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 // Sizes and phases of the skip-blocks, used for distributing search depths across the threads
67 const int skipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
68 const int skipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
70 // Razoring and futility margin based on depth
71 const int razor_margin[4] = { 483, 570, 603, 554 };
72 Value futility_margin(Depth d) { return Value(150 * d / ONE_PLY); }
74 // Futility and reductions lookup tables, initialized at startup
75 int FutilityMoveCounts[2][16]; // [improving][depth]
76 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
78 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
79 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
82 // History and stats update bonus, based on depth
83 Value stat_bonus(Depth depth) {
84 int d = depth / ONE_PLY ;
85 return d > 17 ? VALUE_ZERO : Value(d * d + 2 * d - 2);
88 // Skill structure is used to implement strength limit
90 Skill(int l) : level(l) {}
91 bool enabled() const { return level < 20; }
92 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
93 Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
94 Move pick_best(size_t multiPV);
97 Move best = MOVE_NONE;
100 // EasyMoveManager structure is used to detect an 'easy move'. When the PV is
101 // stable across multiple search iterations, we can quickly return the best move.
102 struct EasyMoveManager {
107 pv[0] = pv[1] = pv[2] = MOVE_NONE;
110 Move get(Key key) const {
111 return expectedPosKey == key ? pv[2] : MOVE_NONE;
114 void update(Position& pos, const std::vector<Move>& newPv) {
116 assert(newPv.size() >= 3);
118 // Keep track of how many times in a row the 3rd ply remains stable
119 stableCnt = (newPv[2] == pv[2]) ? stableCnt + 1 : 0;
121 if (!std::equal(newPv.begin(), newPv.begin() + 3, pv))
123 std::copy(newPv.begin(), newPv.begin() + 3, pv);
126 pos.do_move(newPv[0], st[0]);
127 pos.do_move(newPv[1], st[1]);
128 expectedPosKey = pos.key();
129 pos.undo_move(newPv[1]);
130 pos.undo_move(newPv[0]);
139 EasyMoveManager EasyMove;
140 Value DrawValue[COLOR_NB];
142 template <NodeType NT>
143 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning);
145 template <NodeType NT, bool InCheck>
146 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
148 Value value_to_tt(Value v, int ply);
149 Value value_from_tt(Value v, int ply);
150 void update_pv(Move* pv, Move move, Move* childPv);
151 void update_cm_stats(Stack* ss, Piece pc, Square s, Value bonus);
152 void update_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, Value bonus);
158 /// Search::init() is called during startup to initialize various lookup tables
160 void Search::init() {
162 for (int imp = 0; imp <= 1; ++imp)
163 for (int d = 1; d < 64; ++d)
164 for (int mc = 1; mc < 64; ++mc)
166 double r = log(d) * log(mc) / 1.95;
168 Reductions[NonPV][imp][d][mc] = int(std::round(r));
169 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
171 // Increase reduction for non-PV nodes when eval is not improving
172 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
173 Reductions[NonPV][imp][d][mc]++;
176 for (int d = 0; d < 16; ++d)
178 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
179 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
184 /// Search::clear() resets search state to zero, to obtain reproducible results
186 void Search::clear() {
190 for (Thread* th : Threads)
192 th->counterMoves.clear();
194 th->counterMoveHistory.clear();
195 th->resetCalls = true;
198 Threads.main()->previousScore = VALUE_INFINITE;
202 /// Search::perft() is our utility to verify move generation. All the leaf nodes
203 /// up to the given depth are generated and counted, and the sum is returned.
205 uint64_t Search::perft(Position& pos, Depth depth) {
208 uint64_t cnt, nodes = 0;
209 const bool leaf = (depth == 2 * ONE_PLY);
211 for (const auto& m : MoveList<LEGAL>(pos))
213 if (Root && depth <= ONE_PLY)
218 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
223 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
228 template uint64_t Search::perft<true>(Position&, Depth);
231 /// MainThread::search() is called by the main thread when the program receives
232 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
234 void MainThread::search() {
236 Color us = rootPos.side_to_move();
237 Time.init(Limits, us, rootPos.game_ply());
239 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
240 DrawValue[ us] = VALUE_DRAW - Value(contempt);
241 DrawValue[~us] = VALUE_DRAW + Value(contempt);
243 if (rootMoves.empty())
245 rootMoves.push_back(RootMove(MOVE_NONE));
246 sync_cout << "info depth 0 score "
247 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
252 for (Thread* th : Threads)
254 th->start_searching();
256 Thread::search(); // Let's start searching!
259 // When playing in 'nodes as time' mode, subtract the searched nodes from
260 // the available ones before exiting.
262 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
264 // When we reach the maximum depth, we can arrive here without a raise of
265 // Signals.stop. However, if we are pondering or in an infinite search,
266 // the UCI protocol states that we shouldn't print the best move before the
267 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
268 // until the GUI sends one of those commands (which also raises Signals.stop).
269 if (!Signals.stop && (Limits.ponder || Limits.infinite))
271 Signals.stopOnPonderhit = true;
275 // Stop the threads if not already stopped
278 // Wait until all threads have finished
279 for (Thread* th : Threads)
281 th->wait_for_search_finished();
283 // Check if there are threads with a better score than main thread
284 Thread* bestThread = this;
285 if ( !this->easyMovePlayed
286 && Options["MultiPV"] == 1
288 && !Skill(Options["Skill Level"]).enabled()
289 && rootMoves[0].pv[0] != MOVE_NONE)
291 for (Thread* th : Threads)
293 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
294 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
296 if (scoreDiff > 0 && depthDiff >= 0)
301 previousScore = bestThread->rootMoves[0].score;
303 // Send new PV when needed
304 if (bestThread != this)
305 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
307 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
309 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
310 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
312 std::cout << sync_endl;
316 /// Thread::search() is the main iterative deepening loop. It calls search()
317 /// repeatedly with increasing depth until the allocated thinking time has been
318 /// consumed, the user stops the search, or the maximum search depth is reached.
320 void Thread::search() {
322 Stack stack[MAX_PLY+7], *ss = stack+4; // To allow referencing (ss-4) and (ss+2)
323 Value bestValue, alpha, beta, delta;
324 Move easyMove = MOVE_NONE;
325 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
327 std::memset(ss-4, 0, 7 * sizeof(Stack));
328 for(int i = 4; i > 0; i--)
329 (ss-i)->counterMoves = &this->counterMoveHistory[NO_PIECE][0]; // Use as sentinel
331 bestValue = delta = alpha = -VALUE_INFINITE;
332 beta = VALUE_INFINITE;
333 completedDepth = DEPTH_ZERO;
337 easyMove = EasyMove.get(rootPos.key());
339 mainThread->easyMovePlayed = mainThread->failedLow = false;
340 mainThread->bestMoveChanges = 0;
344 size_t multiPV = Options["MultiPV"];
345 Skill skill(Options["Skill Level"]);
347 // When playing with strength handicap enable MultiPV search that we will
348 // use behind the scenes to retrieve a set of possible moves.
350 multiPV = std::max(multiPV, (size_t)4);
352 multiPV = std::min(multiPV, rootMoves.size());
354 // Iterative deepening loop until requested to stop or the target depth is reached
355 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
357 && (!Limits.depth || Threads.main()->rootDepth / ONE_PLY <= Limits.depth))
359 // Distribute search depths across the threads
362 int i = (idx - 1) % 20;
363 if (((rootDepth / ONE_PLY + rootPos.game_ply() + skipPhase[i]) / skipSize[i]) % 2)
367 // Age out PV variability metric
369 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
371 // Save the last iteration's scores before first PV line is searched and
372 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
373 for (RootMove& rm : rootMoves)
374 rm.previousScore = rm.score;
376 // MultiPV loop. We perform a full root search for each PV line
377 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
379 // Reset aspiration window starting size
380 if (rootDepth >= 5 * ONE_PLY)
383 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
384 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
387 // Start with a small aspiration window and, in the case of a fail
388 // high/low, re-search with a bigger window until we're not failing
392 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
394 // Bring the best move to the front. It is critical that sorting
395 // is done with a stable algorithm because all the values but the
396 // first and eventually the new best one are set to -VALUE_INFINITE
397 // and we want to keep the same order for all the moves except the
398 // new PV that goes to the front. Note that in case of MultiPV
399 // search the already searched PV lines are preserved.
400 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
402 // If search has been stopped, we break immediately. Sorting and
403 // writing PV back to TT is safe because RootMoves is still
404 // valid, although it refers to the previous iteration.
408 // When failing high/low give some update (without cluttering
409 // the UI) before a re-search.
412 && (bestValue <= alpha || bestValue >= beta)
413 && Time.elapsed() > 3000)
414 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
416 // In case of failing low/high increase aspiration window and
417 // re-search, otherwise exit the loop.
418 if (bestValue <= alpha)
420 beta = (alpha + beta) / 2;
421 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
425 mainThread->failedLow = true;
426 Signals.stopOnPonderhit = false;
429 else if (bestValue >= beta)
431 alpha = (alpha + beta) / 2;
432 beta = std::min(bestValue + delta, VALUE_INFINITE);
437 delta += delta / 4 + 5;
439 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
442 // Sort the PV lines searched so far and update the GUI
443 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
448 if (Signals.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000)
449 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
453 completedDepth = rootDepth;
458 // If skill level is enabled and time is up, pick a sub-optimal best move
459 if (skill.enabled() && skill.time_to_pick(rootDepth))
460 skill.pick_best(multiPV);
462 // Have we found a "mate in x"?
464 && bestValue >= VALUE_MATE_IN_MAX_PLY
465 && VALUE_MATE - bestValue <= 2 * Limits.mate)
468 // Do we have time for the next iteration? Can we stop searching now?
469 if (Limits.use_time_management())
471 if (!Signals.stop && !Signals.stopOnPonderhit)
473 // Stop the search if only one legal move is available, or if all
474 // of the available time has been used, or if we matched an easyMove
475 // from the previous search and just did a fast verification.
476 const int F[] = { mainThread->failedLow,
477 bestValue - mainThread->previousScore };
479 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
480 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
482 bool doEasyMove = rootMoves[0].pv[0] == easyMove
483 && mainThread->bestMoveChanges < 0.03
484 && Time.elapsed() > Time.optimum() * 5 / 44;
486 if ( rootMoves.size() == 1
487 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628
488 || (mainThread->easyMovePlayed = doEasyMove, doEasyMove))
490 // If we are allowed to ponder do not stop the search now but
491 // keep pondering until the GUI sends "ponderhit" or "stop".
493 Signals.stopOnPonderhit = true;
499 if (rootMoves[0].pv.size() >= 3)
500 EasyMove.update(rootPos, rootMoves[0].pv);
509 // Clear any candidate easy move that wasn't stable for the last search
510 // iterations; the second condition prevents consecutive fast moves.
511 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
514 // If skill level is enabled, swap best PV line with the sub-optimal one
516 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
517 rootMoves.end(), skill.best_move(multiPV)));
523 // search<>() is the main search function for both PV and non-PV nodes
525 template <NodeType NT>
526 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
528 const bool PvNode = NT == PV;
529 const bool rootNode = PvNode && (ss-1)->ply == 0;
531 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
532 assert(PvNode || (alpha == beta - 1));
533 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
534 assert(!(PvNode && cutNode));
535 assert(depth / ONE_PLY * ONE_PLY == depth);
537 Move pv[MAX_PLY+1], quietsSearched[64];
541 Move ttMove, move, excludedMove, bestMove;
542 Depth extension, newDepth;
543 Value bestValue, value, ttValue, eval;
544 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
545 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets;
547 int moveCount, quietCount;
549 // Step 1. Initialize node
550 Thread* thisThread = pos.this_thread();
551 inCheck = pos.checkers();
552 moveCount = quietCount = ss->moveCount = 0;
553 ss->history = VALUE_ZERO;
554 bestValue = -VALUE_INFINITE;
555 ss->ply = (ss-1)->ply + 1;
557 // Check for the available remaining time
558 if (thisThread->resetCalls.load(std::memory_order_relaxed))
560 thisThread->resetCalls = false;
561 // At low node count increase the checking rate to about 0.1% of nodes
562 // otherwise use a default value.
563 thisThread->callsCnt = Limits.nodes ? std::min((int64_t)4096, Limits.nodes / 1024)
567 if (--thisThread->callsCnt <= 0)
569 for (Thread* th : Threads)
570 th->resetCalls = true;
575 // Used to send selDepth info to GUI
576 if (PvNode && thisThread->maxPly < ss->ply)
577 thisThread->maxPly = ss->ply;
581 // Step 2. Check for aborted search and immediate draw
582 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
583 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
584 : DrawValue[pos.side_to_move()];
586 // Step 3. Mate distance pruning. Even if we mate at the next move our score
587 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
588 // a shorter mate was found upward in the tree then there is no need to search
589 // because we will never beat the current alpha. Same logic but with reversed
590 // signs applies also in the opposite condition of being mated instead of giving
591 // mate. In this case return a fail-high score.
592 alpha = std::max(mated_in(ss->ply), alpha);
593 beta = std::min(mate_in(ss->ply+1), beta);
598 assert(0 <= ss->ply && ss->ply < MAX_PLY);
600 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
601 ss->counterMoves = &thisThread->counterMoveHistory[NO_PIECE][0];
602 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
603 Square prevSq = to_sq((ss-1)->currentMove);
605 // Step 4. Transposition table lookup. We don't want the score of a partial
606 // search to overwrite a previous full search TT value, so we use a different
607 // position key in case of an excluded move.
608 excludedMove = ss->excludedMove;
609 posKey = pos.key() ^ Key(excludedMove);
610 tte = TT.probe(posKey, ttHit);
611 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
612 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
613 : ttHit ? tte->move() : MOVE_NONE;
615 // At non-PV nodes we check for an early TT cutoff
618 && tte->depth() >= depth
619 && ttValue != VALUE_NONE // Possible in case of TT access race
620 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
621 : (tte->bound() & BOUND_UPPER)))
623 // If ttMove is quiet, update move sorting heuristics on TT hit
628 if (!pos.capture_or_promotion(ttMove))
629 update_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
631 // Extra penalty for a quiet TT move in previous ply when it gets refuted
632 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
633 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
635 // Penalty for a quiet ttMove that fails low
636 else if (!pos.capture_or_promotion(ttMove))
638 Value penalty = -stat_bonus(depth + ONE_PLY);
639 thisThread->history.update(pos.side_to_move(), ttMove, penalty);
640 update_cm_stats(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
646 // Step 4a. Tablebase probe
647 if (!rootNode && TB::Cardinality)
649 int piecesCount = pos.count<ALL_PIECES>();
651 if ( piecesCount <= TB::Cardinality
652 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
653 && pos.rule50_count() == 0
654 && !pos.can_castle(ANY_CASTLING))
657 TB::WDLScore v = Tablebases::probe_wdl(pos, &err);
659 if (err != TB::ProbeState::FAIL)
661 thisThread->tbHits++;
663 int drawScore = TB::UseRule50 ? 1 : 0;
665 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
666 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
667 : VALUE_DRAW + 2 * v * drawScore;
669 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
670 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
671 MOVE_NONE, VALUE_NONE, TT.generation());
678 // Step 5. Evaluate the position statically
681 ss->staticEval = eval = VALUE_NONE;
687 // Never assume anything on values stored in TT
688 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
689 eval = ss->staticEval = evaluate(pos);
691 // Can ttValue be used as a better position evaluation?
692 if (ttValue != VALUE_NONE)
693 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
698 eval = ss->staticEval =
699 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
700 : -(ss-1)->staticEval + 2 * Eval::Tempo;
702 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
703 ss->staticEval, TT.generation());
706 if (skipEarlyPruning)
709 // Step 6. Razoring (skipped when in check)
711 && depth < 4 * ONE_PLY
712 && eval + razor_margin[depth / ONE_PLY] <= alpha)
714 if (depth <= ONE_PLY)
715 return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
717 Value ralpha = alpha - razor_margin[depth / ONE_PLY];
718 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1);
723 // Step 7. Futility pruning: child node (skipped when in check)
725 && depth < 7 * ONE_PLY
726 && eval - futility_margin(depth) >= beta
727 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
728 && pos.non_pawn_material(pos.side_to_move()))
731 // Step 8. Null move search with verification search (is omitted in PV nodes)
734 && (ss->staticEval >= beta - 35 * (depth / ONE_PLY - 6) || depth >= 13 * ONE_PLY)
735 && pos.non_pawn_material(pos.side_to_move()))
738 assert(eval - beta >= 0);
740 // Null move dynamic reduction based on depth and value
741 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
743 ss->currentMove = MOVE_NULL;
744 ss->counterMoves = &thisThread->counterMoveHistory[NO_PIECE][0];
746 pos.do_null_move(st);
747 Value nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1)
748 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
749 pos.undo_null_move();
751 if (nullValue >= beta)
753 // Do not return unproven mate scores
754 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
757 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
760 // Do verification search at high depths
761 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
762 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
769 // Step 9. ProbCut (skipped when in check)
770 // If we have a good enough capture and a reduced search returns a value
771 // much above beta, we can (almost) safely prune the previous move.
773 && depth >= 5 * ONE_PLY
774 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
776 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
777 Depth rdepth = depth - 4 * ONE_PLY;
779 assert(rdepth >= ONE_PLY);
780 assert(is_ok((ss-1)->currentMove));
782 MovePicker mp(pos, ttMove, rbeta - ss->staticEval);
784 while ((move = mp.next_move()) != MOVE_NONE)
787 ss->currentMove = move;
788 ss->counterMoves = &thisThread->counterMoveHistory[pos.moved_piece(move)][to_sq(move)];
790 pos.do_move(move, st);
791 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode, false);
798 // Step 10. Internal iterative deepening (skipped when in check)
799 if ( depth >= 6 * ONE_PLY
801 && (PvNode || ss->staticEval + 256 >= beta))
803 Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
804 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
806 tte = TT.probe(posKey, ttHit);
807 ttMove = ttHit ? tte->move() : MOVE_NONE;
810 moves_loop: // When in check search starts from here
812 const CounterMoveStats& cmh = *(ss-1)->counterMoves;
813 const CounterMoveStats& fmh = *(ss-2)->counterMoves;
814 const CounterMoveStats& fm2 = *(ss-4)->counterMoves;
815 const bool cm_ok = is_ok((ss-1)->currentMove);
816 const bool fm_ok = is_ok((ss-2)->currentMove);
817 const bool f2_ok = is_ok((ss-4)->currentMove);
819 MovePicker mp(pos, ttMove, depth, ss);
820 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
821 improving = ss->staticEval >= (ss-2)->staticEval
822 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
823 ||(ss-2)->staticEval == VALUE_NONE;
825 singularExtensionNode = !rootNode
826 && depth >= 8 * ONE_PLY
827 && ttMove != MOVE_NONE
828 && ttValue != VALUE_NONE
829 && !excludedMove // Recursive singular search is not allowed
830 && (tte->bound() & BOUND_LOWER)
831 && tte->depth() >= depth - 3 * ONE_PLY;
834 // Step 11. Loop through moves
835 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
836 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
840 if (move == excludedMove)
843 // At root obey the "searchmoves" option and skip moves not listed in Root
844 // Move List. As a consequence any illegal move is also skipped. In MultiPV
845 // mode we also skip PV moves which have been already searched.
846 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
847 thisThread->rootMoves.end(), move))
850 ss->moveCount = ++moveCount;
852 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
853 sync_cout << "info depth " << depth / ONE_PLY
854 << " currmove " << UCI::move(move, pos.is_chess960())
855 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
858 (ss+1)->pv = nullptr;
860 extension = DEPTH_ZERO;
861 captureOrPromotion = pos.capture_or_promotion(move);
862 moved_piece = pos.moved_piece(move);
864 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
865 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
866 : pos.gives_check(move);
868 moveCountPruning = depth < 16 * ONE_PLY
869 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
871 // Step 12. Singular and Gives Check Extensions
873 // Singular extension search. If all moves but one fail low on a search of
874 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
875 // is singular and should be extended. To verify this we do a reduced search
876 // on all the other moves but the ttMove and if the result is lower than
877 // ttValue minus a margin then we extend the ttMove.
878 if ( singularExtensionNode
882 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
883 Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
884 ss->excludedMove = move;
885 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode, true);
886 ss->excludedMove = MOVE_NONE;
893 && pos.see_ge(move, VALUE_ZERO))
896 // Calculate new depth for this move
897 newDepth = depth - ONE_PLY + extension;
899 // Step 13. Pruning at shallow depth
901 && bestValue > VALUE_MATED_IN_MAX_PLY)
903 if ( !captureOrPromotion
905 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= 5000))
907 // Move count based pruning
908 if (moveCountPruning) {
913 // Reduced depth of the next LMR search
914 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
916 // Countermoves based pruning
918 && ((cmh[moved_piece][to_sq(move)] < VALUE_ZERO) || !cm_ok)
919 && ((fmh[moved_piece][to_sq(move)] < VALUE_ZERO) || !fm_ok)
920 && ((fm2[moved_piece][to_sq(move)] < VALUE_ZERO) || !f2_ok || (cm_ok && fm_ok)))
923 // Futility pruning: parent node
926 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
929 // Prune moves with negative SEE
931 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
934 else if ( depth < 7 * ONE_PLY
936 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
940 // Speculative prefetch as early as possible
941 prefetch(TT.first_entry(pos.key_after(move)));
943 // Check for legality just before making the move
944 if (!rootNode && !pos.legal(move))
946 ss->moveCount = --moveCount;
950 // Update the current move (this must be done after singular extension search)
951 ss->currentMove = move;
952 ss->counterMoves = &thisThread->counterMoveHistory[moved_piece][to_sq(move)];
954 // Step 14. Make the move
955 pos.do_move(move, st, givesCheck);
957 // Step 15. Reduced depth search (LMR). If the move fails high it will be
958 // re-searched at full depth.
959 if ( depth >= 3 * ONE_PLY
961 && (!captureOrPromotion || moveCountPruning))
963 Depth r = reduction<PvNode>(improving, depth, moveCount);
965 if (captureOrPromotion)
966 r -= r ? ONE_PLY : DEPTH_ZERO;
969 // Increase reduction for cut nodes
973 // Decrease reduction for moves that escape a capture. Filter out
974 // castling moves, because they are coded as "king captures rook" and
975 // hence break make_move().
976 else if ( type_of(move) == NORMAL
977 && !pos.see_ge(make_move(to_sq(move), from_sq(move)), VALUE_ZERO))
980 ss->history = cmh[moved_piece][to_sq(move)]
981 + fmh[moved_piece][to_sq(move)]
982 + fm2[moved_piece][to_sq(move)]
983 + thisThread->history.get(~pos.side_to_move(), move)
984 - 4000; // Correction factor
986 // Decrease/increase reduction by comparing opponent's stat score
987 if (ss->history > VALUE_ZERO && (ss-1)->history < VALUE_ZERO)
990 else if (ss->history < VALUE_ZERO && (ss-1)->history > VALUE_ZERO)
993 // Decrease/increase reduction for moves with a good/bad history
994 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->history / 20000) * ONE_PLY);
997 Depth d = std::max(newDepth - r, ONE_PLY);
999 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
1001 doFullDepthSearch = (value > alpha && d != newDepth);
1004 doFullDepthSearch = !PvNode || moveCount > 1;
1006 // Step 16. Full depth search when LMR is skipped or fails high
1007 if (doFullDepthSearch)
1008 value = newDepth < ONE_PLY ?
1009 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha)
1010 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha)
1011 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
1013 // For PV nodes only, do a full PV search on the first move or after a fail
1014 // high (in the latter case search only if value < beta), otherwise let the
1015 // parent node fail low with value <= alpha and try another move.
1016 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1019 (ss+1)->pv[0] = MOVE_NONE;
1021 value = newDepth < ONE_PLY ?
1022 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha)
1023 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha)
1024 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1027 // Step 17. Undo move
1028 pos.undo_move(move);
1030 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1032 // Step 18. Check for a new best move
1033 // Finished searching the move. If a stop occurred, the return value of
1034 // the search cannot be trusted, and we return immediately without
1035 // updating best move, PV and TT.
1036 if (Signals.stop.load(std::memory_order_relaxed))
1041 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1042 thisThread->rootMoves.end(), move);
1044 // PV move or new best move ?
1045 if (moveCount == 1 || value > alpha)
1052 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1053 rm.pv.push_back(*m);
1055 // We record how often the best move has been changed in each
1056 // iteration. This information is used for time management: When
1057 // the best move changes frequently, we allocate some more time.
1058 if (moveCount > 1 && thisThread == Threads.main())
1059 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1062 // All other moves but the PV are set to the lowest value: this is
1063 // not a problem when sorting because the sort is stable and the
1064 // move position in the list is preserved - just the PV is pushed up.
1065 rm.score = -VALUE_INFINITE;
1068 if (value > bestValue)
1076 if (PvNode && !rootNode) // Update pv even in fail-high case
1077 update_pv(ss->pv, move, (ss+1)->pv);
1079 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1083 assert(value >= beta); // Fail high
1089 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1090 quietsSearched[quietCount++] = move;
1093 // The following condition would detect a stop only after move loop has been
1094 // completed. But in this case bestValue is valid because we have fully
1095 // searched our subtree, and we can anyhow save the result in TT.
1101 // Step 20. Check for mate and stalemate
1102 // All legal moves have been searched and if there are no legal moves, it
1103 // must be a mate or a stalemate. If we are in a singular extension search then
1104 // return a fail low score.
1106 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1109 bestValue = excludedMove ? alpha
1110 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1114 // Quiet best move: update move sorting heuristics
1115 if (!pos.capture_or_promotion(bestMove))
1116 update_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1118 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1119 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1120 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1122 // Bonus for prior countermove that caused the fail low
1123 else if ( depth >= 3 * ONE_PLY
1124 && !pos.captured_piece()
1126 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1128 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1129 bestValue >= beta ? BOUND_LOWER :
1130 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1131 depth, bestMove, ss->staticEval, TT.generation());
1133 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1139 // qsearch() is the quiescence search function, which is called by the main
1140 // search function with depth zero, or recursively with depth less than ONE_PLY.
1142 template <NodeType NT, bool InCheck>
1143 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1145 const bool PvNode = NT == PV;
1147 assert(InCheck == !!pos.checkers());
1148 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1149 assert(PvNode || (alpha == beta - 1));
1150 assert(depth <= DEPTH_ZERO);
1151 assert(depth / ONE_PLY * ONE_PLY == depth);
1157 Move ttMove, move, bestMove;
1158 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1159 bool ttHit, givesCheck, evasionPrunable;
1164 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1166 ss->pv[0] = MOVE_NONE;
1169 ss->currentMove = bestMove = MOVE_NONE;
1170 ss->ply = (ss-1)->ply + 1;
1172 // Check for an instant draw or if the maximum ply has been reached
1173 if (pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
1174 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1175 : DrawValue[pos.side_to_move()];
1177 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1179 // Decide whether or not to include checks: this fixes also the type of
1180 // TT entry depth that we are going to use. Note that in qsearch we use
1181 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1182 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1183 : DEPTH_QS_NO_CHECKS;
1185 // Transposition table lookup
1187 tte = TT.probe(posKey, ttHit);
1188 ttMove = ttHit ? tte->move() : MOVE_NONE;
1189 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1193 && tte->depth() >= ttDepth
1194 && ttValue != VALUE_NONE // Only in case of TT access race
1195 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1196 : (tte->bound() & BOUND_UPPER)))
1199 // Evaluate the position statically
1202 ss->staticEval = VALUE_NONE;
1203 bestValue = futilityBase = -VALUE_INFINITE;
1209 // Never assume anything on values stored in TT
1210 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1211 ss->staticEval = bestValue = evaluate(pos);
1213 // Can ttValue be used as a better position evaluation?
1214 if (ttValue != VALUE_NONE)
1215 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1216 bestValue = ttValue;
1219 ss->staticEval = bestValue =
1220 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1221 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1223 // Stand pat. Return immediately if static value is at least beta
1224 if (bestValue >= beta)
1227 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1228 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1233 if (PvNode && bestValue > alpha)
1236 futilityBase = bestValue + 128;
1239 // Initialize a MovePicker object for the current position, and prepare
1240 // to search the moves. Because the depth is <= 0 here, only captures,
1241 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1243 MovePicker mp(pos, ttMove, depth, to_sq((ss-1)->currentMove));
1245 // Loop through the moves until no moves remain or a beta cutoff occurs
1246 while ((move = mp.next_move()) != MOVE_NONE)
1248 assert(is_ok(move));
1250 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
1251 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
1252 : pos.gives_check(move);
1257 && futilityBase > -VALUE_KNOWN_WIN
1258 && !pos.advanced_pawn_push(move))
1260 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1262 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1264 if (futilityValue <= alpha)
1266 bestValue = std::max(bestValue, futilityValue);
1270 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1272 bestValue = std::max(bestValue, futilityBase);
1277 // Detect non-capture evasions that are candidates to be pruned
1278 evasionPrunable = InCheck
1279 && bestValue > VALUE_MATED_IN_MAX_PLY
1280 && !pos.capture(move);
1282 // Don't search moves with negative SEE values
1283 if ( (!InCheck || evasionPrunable)
1284 && type_of(move) != PROMOTION
1285 && !pos.see_ge(move, VALUE_ZERO))
1288 // Speculative prefetch as early as possible
1289 prefetch(TT.first_entry(pos.key_after(move)));
1291 // Check for legality just before making the move
1292 if (!pos.legal(move))
1295 ss->currentMove = move;
1297 // Make and search the move
1298 pos.do_move(move, st, givesCheck);
1299 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1300 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1301 pos.undo_move(move);
1303 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1305 // Check for a new best move
1306 if (value > bestValue)
1312 if (PvNode) // Update pv even in fail-high case
1313 update_pv(ss->pv, move, (ss+1)->pv);
1315 if (PvNode && value < beta) // Update alpha here!
1322 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1323 ttDepth, move, ss->staticEval, TT.generation());
1331 // All legal moves have been searched. A special case: If we're in check
1332 // and no legal moves were found, it is checkmate.
1333 if (InCheck && bestValue == -VALUE_INFINITE)
1334 return mated_in(ss->ply); // Plies to mate from the root
1336 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1337 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1338 ttDepth, bestMove, ss->staticEval, TT.generation());
1340 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1346 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1347 // "plies to mate from the current position". Non-mate scores are unchanged.
1348 // The function is called before storing a value in the transposition table.
1350 Value value_to_tt(Value v, int ply) {
1352 assert(v != VALUE_NONE);
1354 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1355 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1359 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1360 // from the transposition table (which refers to the plies to mate/be mated
1361 // from current position) to "plies to mate/be mated from the root".
1363 Value value_from_tt(Value v, int ply) {
1365 return v == VALUE_NONE ? VALUE_NONE
1366 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1367 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1371 // update_pv() adds current move and appends child pv[]
1373 void update_pv(Move* pv, Move move, Move* childPv) {
1375 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1381 // update_cm_stats() updates countermove and follow-up move history
1383 void update_cm_stats(Stack* ss, Piece pc, Square s, Value bonus) {
1385 for (int i : {1, 2, 4})
1386 if (is_ok((ss-i)->currentMove))
1387 (ss-i)->counterMoves->update(pc, s, bonus);
1391 // update_stats() updates move sorting heuristics when a new quiet best move is found
1393 void update_stats(const Position& pos, Stack* ss, Move move,
1394 Move* quiets, int quietsCnt, Value bonus) {
1396 if (ss->killers[0] != move)
1398 ss->killers[1] = ss->killers[0];
1399 ss->killers[0] = move;
1402 Color c = pos.side_to_move();
1403 Thread* thisThread = pos.this_thread();
1404 thisThread->history.update(c, move, bonus);
1405 update_cm_stats(ss, pos.moved_piece(move), to_sq(move), bonus);
1407 if (is_ok((ss-1)->currentMove))
1409 Square prevSq = to_sq((ss-1)->currentMove);
1410 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1413 // Decrease all the other played quiet moves
1414 for (int i = 0; i < quietsCnt; ++i)
1416 thisThread->history.update(c, quiets[i], -bonus);
1417 update_cm_stats(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1422 // When playing with strength handicap, choose best move among a set of RootMoves
1423 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1425 Move Skill::pick_best(size_t multiPV) {
1427 const RootMoves& rootMoves = Threads.main()->rootMoves;
1428 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1430 // RootMoves are already sorted by score in descending order
1431 Value topScore = rootMoves[0].score;
1432 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1433 int weakness = 120 - 2 * level;
1434 int maxScore = -VALUE_INFINITE;
1436 // Choose best move. For each move score we add two terms, both dependent on
1437 // weakness. One is deterministic and bigger for weaker levels, and one is
1438 // random. Then we choose the move with the resulting highest score.
1439 for (size_t i = 0; i < multiPV; ++i)
1441 // This is our magic formula
1442 int push = ( weakness * int(topScore - rootMoves[i].score)
1443 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1445 if (rootMoves[i].score + push > maxScore)
1447 maxScore = rootMoves[i].score + push;
1448 best = rootMoves[i].pv[0];
1456 // check_time() is used to print debug info and, more importantly, to detect
1457 // when we are out of available time and thus stop the search.
1461 static TimePoint lastInfoTime = now();
1463 int elapsed = Time.elapsed();
1464 TimePoint tick = Limits.startTime + elapsed;
1466 if (tick - lastInfoTime >= 1000)
1468 lastInfoTime = tick;
1472 // An engine may not stop pondering until told so by the GUI
1476 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1477 || (Limits.movetime && elapsed >= Limits.movetime)
1478 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1479 Signals.stop = true;
1485 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1486 /// that all (if any) unsearched PV lines are sent using a previous search score.
1488 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1490 std::stringstream ss;
1491 int elapsed = Time.elapsed() + 1;
1492 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1493 size_t PVIdx = pos.this_thread()->PVIdx;
1494 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1495 uint64_t nodesSearched = Threads.nodes_searched();
1496 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1498 for (size_t i = 0; i < multiPV; ++i)
1500 bool updated = (i <= PVIdx);
1502 if (depth == ONE_PLY && !updated)
1505 Depth d = updated ? depth : depth - ONE_PLY;
1506 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1508 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1509 v = tb ? TB::Score : v;
1511 if (ss.rdbuf()->in_avail()) // Not at first line
1515 << " depth " << d / ONE_PLY
1516 << " seldepth " << pos.this_thread()->maxPly
1517 << " multipv " << i + 1
1518 << " score " << UCI::value(v);
1520 if (!tb && i == PVIdx)
1521 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1523 ss << " nodes " << nodesSearched
1524 << " nps " << nodesSearched * 1000 / elapsed;
1526 if (elapsed > 1000) // Earlier makes little sense
1527 ss << " hashfull " << TT.hashfull();
1529 ss << " tbhits " << tbHits
1530 << " time " << elapsed
1533 for (Move m : rootMoves[i].pv)
1534 ss << " " << UCI::move(m, pos.is_chess960());
1541 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1542 /// before exiting the search, for instance, in case we stop the search during a
1543 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1544 /// otherwise in case of 'ponder on' we have nothing to think on.
1546 bool RootMove::extract_ponder_from_tt(Position& pos) {
1551 assert(pv.size() == 1);
1556 pos.do_move(pv[0], st);
1557 TTEntry* tte = TT.probe(pos.key(), ttHit);
1561 Move m = tte->move(); // Local copy to be SMP safe
1562 if (MoveList<LEGAL>(pos).contains(m))
1566 pos.undo_move(pv[0]);
1567 return pv.size() > 1;
1570 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1573 UseRule50 = Options["Syzygy50MoveRule"];
1574 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1575 Cardinality = Options["SyzygyProbeLimit"];
1577 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1578 if (Cardinality > MaxCardinality)
1580 Cardinality = MaxCardinality;
1581 ProbeDepth = DEPTH_ZERO;
1584 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1587 // If the current root position is in the tablebases, then RootMoves
1588 // contains only moves that preserve the draw or the win.
1589 RootInTB = root_probe(pos, rootMoves, TB::Score);
1592 Cardinality = 0; // Do not probe tablebases during the search
1594 else // If DTZ tables are missing, use WDL tables as a fallback
1596 // Filter out moves that do not preserve the draw or the win.
1597 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1599 // Only probe during search if winning
1600 if (RootInTB && TB::Score <= VALUE_DRAW)
1604 if (RootInTB && !UseRule50)
1605 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1606 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1