
581 : DrawValue[pos.side_to_move()];
583 // Step 3. Mate distance pruning. Even if we mate at the next move our score
584 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
585 // a shorter mate was found upward in the tree then there is no need to search
586 // because we will never beat the current alpha. Same logic but with reversed
587 // signs applies also in the opposite condition of being mated instead of giving
588 // mate. In this case return a fail-high score.
589 alpha = std::max(mated_in(ss->ply), alpha);
590 beta = std::min(mate_in(ss->ply+1), beta);
595 assert(0 <= ss->ply && ss->ply < MAX_PLY);
597 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
598 ss->counterMoves = &thisThread->counterMoveHistory[NO_PIECE][0];
599 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
600 Square prevSq = to_sq((ss-1)->currentMove);
602 // Step 4. Transposition table lookup. We don't want the score of a partial
603 // search to overwrite a previous full search TT value, so we use a different
604 // position key in case of an excluded move.
605 excludedMove = ss->excludedMove;
606 posKey = pos.key() ^ Key(excludedMove);
607 tte = TT.probe(posKey, ttHit);
608 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
609 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
610 : ttHit ? tte->move() : MOVE_NONE;
612 // At non-PV nodes we check for an early TT cutoff
615 && tte->depth() >= depth
616 && ttValue != VALUE_NONE // Possible in case of TT access race
617 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
618 : (tte->bound() & BOUND_UPPER)))
620 // If ttMove is quiet, update move sorting heuristics on TT hit
625 if (!pos.capture_or_promotion(ttMove))
626 update_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
628 // Extra penalty for a quiet TT move in previous ply when it gets refuted
629 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
630 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
632 // Penalty for a quiet ttMove that fails low
633 else if (!pos.capture_or_promotion(ttMove))
635 Value penalty = -stat_bonus(depth + ONE_PLY);
636 thisThread->history.update(pos.side_to_move(), ttMove, penalty);
637 update_cm_stats(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
643 // Step 4a. Tablebase probe
644 if (!rootNode && TB::Cardinality)
646 int piecesCount = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
648 if ( piecesCount <= TB::Cardinality
649 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
650 && pos.rule50_count() == 0
651 && !pos.can_castle(ANY_CASTLING))
654 TB::WDLScore v = Tablebases::probe_wdl(pos, &err);
656 if (err != TB::ProbeState::FAIL)
658 thisThread->tbHits++;
660 int drawScore = TB::UseRule50 ? 1 : 0;
662 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
663 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
664 : VALUE_DRAW + 2 * v * drawScore;
666 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
667 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
668 MOVE_NONE, VALUE_NONE, TT.generation());
675 // Step 5. Evaluate the position statically
678 ss->staticEval = eval = VALUE_NONE;
684 // Never assume anything on values stored in TT
685 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
686 eval = ss->staticEval = evaluate(pos);
688 // Can ttValue be used as a better position evaluation?
689 if (ttValue != VALUE_NONE)
690 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
695 eval = ss->staticEval =
696 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
697 : -(ss-1)->staticEval + 2 * Eval::Tempo;
699 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
700 ss->staticEval, TT.generation());
703 if (skipEarlyPruning)
706 // Step 6. Razoring (skipped when in check)
708 && depth < 4 * ONE_PLY
709 && eval + razor_margin[depth / ONE_PLY] <= alpha)
711 if (depth <= ONE_PLY)
712 return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
714 Value ralpha = alpha - razor_margin[depth / ONE_PLY];
715 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1);
720 // Step 7. Futility pruning: child node (skipped when in check)
722 && depth < 7 * ONE_PLY
723 && eval - futility_margin(depth) >= beta
724 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
725 && pos.non_pawn_material(pos.side_to_move()))
728 // Step 8. Null move search with verification search (is omitted in PV nodes)
731 && (ss->staticEval >= beta - 35 * (depth / ONE_PLY - 6) || depth >= 13 * ONE_PLY)
732 && pos.non_pawn_material(pos.side_to_move()))
735 assert(eval - beta >= 0);
737 // Null move dynamic reduction based on depth and value
738 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
740 ss->currentMove = MOVE_NULL;
741 ss->counterMoves = &thisThread->counterMoveHistory[NO_PIECE][0];
743 pos.do_null_move(st);
744 Value nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1)
745 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
746 pos.undo_null_move();
748 if (nullValue >= beta)
750 // Do not return unproven mate scores
751 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
754 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
757 // Do verification search at high depths
758 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
759 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
766 // Step 9. ProbCut (skipped when in check)
767 // If we have a good enough capture and a reduced search returns a value
768 // much above beta, we can (almost) safely prune the previous move.
770 && depth >= 5 * ONE_PLY
771 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
773 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
774 Depth rdepth = depth - 4 * ONE_PLY;
776 assert(rdepth >= ONE_PLY);
777 assert(is_ok((ss-1)->currentMove));
779 MovePicker mp(pos, ttMove, rbeta - ss->staticEval);
781 while ((move = mp.next_move()) != MOVE_NONE)
784 ss->currentMove = move;
785 ss->counterMoves = &thisThread->counterMoveHistory[pos.moved_piece(move)][to_sq(move)];
787 pos.do_move(move, st);
788 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode, false);
795 // Step 10. Internal iterative deepening (skipped when in check)
796 if ( depth >= 6 * ONE_PLY
798 && (PvNode || ss->staticEval + 256 >= beta))
800 Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
801 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
803 tte = TT.probe(posKey, ttHit);
804 ttMove = ttHit ? tte->move() : MOVE_NONE;
807 moves_loop: // When in check search starts from here
809 const CounterMoveStats* cmh = (ss-1)->counterMoves;
810 const CounterMoveStats* fmh = (ss-2)->counterMoves;
811 const CounterMoveStats* fmh2 = (ss-4)->counterMoves;
812 const bool cm_ok = is_ok((ss-1)->currentMove);
813 const bool fm_ok = is_ok((ss-2)->currentMove);
814 const bool fm2_ok = is_ok((ss-4)->currentMove);
816 MovePicker mp(pos, ttMove, depth, ss);
817 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
818 improving = ss->staticEval >= (ss-2)->staticEval
819 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
820 ||(ss-2)->staticEval == VALUE_NONE;
822 singularExtensionNode = !rootNode
823 && depth >= 8 * ONE_PLY
824 && ttMove != MOVE_NONE
825 && ttValue != VALUE_NONE
826 && !excludedMove // Recursive singular search is not allowed
827 && (tte->bound() & BOUND_LOWER)
828 && tte->depth() >= depth - 3 * ONE_PLY;
830 // Step 11. Loop through moves
831 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
832 while ((move = mp.next_move()) != MOVE_NONE)
836 if (move == excludedMove)
839 // At root obey the "searchmoves" option and skip moves not listed in Root
840 // Move List. As a consequence any illegal move is also skipped. In MultiPV
841 // mode we also skip PV moves which have been already searched.
842 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
843 thisThread->rootMoves.end(), move))
846 ss->moveCount = ++moveCount;
848 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
849 sync_cout << "info depth " << depth / ONE_PLY
850 << " currmove " << UCI::move(move, pos.is_chess960())
851 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
854 (ss+1)->pv = nullptr;
856 extension = DEPTH_ZERO;
857 captureOrPromotion = pos.capture_or_promotion(move);
858 moved_piece = pos.moved_piece(move);
860 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
861 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
862 : pos.gives_check(move);
864 moveCountPruning = depth < 16 * ONE_PLY
865 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
867 // Step 12. Extensions
871 && pos.see_ge(move, VALUE_ZERO))
874 // Singular extension search. If all moves but one fail low on a search of
875 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
876 // is singular and should be extended. To verify this we do a reduced search
877 // on all the other moves but the ttMove and if the result is lower than
878 // ttValue minus a margin then we extend the ttMove.
879 if ( singularExtensionNode
884 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
885 Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
886 ss->excludedMove = move;
887 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode, true);
888 ss->excludedMove = MOVE_NONE;
894 // Calculate new depth for this move
895 newDepth = depth - ONE_PLY + extension;
897 // Step 13. Pruning at shallow depth
899 && bestValue > VALUE_MATED_IN_MAX_PLY)
901 if ( !captureOrPromotion
903 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK) >= 5000))
905 // Move count based pruning
906 if (moveCountPruning)
909 // Reduced depth of the next LMR search
910 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
912 // Countermoves based pruning
914 && (((*cmh )[moved_piece][to_sq(move)] < VALUE_ZERO) || !cm_ok)
915 && (((*fmh )[moved_piece][to_sq(move)] < VALUE_ZERO) || !fm_ok)
916 && (((*fmh2)[moved_piece][to_sq(move)] < VALUE_ZERO) || !fm2_ok || (cm_ok && fm_ok)))
919 // Futility pruning: parent node
922 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
925 // Prune moves with negative SEE
927 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
930 else if ( depth < 7 * ONE_PLY
932 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
936 // Speculative prefetch as early as possible
937 prefetch(TT.first_entry(pos.key_after(move)));
939 // Check for legality just before making the move
940 if (!rootNode && !pos.legal(move))
942 ss->moveCount = --moveCount;
946 // Update the current move (this must be done after singular extension search)
947 ss->currentMove = move;
948 ss->counterMoves = &thisThread->counterMoveHistory[moved_piece][to_sq(move)];
950 // Step 14. Make the move
951 pos.do_move(move, st, givesCheck);
953 // Step 15. Reduced depth search (LMR). If the move fails high it will be
954 // re-searched at full depth.
955 if ( depth >= 3 * ONE_PLY
957 && (!captureOrPromotion || moveCountPruning))
959 Depth r = reduction<PvNode>(improving, depth, moveCount);
961 if (captureOrPromotion)
962 r -= r ? ONE_PLY : DEPTH_ZERO;
965 // Increase reduction for cut nodes
969 // Decrease reduction for moves that escape a capture. Filter out
970 // castling moves, because they are coded as "king captures rook" and
971 // hence break make_move().
972 else if ( type_of(move) == NORMAL
973 && !pos.see_ge(make_move(to_sq(move), from_sq(move)), VALUE_ZERO))
976 ss->history = (*cmh )[moved_piece][to_sq(move)]
977 + (*fmh )[moved_piece][to_sq(move)]
978 + (*fmh2)[moved_piece][to_sq(move)]
979 + thisThread->history.get(~pos.side_to_move(), move)
980 - 4000; // Correction factor
982 // Decrease/increase reduction by comparing opponent's stat score
983 if (ss->history > VALUE_ZERO && (ss-1)->history < VALUE_ZERO)
986 else if (ss->history < VALUE_ZERO && (ss-1)->history > VALUE_ZERO)
989 // Decrease/increase reduction for moves with a good/bad history
990 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->history / 20000) * ONE_PLY);
993 Depth d = std::max(newDepth - r, ONE_PLY);
995 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
997 doFullDepthSearch = (value > alpha && d != newDepth);
1000 doFullDepthSearch = !PvNode || moveCount > 1;
1002 // Step 16. Full depth search when LMR is skipped or fails high
1003 if (doFullDepthSearch)
1004 value = newDepth < ONE_PLY ?
1005 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha)
1006 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha)
1007 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
1009 // For PV nodes only, do a full PV search on the first move or after a fail
1010 // high (in the latter case search only if value < beta), otherwise let the
1011 // parent node fail low with value <= alpha and try another move.
1012 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1015 (ss+1)->pv[0] = MOVE_NONE;
1017 value = newDepth < ONE_PLY ?
1018 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha)
1019 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha)
1020 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1023 // Step 17. Undo move
1024 pos.undo_move(move);
1026 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1028 // Step 18. Check for a new best move
1029 // Finished searching the move. If a stop occurred, the return value of
1030 // the search cannot be trusted, and we return immediately without
1031 // updating best move, PV and TT.
1032 if (Signals.stop.load(std::memory_order_relaxed))
1037 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1038 thisThread->rootMoves.end(), move);
1040 // PV move or new best move ?
1041 if (moveCount == 1 || value > alpha)
1048 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1049 rm.pv.push_back(*m);
1051 // We record how often the best move has been changed in each
1052 // iteration. This information is used for time management: When
1053 // the best move changes frequently, we allocate some more time.
1054 if (moveCount > 1 && thisThread == Threads.main())
1055 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1058 // All other moves but the PV are set to the lowest value: this is
1059 // not a problem when sorting because the sort is stable and the
1060 // move position in the list is preserved - just the PV is pushed up.
1061 rm.score = -VALUE_INFINITE;
1064 if (value > bestValue)
1072 if (PvNode && !rootNode) // Update pv even in fail-high case
1073 update_pv(ss->pv, move, (ss+1)->pv);
1075 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1079 assert(value >= beta); // Fail high
1085 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1086 quietsSearched[quietCount++] = move;
1089 // The following condition would detect a stop only after move loop has been
1090 // completed. But in this case bestValue is valid because we have fully
1091 // searched our subtree, and we can anyhow save the result in TT.
1097 // Step 20. Check for mate and stalemate
1098 // All legal moves have been searched and if there are no legal moves, it
1099 // must be a mate or a stalemate. If we are in a singular extension search then
1100 // return a fail low score.
1102 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1105 bestValue = excludedMove ? alpha
1106 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1110 // Quiet best move: update move sorting heuristics
1111 if (!pos.capture_or_promotion(bestMove))
1112 update_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1114 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1115 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1116 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1118 // Bonus for prior countermove that caused the fail low
1119 else if ( depth >= 3 * ONE_PLY
1120 && !pos.captured_piece()
1122 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1124 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1125 bestValue >= beta ? BOUND_LOWER :
1126 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1127 depth, bestMove, ss->staticEval, TT.generation());
1129 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1135 // qsearch() is the quiescence search function, which is called by the main
1136 // search function with depth zero, or recursively with depth less than ONE_PLY.
1138 template <NodeType NT, bool InCheck>
1139 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1141 const bool PvNode = NT == PV;
1143 assert(InCheck == !!pos.checkers());
1144 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1145 assert(PvNode || (alpha == beta - 1));
1146 assert(depth <= DEPTH_ZERO);
1147 assert(depth / ONE_PLY * ONE_PLY == depth);
1153 Move ttMove, move, bestMove;
1154 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1155 bool ttHit, givesCheck, evasionPrunable;
1160 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1162 ss->pv[0] = MOVE_NONE;
1165 ss->currentMove = bestMove = MOVE_NONE;
1166 ss->ply = (ss-1)->ply + 1;
1168 // Check for an instant draw or if the maximum ply has been reached
1169 if (pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
1170 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1171 : DrawValue[pos.side_to_move()];
1173 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1175 // Decide whether or not to include checks: this fixes also the type of
1176 // TT entry depth that we are going to use. Note that in qsearch we use
1177 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1178 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1179 : DEPTH_QS_NO_CHECKS;
1181 // Transposition table lookup
1183 tte = TT.probe(posKey, ttHit);
1184 ttMove = ttHit ? tte->move() : MOVE_NONE;
1185 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1189 && tte->depth() >= ttDepth
1190 && ttValue != VALUE_NONE // Only in case of TT access race
1191 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1192 : (tte->bound() & BOUND_UPPER)))
1195 // Evaluate the position statically
1198 ss->staticEval = VALUE_NONE;
1199 bestValue = futilityBase = -VALUE_INFINITE;
1205 // Never assume anything on values stored in TT
1206 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1207 ss->staticEval = bestValue = evaluate(pos);
1209 // Can ttValue be used as a better position evaluation?
1210 if (ttValue != VALUE_NONE)
1211 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1212 bestValue = ttValue;
1215 ss->staticEval = bestValue =
1216 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1217 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1219 // Stand pat. Return immediately if static value is at least beta
1220 if (bestValue >= beta)
1223 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1224 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1229 if (PvNode && bestValue > alpha)
1232 futilityBase = bestValue + 128;
1235 // Initialize a MovePicker object for the current position, and prepare
1236 // to search the moves. Because the depth is <= 0 here, only captures,
1237 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1239 MovePicker mp(pos, ttMove, depth, to_sq((ss-1)->currentMove));
1241 // Loop through the moves until no moves remain or a beta cutoff occurs
1242 while ((move = mp.next_move()) != MOVE_NONE)
1244 assert(is_ok(move));
1246 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
1247 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
1248 : pos.gives_check(move);
1253 && futilityBase > -VALUE_KNOWN_WIN
1254 && !pos.advanced_pawn_push(move))
1256 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1258 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1260 if (futilityValue <= alpha)
1262 bestValue = std::max(bestValue, futilityValue);
1266 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1268 bestValue = std::max(bestValue, futilityBase);
1273 // Detect non-capture evasions that are candidates to be pruned
1274 evasionPrunable = InCheck
1275 && bestValue > VALUE_MATED_IN_MAX_PLY
1276 && !pos.capture(move);
1278 // Don't search moves with negative SEE values
1279 if ( (!InCheck || evasionPrunable)
1280 && type_of(move) != PROMOTION
1281 && !pos.see_ge(move, VALUE_ZERO))
1284 // Speculative prefetch as early as possible
1285 prefetch(TT.first_entry(pos.key_after(move)));
1287 // Check for legality just before making the move
1288 if (!pos.legal(move))
1291 ss->currentMove = move;
1293 // Make and search the move
1294 pos.do_move(move, st, givesCheck);
1295 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1296 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1297 pos.undo_move(move);
1299 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1301 // Check for a new best move
1302 if (value > bestValue)
1308 if (PvNode) // Update pv even in fail-high case
1309 update_pv(ss->pv, move, (ss+1)->pv);
1311 if (PvNode && value < beta) // Update alpha here!
1318 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1319 ttDepth, move, ss->staticEval, TT.generation());
1327 // All legal moves have been searched. A special case: If we're in check
1328 // and no legal moves were found, it is checkmate.
1329 if (InCheck && bestValue == -VALUE_INFINITE)
1330 return mated_in(ss->ply); // Plies to mate from the root
1332 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1333 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1334 ttDepth, bestMove, ss->staticEval, TT.generation());
1336 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1342 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1343 // "plies to mate from the current position". Non-mate scores are unchanged.
1344 // The function is called before storing a value in the transposition table.
1346 Value value_to_tt(Value v, int ply) {
1348 assert(v != VALUE_NONE);
1350 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1351 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1355 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1356 // from the transposition table (which refers to the plies to mate/be mated
1357 // from current position) to "plies to mate/be mated from the root".
1359 Value value_from_tt(Value v, int ply) {
1361 return v == VALUE_NONE ? VALUE_NONE
1362 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1363 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1367 // update_pv() adds current move and appends child pv[]
1369 void update_pv(Move* pv, Move move, Move* childPv) {
1371 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1377 // update_cm_stats() updates countermove and follow-up move history
1379 void update_cm_stats(Stack* ss, Piece pc, Square s, Value bonus) {
1381 for (int i : {1, 2, 4})
1382 if (is_ok((ss-i)->currentMove))
1383 (ss-i)->counterMoves->update(pc, s, bonus);
1387 // update_stats() updates move sorting heuristics when a new quiet best move is found
1389 void update_stats(const Position& pos, Stack* ss, Move move,
1390 Move* quiets, int quietsCnt, Value bonus) {
1392 if (ss->killers[0] != move)
1394 ss->killers[1] = ss->killers[0];
1395 ss->killers[0] = move;
1398 Color c = pos.side_to_move();
1399 Thread* thisThread = pos.this_thread();
1400 thisThread->history.update(c, move, bonus);
1401 update_cm_stats(ss, pos.moved_piece(move), to_sq(move), bonus);
1403 if (is_ok((ss-1)->currentMove))
1405 Square prevSq = to_sq((ss-1)->currentMove);
1406 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1409 // Decrease all the other played quiet moves
1410 for (int i = 0; i < quietsCnt; ++i)
1412 thisThread->history.update(c, quiets[i], -bonus);
1413 update_cm_stats(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1418 // When playing with strength handicap, choose best move among a set of RootMoves
1419 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1421 Move Skill::pick_best(size_t multiPV) {
1423 const RootMoves& rootMoves = Threads.main()->rootMoves;
1424 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1426 // RootMoves are already sorted by score in descending order
1427 Value topScore = rootMoves[0].score;
1428 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1429 int weakness = 120 - 2 * level;
1430 int maxScore = -VALUE_INFINITE;
1432 // Choose best move. For each move score we add two terms, both dependent on
1433 // weakness. One is deterministic and bigger for weaker levels, and one is
1434 // random. Then we choose the move with the resulting highest score.
1435 for (size_t i = 0; i < multiPV; ++i)
1437 // This is our magic formula
1438 int push = ( weakness * int(topScore - rootMoves[i].score)
1439 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1441 if (rootMoves[i].score + push > maxScore)
1443 maxScore = rootMoves[i].score + push;
1444 best = rootMoves[i].pv[0];
1452 // check_time() is used to print debug info and, more importantly, to detect
1453 // when we are out of available time and thus stop the search.
1457 static TimePoint lastInfoTime = now();
1459 int elapsed = Time.elapsed();
1460 TimePoint tick = Limits.startTime + elapsed;
1462 if (tick - lastInfoTime >= 1000)
1464 lastInfoTime = tick;
1468 // An engine may not stop pondering until told so by the GUI
1472 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1473 || (Limits.movetime && elapsed >= Limits.movetime)
1474 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1475 Signals.stop = true;
1481 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1482 /// that all (if any) unsearched PV lines are sent using a previous search score.
1484 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1486 std::stringstream ss;
1487 int elapsed = Time.elapsed() + 1;
1488 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1489 size_t PVIdx = pos.this_thread()->PVIdx;
1490 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1491 uint64_t nodesSearched = Threads.nodes_searched();
1492 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1494 for (size_t i = 0; i < multiPV; ++i)
1496 bool updated = (i <= PVIdx);
1498 if (depth == ONE_PLY && !updated)
1501 Depth d = updated ? depth : depth - ONE_PLY;
1502 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1504 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1505 v = tb ? TB::Score : v;
1507 if (ss.rdbuf()->in_avail()) // Not at first line
1511 << " depth " << d / ONE_PLY
1512 << " seldepth " << pos.this_thread()->maxPly
1513 << " multipv " << i + 1
1514 << " score " << UCI::value(v);
1516 if (!tb && i == PVIdx)
1517 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1519 ss << " nodes " << nodesSearched
1520 << " nps " << nodesSearched * 1000 / elapsed;
1522 if (elapsed > 1000) // Earlier makes little sense
1523 ss << " hashfull " << TT.hashfull();
1525 ss << " tbhits " << tbHits
1526 << " time " << elapsed
1529 for (Move m : rootMoves[i].pv)
1530 ss << " " << UCI::move(m, pos.is_chess960());
1537 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1538 /// before exiting the search, for instance, in case we stop the search during a
1539 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1540 /// otherwise in case of 'ponder on' we have nothing to think on.
1542 bool RootMove::extract_ponder_from_tt(Position& pos) {
1547 assert(pv.size() == 1);
1552 pos.do_move(pv[0], st);
1553 TTEntry* tte = TT.probe(pos.key(), ttHit);
1557 Move m = tte->move(); // Local copy to be SMP safe
1558 if (MoveList<LEGAL>(pos).contains(m))
1562 pos.undo_move(pv[0]);
1563 return pv.size() > 1;
1566 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1569 UseRule50 = Options["Syzygy50MoveRule"];
1570 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1571 Cardinality = Options["SyzygyProbeLimit"];
1573 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1574 if (Cardinality > MaxCardinality)
1576 Cardinality = MaxCardinality;
1577 ProbeDepth = DEPTH_ZERO;
1580 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1583 // If the current root position is in the tablebases, then RootMoves
1584 // contains only moves that preserve the draw or the win.
1585 RootInTB = root_probe(pos, rootMoves, TB::Score);
1588 Cardinality = 0; // Do not probe tablebases during the search
1590 else // If DTZ tables are missing, use WDL tables as a fallback
1592 // Filter out moves that do not preserve the draw or the win.
1593 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1595 // Only probe during search if winning
1596 if (RootInTB && TB::Score <= VALUE_DRAW)
1600 if (RootInTB && !UseRule50)
1601 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1602 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1