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) 2015-2016 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 // Skill structure is used to implement strength limit
80 Skill(int l) : level(l) {}
81 bool enabled() const { return level < 20; }
82 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
83 Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
84 Move pick_best(size_t multiPV);
87 Move best = MOVE_NONE;
90 // EasyMoveManager structure is used to detect an 'easy move'. When the PV is
91 // stable across multiple search iterations, we can quickly return the best move.
92 struct EasyMoveManager {
97 pv[0] = pv[1] = pv[2] = MOVE_NONE;
100 Move get(Key key) const {
101 return expectedPosKey == key ? pv[2] : MOVE_NONE;
104 void update(Position& pos, const std::vector<Move>& newPv) {
106 assert(newPv.size() >= 3);
108 // Keep track of how many times in a row the 3rd ply remains stable
109 stableCnt = (newPv[2] == pv[2]) ? stableCnt + 1 : 0;
111 if (!std::equal(newPv.begin(), newPv.begin() + 3, pv))
113 std::copy(newPv.begin(), newPv.begin() + 3, pv);
116 pos.do_move(newPv[0], st[0]);
117 pos.do_move(newPv[1], st[1]);
118 expectedPosKey = pos.key();
119 pos.undo_move(newPv[1]);
120 pos.undo_move(newPv[0]);
129 // Set of rows with half bits set to 1 and half to 0. It is used to allocate
130 // the search depths across the threads.
131 typedef std::vector<int> Row;
133 const Row HalfDensity[] = {
146 {0, 0, 0, 0, 1, 1, 1, 1},
147 {0, 0, 0, 1, 1, 1, 1, 0},
148 {0, 0, 1, 1, 1, 1, 0 ,0},
149 {0, 1, 1, 1, 1, 0, 0 ,0},
150 {1, 1, 1, 1, 0, 0, 0 ,0},
151 {1, 1, 1, 0, 0, 0, 0 ,1},
152 {1, 1, 0, 0, 0, 0, 1 ,1},
153 {1, 0, 0, 0, 0, 1, 1 ,1},
156 const size_t HalfDensitySize = std::extent<decltype(HalfDensity)>::value;
158 Value bonus(Depth depth) { int d = depth / ONE_PLY ; return Value(d * d + 2 * d - 2); }
159 Value penalty(Depth depth) { int d = depth / ONE_PLY ; return -Value(d * d + 4 * d + 1); }
161 EasyMoveManager EasyMove;
162 Value DrawValue[COLOR_NB];
164 template <NodeType NT>
165 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning);
167 template <NodeType NT, bool InCheck>
168 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
170 Value value_to_tt(Value v, int ply);
171 Value value_from_tt(Value v, int ply);
172 void update_pv(Move* pv, Move move, Move* childPv);
173 void update_cm_stats(Stack* ss, Piece pc, Square s, Value bonus);
174 void update_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, Value bonus);
180 /// Search::init() is called during startup to initialize various lookup tables
182 void Search::init() {
184 for (int imp = 0; imp <= 1; ++imp)
185 for (int d = 1; d < 64; ++d)
186 for (int mc = 1; mc < 64; ++mc)
188 double r = log(d) * log(mc) / 2;
190 Reductions[NonPV][imp][d][mc] = int(std::round(r));
191 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
193 // Increase reduction for non-PV nodes when eval is not improving
194 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
195 Reductions[NonPV][imp][d][mc]++;
198 for (int d = 0; d < 16; ++d)
200 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
201 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
206 /// Search::clear() resets search state to zero, to obtain reproducible results
208 void Search::clear() {
212 for (Thread* th : Threads)
214 th->counterMoves.clear();
216 th->counterMoveHistory.clear();
217 th->resetCalls = true;
220 Threads.main()->previousScore = VALUE_INFINITE;
224 /// Search::perft() is our utility to verify move generation. All the leaf nodes
225 /// up to the given depth are generated and counted, and the sum is returned.
227 uint64_t Search::perft(Position& pos, Depth depth) {
230 uint64_t cnt, nodes = 0;
231 const bool leaf = (depth == 2 * ONE_PLY);
233 for (const auto& m : MoveList<LEGAL>(pos))
235 if (Root && depth <= ONE_PLY)
240 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
245 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
250 template uint64_t Search::perft<true>(Position&, Depth);
253 /// MainThread::search() is called by the main thread when the program receives
254 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
256 void MainThread::search() {
258 Color us = rootPos.side_to_move();
259 Time.init(Limits, us, rootPos.game_ply());
261 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
262 DrawValue[ us] = VALUE_DRAW - Value(contempt);
263 DrawValue[~us] = VALUE_DRAW + Value(contempt);
265 if (rootMoves.empty())
267 rootMoves.push_back(RootMove(MOVE_NONE));
268 sync_cout << "info depth 0 score "
269 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
274 for (Thread* th : Threads)
276 th->start_searching();
278 Thread::search(); // Let's start searching!
281 // When playing in 'nodes as time' mode, subtract the searched nodes from
282 // the available ones before exiting.
284 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
286 // When we reach the maximum depth, we can arrive here without a raise of
287 // Signals.stop. However, if we are pondering or in an infinite search,
288 // the UCI protocol states that we shouldn't print the best move before the
289 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
290 // until the GUI sends one of those commands (which also raises Signals.stop).
291 if (!Signals.stop && (Limits.ponder || Limits.infinite))
293 Signals.stopOnPonderhit = true;
297 // Stop the threads if not already stopped
300 // Wait until all threads have finished
301 for (Thread* th : Threads)
303 th->wait_for_search_finished();
305 // Check if there are threads with a better score than main thread
306 Thread* bestThread = this;
307 if ( !this->easyMovePlayed
308 && Options["MultiPV"] == 1
310 && !Skill(Options["Skill Level"]).enabled()
311 && rootMoves[0].pv[0] != MOVE_NONE)
313 for (Thread* th : Threads)
315 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
316 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
318 if (scoreDiff > 0 && depthDiff >= 0)
323 previousScore = bestThread->rootMoves[0].score;
325 // Send new PV when needed
326 if (bestThread != this)
327 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
329 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
331 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
332 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
334 std::cout << sync_endl;
338 // Thread::search() is the main iterative deepening loop. It calls search()
339 // repeatedly with increasing depth until the allocated thinking time has been
340 // consumed, the user stops the search, or the maximum search depth is reached.
342 void Thread::search() {
344 Stack stack[MAX_PLY+7], *ss = stack+4; // To allow referencing (ss-4) and (ss+2)
345 Value bestValue, alpha, beta, delta;
346 Move easyMove = MOVE_NONE;
347 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
349 std::memset(ss-4, 0, 7 * sizeof(Stack));
351 bestValue = delta = alpha = -VALUE_INFINITE;
352 beta = VALUE_INFINITE;
353 completedDepth = DEPTH_ZERO;
357 easyMove = EasyMove.get(rootPos.key());
359 mainThread->easyMovePlayed = mainThread->failedLow = false;
360 mainThread->bestMoveChanges = 0;
364 size_t multiPV = Options["MultiPV"];
365 Skill skill(Options["Skill Level"]);
367 // When playing with strength handicap enable MultiPV search that we will
368 // use behind the scenes to retrieve a set of possible moves.
370 multiPV = std::max(multiPV, (size_t)4);
372 multiPV = std::min(multiPV, rootMoves.size());
374 // Iterative deepening loop until requested to stop or the target depth is reached
375 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
377 && (!Limits.depth || Threads.main()->rootDepth / ONE_PLY <= Limits.depth))
379 // Set up the new depths for the helper threads skipping on average every
380 // 2nd ply (using a half-density matrix).
383 const Row& row = HalfDensity[(idx - 1) % HalfDensitySize];
384 if (row[(rootDepth / ONE_PLY + rootPos.game_ply()) % row.size()])
388 // Age out PV variability metric
390 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
392 // Save the last iteration's scores before first PV line is searched and
393 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
394 for (RootMove& rm : rootMoves)
395 rm.previousScore = rm.score;
397 // MultiPV loop. We perform a full root search for each PV line
398 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
400 // Reset aspiration window starting size
401 if (rootDepth >= 5 * ONE_PLY)
404 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
405 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
408 // Start with a small aspiration window and, in the case of a fail
409 // high/low, re-search with a bigger window until we're not failing
413 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
415 // Bring the best move to the front. It is critical that sorting
416 // is done with a stable algorithm because all the values but the
417 // first and eventually the new best one are set to -VALUE_INFINITE
418 // and we want to keep the same order for all the moves except the
419 // new PV that goes to the front. Note that in case of MultiPV
420 // search the already searched PV lines are preserved.
421 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
423 // If search has been stopped, break immediately. Sorting and
424 // writing PV back to TT is safe because RootMoves is still
425 // valid, although it refers to the previous iteration.
429 // When failing high/low give some update (without cluttering
430 // the UI) before a re-search.
433 && (bestValue <= alpha || bestValue >= beta)
434 && Time.elapsed() > 3000)
435 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
437 // In case of failing low/high increase aspiration window and
438 // re-search, otherwise exit the loop.
439 if (bestValue <= alpha)
441 beta = (alpha + beta) / 2;
442 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
446 mainThread->failedLow = true;
447 Signals.stopOnPonderhit = false;
450 else if (bestValue >= beta)
452 alpha = (alpha + beta) / 2;
453 beta = std::min(bestValue + delta, VALUE_INFINITE);
458 delta += delta / 4 + 5;
460 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
463 // Sort the PV lines searched so far and update the GUI
464 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
469 if (Signals.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000)
470 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
474 completedDepth = rootDepth;
479 // If skill level is enabled and time is up, pick a sub-optimal best move
480 if (skill.enabled() && skill.time_to_pick(rootDepth))
481 skill.pick_best(multiPV);
483 // Have we found a "mate in x"?
485 && bestValue >= VALUE_MATE_IN_MAX_PLY
486 && VALUE_MATE - bestValue <= 2 * Limits.mate)
489 // Do we have time for the next iteration? Can we stop searching now?
490 if (Limits.use_time_management())
492 if (!Signals.stop && !Signals.stopOnPonderhit)
494 // Stop the search if only one legal move is available, or if all
495 // of the available time has been used, or if we matched an easyMove
496 // from the previous search and just did a fast verification.
497 const int F[] = { mainThread->failedLow,
498 bestValue - mainThread->previousScore };
500 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
501 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
503 bool doEasyMove = rootMoves[0].pv[0] == easyMove
504 && mainThread->bestMoveChanges < 0.03
505 && Time.elapsed() > Time.optimum() * 5 / 42;
507 if ( rootMoves.size() == 1
508 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628
509 || (mainThread->easyMovePlayed = doEasyMove, doEasyMove))
511 // If we are allowed to ponder do not stop the search now but
512 // keep pondering until the GUI sends "ponderhit" or "stop".
514 Signals.stopOnPonderhit = true;
520 if (rootMoves[0].pv.size() >= 3)
521 EasyMove.update(rootPos, rootMoves[0].pv);
530 // Clear any candidate easy move that wasn't stable for the last search
531 // iterations; the second condition prevents consecutive fast moves.
532 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
535 // If skill level is enabled, swap best PV line with the sub-optimal one
537 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
538 rootMoves.end(), skill.best_move(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, bool skipEarlyPruning) {
549 const bool PvNode = NT == PV;
550 const bool rootNode = PvNode && (ss-1)->ply == 0;
552 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
553 assert(PvNode || (alpha == beta - 1));
554 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
555 assert(!(PvNode && cutNode));
556 assert(depth / ONE_PLY * ONE_PLY == depth);
558 Move pv[MAX_PLY+1], quietsSearched[64];
562 Move ttMove, move, excludedMove, bestMove;
563 Depth extension, newDepth;
564 Value bestValue, value, ttValue, eval, nullValue;
565 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
566 bool captureOrPromotion, doFullDepthSearch, moveCountPruning;
568 int moveCount, quietCount;
570 // Step 1. Initialize node
571 Thread* thisThread = pos.this_thread();
572 inCheck = pos.checkers();
573 moveCount = quietCount = ss->moveCount = 0;
574 ss->history = VALUE_ZERO;
575 bestValue = -VALUE_INFINITE;
576 ss->ply = (ss-1)->ply + 1;
578 // Check for the available remaining time
579 if (thisThread->resetCalls.load(std::memory_order_relaxed))
581 thisThread->resetCalls = false;
582 // At low node count increase the checking rate to about 0.1% of nodes
583 // otherwise use a default value.
584 thisThread->callsCnt = Limits.nodes ? std::min((int64_t)4096, Limits.nodes / 1024)
588 if (--thisThread->callsCnt <= 0)
590 for (Thread* th : Threads)
591 th->resetCalls = true;
596 // Used to send selDepth info to GUI
597 if (PvNode && thisThread->maxPly < ss->ply)
598 thisThread->maxPly = ss->ply;
602 // Step 2. Check for aborted search and immediate draw
603 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
604 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
605 : DrawValue[pos.side_to_move()];
607 // Step 3. Mate distance pruning. Even if we mate at the next move our score
608 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
609 // a shorter mate was found upward in the tree then there is no need to search
610 // because we will never beat the current alpha. Same logic but with reversed
611 // signs applies also in the opposite condition of being mated instead of giving
612 // mate. In this case return a fail-high score.
613 alpha = std::max(mated_in(ss->ply), alpha);
614 beta = std::min(mate_in(ss->ply+1), beta);
619 assert(0 <= ss->ply && ss->ply < MAX_PLY);
621 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
622 ss->counterMoves = nullptr;
623 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
624 Square prevSq = to_sq((ss-1)->currentMove);
626 // Step 4. Transposition table lookup. We don't want the score of a partial
627 // search to overwrite a previous full search TT value, so we use a different
628 // position key in case of an excluded move.
629 excludedMove = ss->excludedMove;
630 posKey = pos.key() ^ Key(excludedMove);
631 tte = TT.probe(posKey, ttHit);
632 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
633 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
634 : ttHit ? tte->move() : MOVE_NONE;
636 // At non-PV nodes we check for an early TT cutoff
639 && tte->depth() >= depth
640 && ttValue != VALUE_NONE // Possible in case of TT access race
641 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
642 : (tte->bound() & BOUND_UPPER)))
644 // If ttMove is quiet, update move sorting heuristics on TT hit
645 if (ttValue >= beta && ttMove)
647 if (!pos.capture_or_promotion(ttMove))
648 update_stats(pos, ss, ttMove, nullptr, 0, bonus(depth));
650 // Extra penalty for a quiet TT move in previous ply when it gets refuted
651 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
652 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, penalty(depth));
657 // Step 4a. Tablebase probe
658 if (!rootNode && TB::Cardinality)
660 int piecesCount = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
662 if ( piecesCount <= TB::Cardinality
663 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
664 && pos.rule50_count() == 0
665 && !pos.can_castle(ANY_CASTLING))
668 TB::WDLScore v = Tablebases::probe_wdl(pos, &err);
670 if (err != TB::ProbeState::FAIL)
672 thisThread->tbHits++;
674 int drawScore = TB::UseRule50 ? 1 : 0;
676 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
677 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
678 : VALUE_DRAW + 2 * v * drawScore;
680 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
681 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
682 MOVE_NONE, VALUE_NONE, TT.generation());
689 // Step 5. Evaluate the position statically
692 ss->staticEval = eval = VALUE_NONE;
698 // Never assume anything on values stored in TT
699 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
700 eval = ss->staticEval = evaluate(pos);
702 // Can ttValue be used as a better position evaluation?
703 if (ttValue != VALUE_NONE)
704 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
709 eval = ss->staticEval =
710 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
711 : -(ss-1)->staticEval + 2 * Eval::Tempo;
713 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
714 ss->staticEval, TT.generation());
717 if (skipEarlyPruning)
720 // Step 6. Razoring (skipped when in check)
722 && depth < 4 * ONE_PLY
723 && ttMove == MOVE_NONE
724 && eval + razor_margin[depth / ONE_PLY] <= alpha)
726 if (depth <= ONE_PLY)
727 return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
729 Value ralpha = alpha - razor_margin[depth / ONE_PLY];
730 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1);
735 // Step 7. Futility pruning: child node (skipped when in check)
737 && depth < 7 * ONE_PLY
738 && eval - futility_margin(depth) >= beta
739 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
740 && pos.non_pawn_material(pos.side_to_move()))
743 // Step 8. Null move search with verification search (is omitted in PV nodes)
746 && (ss->staticEval >= beta - 35 * (depth / ONE_PLY - 6) || depth >= 13 * ONE_PLY)
747 && pos.non_pawn_material(pos.side_to_move()))
749 ss->currentMove = MOVE_NULL;
750 ss->counterMoves = nullptr;
752 assert(eval - beta >= 0);
754 // Null move dynamic reduction based on depth and value
755 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
757 pos.do_null_move(st);
758 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1)
759 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
760 pos.undo_null_move();
762 if (nullValue >= beta)
764 // Do not return unproven mate scores
765 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
768 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
771 // Do verification search at high depths
772 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
773 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
780 // Step 9. ProbCut (skipped when in check)
781 // If we have a good enough capture and a reduced search returns a value
782 // much above beta, we can (almost) safely prune the previous move.
784 && depth >= 5 * ONE_PLY
785 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
787 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
788 Depth rdepth = depth - 4 * ONE_PLY;
790 assert(rdepth >= ONE_PLY);
791 assert((ss-1)->currentMove != MOVE_NONE);
792 assert((ss-1)->currentMove != MOVE_NULL);
794 MovePicker mp(pos, ttMove, rbeta - ss->staticEval);
796 while ((move = mp.next_move()) != MOVE_NONE)
799 ss->currentMove = move;
800 ss->counterMoves = &thisThread->counterMoveHistory[pos.moved_piece(move)][to_sq(move)];
801 pos.do_move(move, st);
802 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode, false);
809 // Step 10. Internal iterative deepening (skipped when in check)
810 if ( depth >= 6 * ONE_PLY
812 && (PvNode || ss->staticEval + 256 >= beta))
814 Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
815 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
817 tte = TT.probe(posKey, ttHit);
818 ttMove = ttHit ? tte->move() : MOVE_NONE;
821 moves_loop: // When in check search starts from here
823 const CounterMoveStats* cmh = (ss-1)->counterMoves;
824 const CounterMoveStats* fmh = (ss-2)->counterMoves;
825 const CounterMoveStats* fmh2 = (ss-4)->counterMoves;
827 MovePicker mp(pos, ttMove, depth, ss);
828 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
829 improving = ss->staticEval >= (ss-2)->staticEval
830 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
831 ||(ss-2)->staticEval == VALUE_NONE;
833 singularExtensionNode = !rootNode
834 && depth >= 8 * ONE_PLY
835 && ttMove != MOVE_NONE
836 && ttValue != VALUE_NONE
837 && !excludedMove // Recursive singular search is not allowed
838 && (tte->bound() & BOUND_LOWER)
839 && tte->depth() >= depth - 3 * ONE_PLY;
841 // Step 11. Loop through moves
842 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
843 while ((move = mp.next_move()) != MOVE_NONE)
847 if (move == excludedMove)
850 // At root obey the "searchmoves" option and skip moves not listed in Root
851 // Move List. As a consequence any illegal move is also skipped. In MultiPV
852 // mode we also skip PV moves which have been already searched.
853 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
854 thisThread->rootMoves.end(), move))
857 ss->moveCount = ++moveCount;
859 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
860 sync_cout << "info depth " << depth / ONE_PLY
861 << " currmove " << UCI::move(move, pos.is_chess960())
862 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
865 (ss+1)->pv = nullptr;
867 extension = DEPTH_ZERO;
868 captureOrPromotion = pos.capture_or_promotion(move);
869 moved_piece = pos.moved_piece(move);
871 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
872 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
873 : pos.gives_check(move);
875 moveCountPruning = depth < 16 * ONE_PLY
876 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
878 // Step 12. Extend checks
881 && pos.see_ge(move, VALUE_ZERO))
884 // Singular extension search. If all moves but one fail low on a search of
885 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
886 // is singular and should be extended. To verify this we do a reduced search
887 // on all the other moves but the ttMove and if the result is lower than
888 // ttValue minus a margin then we extend the ttMove.
889 if ( singularExtensionNode
894 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
895 Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
896 ss->excludedMove = move;
897 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode, true);
898 ss->excludedMove = MOVE_NONE;
904 // Update the current move (this must be done after singular extension search)
905 newDepth = depth - ONE_PLY + extension;
907 // Step 13. Pruning at shallow depth
909 && bestValue > VALUE_MATED_IN_MAX_PLY)
911 if ( !captureOrPromotion
913 && !pos.advanced_pawn_push(move))
915 // Move count based pruning
916 if (moveCountPruning)
919 // Reduced depth of the next LMR search
920 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
922 // Countermoves based pruning
924 && (!cmh || (*cmh )[moved_piece][to_sq(move)] < VALUE_ZERO)
925 && (!fmh || (*fmh )[moved_piece][to_sq(move)] < VALUE_ZERO)
926 && (!fmh2 || (*fmh2)[moved_piece][to_sq(move)] < VALUE_ZERO || (cmh && fmh)))
929 // Futility pruning: parent node
932 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
935 // Prune moves with negative SEE
937 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
940 else if ( depth < 7 * ONE_PLY
942 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
946 // Speculative prefetch as early as possible
947 prefetch(TT.first_entry(pos.key_after(move)));
949 // Check for legality just before making the move
950 if (!rootNode && !pos.legal(move))
952 ss->moveCount = --moveCount;
956 ss->currentMove = move;
957 ss->counterMoves = &thisThread->counterMoveHistory[moved_piece][to_sq(move)];
959 // Step 14. Make the move
960 pos.do_move(move, st, givesCheck);
962 // Step 15. Reduced depth search (LMR). If the move fails high it will be
963 // re-searched at full depth.
964 if ( depth >= 3 * ONE_PLY
966 && (!captureOrPromotion || moveCountPruning))
968 Depth r = reduction<PvNode>(improving, depth, moveCount);
970 if (captureOrPromotion)
971 r -= r ? ONE_PLY : DEPTH_ZERO;
974 // Increase reduction for cut nodes
978 // Decrease reduction for moves that escape a capture. Filter out
979 // castling moves, because they are coded as "king captures rook" and
980 // hence break make_move().
981 else if ( type_of(move) == NORMAL
982 && !pos.see_ge(make_move(to_sq(move), from_sq(move)), VALUE_ZERO))
985 ss->history = (cmh ? (*cmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
986 + (fmh ? (*fmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
987 + (fmh2 ? (*fmh2)[moved_piece][to_sq(move)] : VALUE_ZERO)
988 + thisThread->history.get(~pos.side_to_move(), move)
989 - 8000; // Correction factor
991 // Decrease/increase reduction by comparing opponent's stat score
992 if (ss->history > VALUE_ZERO && (ss-1)->history < VALUE_ZERO)
995 else if (ss->history < VALUE_ZERO && (ss-1)->history > VALUE_ZERO)
998 // Decrease/increase reduction for moves with a good/bad history
999 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->history / 20000) * ONE_PLY);
1002 Depth d = std::max(newDepth - r, ONE_PLY);
1004 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
1006 doFullDepthSearch = (value > alpha && d != newDepth);
1009 doFullDepthSearch = !PvNode || moveCount > 1;
1011 // Step 16. Full depth search when LMR is skipped or fails high
1012 if (doFullDepthSearch)
1013 value = newDepth < ONE_PLY ?
1014 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha)
1015 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha)
1016 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
1018 // For PV nodes only, do a full PV search on the first move or after a fail
1019 // high (in the latter case search only if value < beta), otherwise let the
1020 // parent node fail low with value <= alpha and try another move.
1021 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1024 (ss+1)->pv[0] = MOVE_NONE;
1026 value = newDepth < ONE_PLY ?
1027 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha)
1028 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha)
1029 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1032 // Step 17. Undo move
1033 pos.undo_move(move);
1035 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1037 // Step 18. Check for a new best move
1038 // Finished searching the move. If a stop occurred, the return value of
1039 // the search cannot be trusted, and we return immediately without
1040 // updating best move, PV and TT.
1041 if (Signals.stop.load(std::memory_order_relaxed))
1046 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1047 thisThread->rootMoves.end(), move);
1049 // PV move or new best move ?
1050 if (moveCount == 1 || value > alpha)
1057 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1058 rm.pv.push_back(*m);
1060 // We record how often the best move has been changed in each
1061 // iteration. This information is used for time management: When
1062 // the best move changes frequently, we allocate some more time.
1063 if (moveCount > 1 && thisThread == Threads.main())
1064 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1067 // All other moves but the PV are set to the lowest value: this is
1068 // not a problem when sorting because the sort is stable and the
1069 // move position in the list is preserved - just the PV is pushed up.
1070 rm.score = -VALUE_INFINITE;
1073 if (value > bestValue)
1081 if (PvNode && !rootNode) // Update pv even in fail-high case
1082 update_pv(ss->pv, move, (ss+1)->pv);
1084 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1088 assert(value >= beta); // Fail high
1094 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1095 quietsSearched[quietCount++] = move;
1098 // The following condition would detect a stop only after move loop has been
1099 // completed. But in this case bestValue is valid because we have fully
1100 // searched our subtree, and we can anyhow save the result in TT.
1106 // Step 20. Check for mate and stalemate
1107 // All legal moves have been searched and if there are no legal moves, it
1108 // must be a mate or a stalemate. If we are in a singular extension search then
1109 // return a fail low score.
1111 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1114 bestValue = excludedMove ? alpha
1115 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1119 // Quiet best move: update move sorting heuristics
1120 if (!pos.capture_or_promotion(bestMove))
1121 update_stats(pos, ss, bestMove, quietsSearched, quietCount, bonus(depth));
1123 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1124 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1125 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, penalty(depth));
1127 // Bonus for prior countermove that caused the fail low
1128 else if ( depth >= 3 * ONE_PLY
1129 && !pos.captured_piece()
1130 && is_ok((ss-1)->currentMove))
1131 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, bonus(depth));
1133 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1134 bestValue >= beta ? BOUND_LOWER :
1135 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1136 depth, bestMove, ss->staticEval, TT.generation());
1138 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1144 // qsearch() is the quiescence search function, which is called by the main
1145 // search function with depth zero, or recursively with depth less than ONE_PLY.
1147 template <NodeType NT, bool InCheck>
1148 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1150 const bool PvNode = NT == PV;
1152 assert(InCheck == !!pos.checkers());
1153 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1154 assert(PvNode || (alpha == beta - 1));
1155 assert(depth <= DEPTH_ZERO);
1156 assert(depth / ONE_PLY * ONE_PLY == depth);
1162 Move ttMove, move, bestMove;
1163 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1164 bool ttHit, givesCheck, evasionPrunable;
1169 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1171 ss->pv[0] = MOVE_NONE;
1174 ss->currentMove = bestMove = MOVE_NONE;
1175 ss->ply = (ss-1)->ply + 1;
1177 // Check for an instant draw or if the maximum ply has been reached
1178 if (pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
1179 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1180 : DrawValue[pos.side_to_move()];
1182 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1184 // Decide whether or not to include checks: this fixes also the type of
1185 // TT entry depth that we are going to use. Note that in qsearch we use
1186 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1187 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1188 : DEPTH_QS_NO_CHECKS;
1190 // Transposition table lookup
1192 tte = TT.probe(posKey, ttHit);
1193 ttMove = ttHit ? tte->move() : MOVE_NONE;
1194 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1198 && tte->depth() >= ttDepth
1199 && ttValue != VALUE_NONE // Only in case of TT access race
1200 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1201 : (tte->bound() & BOUND_UPPER)))
1204 // Evaluate the position statically
1207 ss->staticEval = VALUE_NONE;
1208 bestValue = futilityBase = -VALUE_INFINITE;
1214 // Never assume anything on values stored in TT
1215 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1216 ss->staticEval = bestValue = evaluate(pos);
1218 // Can ttValue be used as a better position evaluation?
1219 if (ttValue != VALUE_NONE)
1220 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1221 bestValue = ttValue;
1224 ss->staticEval = bestValue =
1225 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1226 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1228 // Stand pat. Return immediately if static value is at least beta
1229 if (bestValue >= beta)
1232 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1233 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1238 if (PvNode && bestValue > alpha)
1241 futilityBase = bestValue + 128;
1244 // Initialize a MovePicker object for the current position, and prepare
1245 // to search the moves. Because the depth is <= 0 here, only captures,
1246 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1248 MovePicker mp(pos, ttMove, depth, to_sq((ss-1)->currentMove));
1250 // Loop through the moves until no moves remain or a beta cutoff occurs
1251 while ((move = mp.next_move()) != MOVE_NONE)
1253 assert(is_ok(move));
1255 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
1256 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
1257 : pos.gives_check(move);
1262 && futilityBase > -VALUE_KNOWN_WIN
1263 && !pos.advanced_pawn_push(move))
1265 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1267 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1269 if (futilityValue <= alpha)
1271 bestValue = std::max(bestValue, futilityValue);
1275 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1277 bestValue = std::max(bestValue, futilityBase);
1282 // Detect non-capture evasions that are candidates to be pruned
1283 evasionPrunable = InCheck
1284 && bestValue > VALUE_MATED_IN_MAX_PLY
1285 && !pos.capture(move);
1287 // Don't search moves with negative SEE values
1288 if ( (!InCheck || evasionPrunable)
1289 && type_of(move) != PROMOTION
1290 && !pos.see_ge(move, VALUE_ZERO))
1293 // Speculative prefetch as early as possible
1294 prefetch(TT.first_entry(pos.key_after(move)));
1296 // Check for legality just before making the move
1297 if (!pos.legal(move))
1300 ss->currentMove = move;
1302 // Make and search the move
1303 pos.do_move(move, st, givesCheck);
1304 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1305 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1306 pos.undo_move(move);
1308 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1310 // Check for a new best move
1311 if (value > bestValue)
1317 if (PvNode) // Update pv even in fail-high case
1318 update_pv(ss->pv, move, (ss+1)->pv);
1320 if (PvNode && value < beta) // Update alpha here!
1327 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1328 ttDepth, move, ss->staticEval, TT.generation());
1336 // All legal moves have been searched. A special case: If we're in check
1337 // and no legal moves were found, it is checkmate.
1338 if (InCheck && bestValue == -VALUE_INFINITE)
1339 return mated_in(ss->ply); // Plies to mate from the root
1341 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1342 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1343 ttDepth, bestMove, ss->staticEval, TT.generation());
1345 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1351 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1352 // "plies to mate from the current position". Non-mate scores are unchanged.
1353 // The function is called before storing a value in the transposition table.
1355 Value value_to_tt(Value v, int ply) {
1357 assert(v != VALUE_NONE);
1359 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1360 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1364 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1365 // from the transposition table (which refers to the plies to mate/be mated
1366 // from current position) to "plies to mate/be mated from the root".
1368 Value value_from_tt(Value v, int ply) {
1370 return v == VALUE_NONE ? VALUE_NONE
1371 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1372 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1376 // update_pv() adds current move and appends child pv[]
1378 void update_pv(Move* pv, Move move, Move* childPv) {
1380 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1386 // update_cm_stats() updates countermove and follow-up move history
1388 void update_cm_stats(Stack* ss, Piece pc, Square s, Value bonus) {
1390 CounterMoveStats* cmh = (ss-1)->counterMoves;
1391 CounterMoveStats* fmh1 = (ss-2)->counterMoves;
1392 CounterMoveStats* fmh2 = (ss-4)->counterMoves;
1395 cmh->update(pc, s, bonus);
1398 fmh1->update(pc, s, bonus);
1401 fmh2->update(pc, s, bonus);
1405 // update_stats() updates move sorting heuristics when a new quiet best move is found
1407 void update_stats(const Position& pos, Stack* ss, Move move,
1408 Move* quiets, int quietsCnt, Value bonus) {
1410 if (ss->killers[0] != move)
1412 ss->killers[1] = ss->killers[0];
1413 ss->killers[0] = move;
1416 Color c = pos.side_to_move();
1417 Thread* thisThread = pos.this_thread();
1418 thisThread->history.update(c, move, bonus);
1419 update_cm_stats(ss, pos.moved_piece(move), to_sq(move), bonus);
1421 if ((ss-1)->counterMoves)
1423 Square prevSq = to_sq((ss-1)->currentMove);
1424 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1427 // Decrease all the other played quiet moves
1428 for (int i = 0; i < quietsCnt; ++i)
1430 thisThread->history.update(c, quiets[i], -bonus);
1431 update_cm_stats(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1436 // When playing with strength handicap, choose best move among a set of RootMoves
1437 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1439 Move Skill::pick_best(size_t multiPV) {
1441 const RootMoves& rootMoves = Threads.main()->rootMoves;
1442 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1444 // RootMoves are already sorted by score in descending order
1445 Value topScore = rootMoves[0].score;
1446 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1447 int weakness = 120 - 2 * level;
1448 int maxScore = -VALUE_INFINITE;
1450 // Choose best move. For each move score we add two terms, both dependent on
1451 // weakness. One is deterministic and bigger for weaker levels, and one is
1452 // random. Then we choose the move with the resulting highest score.
1453 for (size_t i = 0; i < multiPV; ++i)
1455 // This is our magic formula
1456 int push = ( weakness * int(topScore - rootMoves[i].score)
1457 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1459 if (rootMoves[i].score + push > maxScore)
1461 maxScore = rootMoves[i].score + push;
1462 best = rootMoves[i].pv[0];
1470 // check_time() is used to print debug info and, more importantly, to detect
1471 // when we are out of available time and thus stop the search.
1475 static TimePoint lastInfoTime = now();
1477 int elapsed = Time.elapsed();
1478 TimePoint tick = Limits.startTime + elapsed;
1480 if (tick - lastInfoTime >= 1000)
1482 lastInfoTime = tick;
1486 // An engine may not stop pondering until told so by the GUI
1490 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1491 || (Limits.movetime && elapsed >= Limits.movetime)
1492 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1493 Signals.stop = true;
1499 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1500 /// that all (if any) unsearched PV lines are sent using a previous search score.
1502 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1504 std::stringstream ss;
1505 int elapsed = Time.elapsed() + 1;
1506 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1507 size_t PVIdx = pos.this_thread()->PVIdx;
1508 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1509 uint64_t nodesSearched = Threads.nodes_searched();
1510 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1512 for (size_t i = 0; i < multiPV; ++i)
1514 bool updated = (i <= PVIdx);
1516 if (depth == ONE_PLY && !updated)
1519 Depth d = updated ? depth : depth - ONE_PLY;
1520 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1522 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1523 v = tb ? TB::Score : v;
1525 if (ss.rdbuf()->in_avail()) // Not at first line
1529 << " depth " << d / ONE_PLY
1530 << " seldepth " << pos.this_thread()->maxPly
1531 << " multipv " << i + 1
1532 << " score " << UCI::value(v);
1534 if (!tb && i == PVIdx)
1535 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1537 ss << " nodes " << nodesSearched
1538 << " nps " << nodesSearched * 1000 / elapsed;
1540 if (elapsed > 1000) // Earlier makes little sense
1541 ss << " hashfull " << TT.hashfull();
1543 ss << " tbhits " << tbHits
1544 << " time " << elapsed
1547 for (Move m : rootMoves[i].pv)
1548 ss << " " << UCI::move(m, pos.is_chess960());
1555 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1556 /// before exiting the search, for instance, in case we stop the search during a
1557 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1558 /// otherwise in case of 'ponder on' we have nothing to think on.
1560 bool RootMove::extract_ponder_from_tt(Position& pos) {
1565 assert(pv.size() == 1);
1570 pos.do_move(pv[0], st);
1571 TTEntry* tte = TT.probe(pos.key(), ttHit);
1575 Move m = tte->move(); // Local copy to be SMP safe
1576 if (MoveList<LEGAL>(pos).contains(m))
1580 pos.undo_move(pv[0]);
1581 return pv.size() > 1;
1584 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1587 UseRule50 = Options["Syzygy50MoveRule"];
1588 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1589 Cardinality = Options["SyzygyProbeLimit"];
1591 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1592 if (Cardinality > MaxCardinality)
1594 Cardinality = MaxCardinality;
1595 ProbeDepth = DEPTH_ZERO;
1598 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1601 // If the current root position is in the tablebases, then RootMoves
1602 // contains only moves that preserve the draw or the win.
1603 RootInTB = root_probe(pos, rootMoves, TB::Score);
1606 Cardinality = 0; // Do not probe tablebases during the search
1608 else // If DTZ tables are missing, use WDL tables as a fallback
1610 // Filter out moves that do not preserve the draw or the win.
1611 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1613 // Only probe during search if winning
1614 if (RootInTB && TB::Score <= VALUE_DRAW)
1618 if (RootInTB && !UseRule50)
1619 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1620 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1