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"
45 namespace Tablebases {
54 namespace TB = Tablebases;
58 using namespace Search;
62 // Different node types, used as a template parameter
63 enum NodeType { NonPV, PV };
65 // Sizes and phases of the skip-blocks, used for distributing search depths across the threads
66 const int skipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
67 const int skipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
69 // Razoring and futility margin based on depth
70 // razor_margin[0] is unused as long as depth >= ONE_PLY in search
71 const int razor_margin[] = { 0, 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 int stat_bonus(Depth depth) {
84 int d = depth / ONE_PLY;
85 return d > 17 ? 0 : 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 stable
101 // 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_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
152 void update_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
154 // perft() is our utility to verify move generation. All the leaf nodes up
155 // to the given depth are generated and counted, and the sum is returned.
157 uint64_t perft(Position& pos, Depth depth) {
160 uint64_t cnt, nodes = 0;
161 const bool leaf = (depth == 2 * ONE_PLY);
163 for (const auto& m : MoveList<LEGAL>(pos))
165 if (Root && depth <= ONE_PLY)
170 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
175 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
183 /// Search::init() is called during startup to initialize various lookup tables
185 void Search::init() {
187 for (int imp = 0; imp <= 1; ++imp)
188 for (int d = 1; d < 64; ++d)
189 for (int mc = 1; mc < 64; ++mc)
191 double r = log(d) * log(mc) / 1.95;
193 Reductions[NonPV][imp][d][mc] = int(std::round(r));
194 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
196 // Increase reduction for non-PV nodes when eval is not improving
197 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
198 Reductions[NonPV][imp][d][mc]++;
201 for (int d = 0; d < 16; ++d)
203 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
204 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
209 /// Search::clear() resets search state to its initial value
211 void Search::clear() {
213 Threads.main()->wait_for_search_finished();
215 Time.availableNodes = 0;
218 for (Thread* th : Threads)
221 Threads.main()->callsCnt = 0;
222 Threads.main()->previousScore = VALUE_INFINITE;
226 /// MainThread::search() is called by the main thread when the program receives
227 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
229 void MainThread::search() {
233 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
234 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
238 Color us = rootPos.side_to_move();
239 Time.init(Limits, us, rootPos.game_ply());
242 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
243 DrawValue[ us] = VALUE_DRAW - Value(contempt);
244 DrawValue[~us] = VALUE_DRAW + Value(contempt);
246 if (rootMoves.empty())
248 rootMoves.emplace_back(MOVE_NONE);
249 sync_cout << "info depth 0 score "
250 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
255 for (Thread* th : Threads)
257 th->start_searching();
259 Thread::search(); // Let's start searching!
262 // When we reach the maximum depth, we can arrive here without a raise of
263 // Threads.stop. However, if we are pondering or in an infinite search,
264 // the UCI protocol states that we shouldn't print the best move before the
265 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
266 // until the GUI sends one of those commands (which also raises Threads.stop).
267 Threads.stopOnPonderhit = true;
269 while (!Threads.stop && (Threads.ponder || Limits.infinite))
270 {} // Busy wait for a stop or a ponder reset
272 // Stop the threads if not already stopped (also raise the stop if
273 // "ponderhit" just reset Threads.ponder).
276 // Wait until all threads have finished
277 for (Thread* th : Threads)
279 th->wait_for_search_finished();
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 // Check if there are threads with a better score than main thread
287 Thread* bestThread = this;
288 if ( !this->easyMovePlayed
289 && Options["MultiPV"] == 1
291 && !Skill(Options["Skill Level"]).enabled()
292 && rootMoves[0].pv[0] != MOVE_NONE)
294 for (Thread* th : Threads)
296 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
297 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
299 // Select the thread with the best score, always if it is a mate
301 && (depthDiff >= 0 || th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY))
306 previousScore = bestThread->rootMoves[0].score;
308 // Send new PV when needed
309 if (bestThread != this)
310 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
312 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
314 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
315 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
317 std::cout << sync_endl;
321 /// Thread::search() is the main iterative deepening loop. It calls search()
322 /// repeatedly with increasing depth until the allocated thinking time has been
323 /// consumed, the user stops the search, or the maximum search depth is reached.
325 void Thread::search() {
327 Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
328 Value bestValue, alpha, beta, delta;
329 Move easyMove = MOVE_NONE;
330 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
332 std::memset(ss-4, 0, 7 * sizeof(Stack));
333 for (int i = 4; i > 0; i--)
334 (ss-i)->contHistory = &this->contHistory[NO_PIECE][0]; // Use as sentinel
336 bestValue = delta = alpha = -VALUE_INFINITE;
337 beta = VALUE_INFINITE;
341 easyMove = EasyMove.get(rootPos.key());
343 mainThread->easyMovePlayed = mainThread->failedLow = false;
344 mainThread->bestMoveChanges = 0;
347 size_t multiPV = Options["MultiPV"];
348 Skill skill(Options["Skill Level"]);
350 // When playing with strength handicap enable MultiPV search that we will
351 // use behind the scenes to retrieve a set of possible moves.
353 multiPV = std::max(multiPV, (size_t)4);
355 multiPV = std::min(multiPV, rootMoves.size());
357 // Iterative deepening loop until requested to stop or the target depth is reached
358 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
360 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
362 // Distribute search depths across the threads
365 int i = (idx - 1) % 20;
366 if (((rootDepth / ONE_PLY + rootPos.game_ply() + skipPhase[i]) / skipSize[i]) % 2)
370 // Age out PV variability metric
372 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
374 // Save the last iteration's scores before first PV line is searched and
375 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
376 for (RootMove& rm : rootMoves)
377 rm.previousScore = rm.score;
379 // MultiPV loop. We perform a full root search for each PV line
380 for (PVIdx = 0; PVIdx < multiPV && !Threads.stop; ++PVIdx)
382 // Reset UCI info selDepth for each depth and each PV line
385 // Reset aspiration window starting size
386 if (rootDepth >= 5 * ONE_PLY)
389 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
390 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
393 // Start with a small aspiration window and, in the case of a fail
394 // high/low, re-search with a bigger window until we're not failing
398 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
400 // Bring the best move to the front. It is critical that sorting
401 // is done with a stable algorithm because all the values but the
402 // first and eventually the new best one are set to -VALUE_INFINITE
403 // and we want to keep the same order for all the moves except the
404 // new PV that goes to the front. Note that in case of MultiPV
405 // search the already searched PV lines are preserved.
406 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
408 // If search has been stopped, we break immediately. Sorting and
409 // writing PV back to TT is safe because RootMoves is still
410 // valid, although it refers to the previous iteration.
414 // When failing high/low give some update (without cluttering
415 // the UI) before a re-search.
418 && (bestValue <= alpha || bestValue >= beta)
419 && Time.elapsed() > 3000)
420 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
422 // In case of failing low/high increase aspiration window and
423 // re-search, otherwise exit the loop.
424 if (bestValue <= alpha)
426 beta = (alpha + beta) / 2;
427 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
431 mainThread->failedLow = true;
432 Threads.stopOnPonderhit = false;
435 else if (bestValue >= beta)
436 beta = std::min(bestValue + delta, VALUE_INFINITE);
440 delta += delta / 4 + 5;
442 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
445 // Sort the PV lines searched so far and update the GUI
446 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
449 && (Threads.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000))
450 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
454 completedDepth = rootDepth;
456 // Have we found a "mate in x"?
458 && bestValue >= VALUE_MATE_IN_MAX_PLY
459 && VALUE_MATE - bestValue <= 2 * Limits.mate)
465 // If skill level is enabled and time is up, pick a sub-optimal best move
466 if (skill.enabled() && skill.time_to_pick(rootDepth))
467 skill.pick_best(multiPV);
469 // Do we have time for the next iteration? Can we stop searching now?
470 if (Limits.use_time_management())
472 if (!Threads.stop && !Threads.stopOnPonderhit)
474 // Stop the search if only one legal move is available, or if all
475 // of the available time has been used, or if we matched an easyMove
476 // from the previous search and just did a fast verification.
477 const int F[] = { mainThread->failedLow,
478 bestValue - mainThread->previousScore };
480 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
481 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
483 bool doEasyMove = rootMoves[0].pv[0] == easyMove
484 && mainThread->bestMoveChanges < 0.03
485 && Time.elapsed() > Time.optimum() * 5 / 44;
487 if ( rootMoves.size() == 1
488 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628
489 || (mainThread->easyMovePlayed = doEasyMove, doEasyMove))
491 // If we are allowed to ponder do not stop the search now but
492 // keep pondering until the GUI sends "ponderhit" or "stop".
494 Threads.stopOnPonderhit = true;
500 if (rootMoves[0].pv.size() >= 3)
501 EasyMove.update(rootPos, rootMoves[0].pv);
510 // Clear any candidate easy move that wasn't stable for the last search
511 // iterations; the second condition prevents consecutive fast moves.
512 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
515 // If skill level is enabled, swap best PV line with the sub-optimal one
517 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
518 rootMoves.end(), skill.best_move(multiPV)));
524 // search<>() is the main search function for both PV and non-PV nodes
526 template <NodeType NT>
527 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
529 const bool PvNode = NT == PV;
530 const bool rootNode = PvNode && ss->ply == 0;
532 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
533 assert(PvNode || (alpha == beta - 1));
534 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
535 assert(!(PvNode && cutNode));
536 assert(depth / ONE_PLY * ONE_PLY == depth);
538 Move pv[MAX_PLY+1], quietsSearched[64];
542 Move ttMove, move, excludedMove, bestMove;
543 Depth extension, newDepth;
544 Value bestValue, value, ttValue, eval;
545 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
546 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
548 int moveCount, quietCount;
550 // Step 1. Initialize node
551 Thread* thisThread = pos.this_thread();
552 inCheck = pos.checkers();
553 moveCount = quietCount = ss->moveCount = 0;
555 bestValue = -VALUE_INFINITE;
557 // Check for the available remaining time
558 if (thisThread == Threads.main())
559 static_cast<MainThread*>(thisThread)->check_time();
561 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
562 if (PvNode && thisThread->selDepth < ss->ply + 1)
563 thisThread->selDepth = ss->ply + 1;
567 // Step 2. Check for aborted search and immediate draw
568 if (Threads.stop.load(std::memory_order_relaxed) || pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
569 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
570 : DrawValue[pos.side_to_move()];
572 // Step 3. Mate distance pruning. Even if we mate at the next move our score
573 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
574 // a shorter mate was found upward in the tree then there is no need to search
575 // because we will never beat the current alpha. Same logic but with reversed
576 // signs applies also in the opposite condition of being mated instead of giving
577 // mate. In this case return a fail-high score.
578 alpha = std::max(mated_in(ss->ply), alpha);
579 beta = std::min(mate_in(ss->ply+1), beta);
584 assert(0 <= ss->ply && ss->ply < MAX_PLY);
586 (ss+1)->ply = ss->ply + 1;
587 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
588 ss->contHistory = &thisThread->contHistory[NO_PIECE][0];
589 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
590 Square prevSq = to_sq((ss-1)->currentMove);
592 // Step 4. Transposition table lookup. We don't want the score of a partial
593 // search to overwrite a previous full search TT value, so we use a different
594 // position key in case of an excluded move.
595 excludedMove = ss->excludedMove;
596 posKey = pos.key() ^ Key(excludedMove);
597 tte = TT.probe(posKey, ttHit);
598 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
599 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
600 : ttHit ? tte->move() : MOVE_NONE;
602 // At non-PV nodes we check for an early TT cutoff
605 && tte->depth() >= depth
606 && ttValue != VALUE_NONE // Possible in case of TT access race
607 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
608 : (tte->bound() & BOUND_UPPER)))
610 // If ttMove is quiet, update move sorting heuristics on TT hit
615 if (!pos.capture_or_promotion(ttMove))
616 update_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
618 // Extra penalty for a quiet TT move in previous ply when it gets refuted
619 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
620 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
622 // Penalty for a quiet ttMove that fails low
623 else if (!pos.capture_or_promotion(ttMove))
625 int penalty = -stat_bonus(depth);
626 thisThread->mainHistory.update(pos.side_to_move(), ttMove, penalty);
627 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
633 // Step 4a. Tablebase probe
634 if (!rootNode && TB::Cardinality)
636 int piecesCount = pos.count<ALL_PIECES>();
638 if ( piecesCount <= TB::Cardinality
639 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
640 && pos.rule50_count() == 0
641 && !pos.can_castle(ANY_CASTLING))
644 TB::WDLScore v = Tablebases::probe_wdl(pos, &err);
646 if (err != TB::ProbeState::FAIL)
648 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
650 int drawScore = TB::UseRule50 ? 1 : 0;
652 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
653 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
654 : VALUE_DRAW + 2 * v * drawScore;
656 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
657 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
658 MOVE_NONE, VALUE_NONE, TT.generation());
665 // Step 5. Evaluate the position statically
668 ss->staticEval = eval = VALUE_NONE;
674 // Never assume anything on values stored in TT
675 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
676 eval = ss->staticEval = evaluate(pos);
678 // Can ttValue be used as a better position evaluation?
679 if ( ttValue != VALUE_NONE
680 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
685 eval = ss->staticEval =
686 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
687 : -(ss-1)->staticEval + 2 * Eval::Tempo;
689 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
690 ss->staticEval, TT.generation());
693 if (skipEarlyPruning)
696 // Step 6. Razoring (skipped when in check)
698 && depth < 4 * ONE_PLY
699 && eval + razor_margin[depth / ONE_PLY] <= alpha)
701 if (depth <= ONE_PLY)
702 return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
704 Value ralpha = alpha - razor_margin[depth / ONE_PLY];
705 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1);
710 // Step 7. Futility pruning: child node (skipped when in check)
712 && depth < 7 * ONE_PLY
713 && eval - futility_margin(depth) >= beta
714 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
715 && pos.non_pawn_material(pos.side_to_move()))
718 // Step 8. Null move search with verification search (is omitted in PV nodes)
721 && (ss->staticEval >= beta - 35 * (depth / ONE_PLY - 6) || depth >= 13 * ONE_PLY)
722 && pos.non_pawn_material(pos.side_to_move()))
725 assert(eval - beta >= 0);
727 // Null move dynamic reduction based on depth and value
728 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
730 ss->currentMove = MOVE_NULL;
731 ss->contHistory = &thisThread->contHistory[NO_PIECE][0];
733 pos.do_null_move(st);
734 Value nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1)
735 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
736 pos.undo_null_move();
738 if (nullValue >= beta)
740 // Do not return unproven mate scores
741 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
744 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
747 // Do verification search at high depths
748 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
749 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
756 // Step 9. ProbCut (skipped when in check)
757 // If we have a good enough capture and a reduced search returns a value
758 // much above beta, we can (almost) safely prune the previous move.
760 && depth >= 5 * ONE_PLY
761 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
763 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
765 assert(is_ok((ss-1)->currentMove));
767 MovePicker mp(pos, ttMove, rbeta - ss->staticEval);
769 while ((move = mp.next_move()) != MOVE_NONE)
772 ss->currentMove = move;
773 ss->contHistory = &thisThread->contHistory[pos.moved_piece(move)][to_sq(move)];
775 assert(depth >= 5 * ONE_PLY);
776 pos.do_move(move, st);
777 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode, false);
784 // Step 10. Internal iterative deepening (skipped when in check)
785 if ( depth >= 6 * ONE_PLY
787 && (PvNode || ss->staticEval + 256 >= beta))
789 Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
790 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
792 tte = TT.probe(posKey, ttHit);
793 ttMove = ttHit ? tte->move() : MOVE_NONE;
796 moves_loop: // When in check search starts from here
798 const PieceToHistory* contHist[] = { (ss-1)->contHistory, (ss-2)->contHistory, nullptr, (ss-4)->contHistory };
799 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
801 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, contHist, countermove, ss->killers);
802 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
803 improving = ss->staticEval >= (ss-2)->staticEval
804 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
805 ||(ss-2)->staticEval == VALUE_NONE;
807 singularExtensionNode = !rootNode
808 && depth >= 8 * ONE_PLY
809 && ttMove != MOVE_NONE
810 && ttValue != VALUE_NONE
811 && !excludedMove // Recursive singular search is not allowed
812 && (tte->bound() & BOUND_LOWER)
813 && tte->depth() >= depth - 3 * ONE_PLY;
816 pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
818 // Step 11. Loop through moves
819 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
820 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
824 if (move == excludedMove)
827 // At root obey the "searchmoves" option and skip moves not listed in Root
828 // Move List. As a consequence any illegal move is also skipped. In MultiPV
829 // mode we also skip PV moves which have been already searched.
830 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
831 thisThread->rootMoves.end(), move))
834 ss->moveCount = ++moveCount;
836 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
837 sync_cout << "info depth " << depth / ONE_PLY
838 << " currmove " << UCI::move(move, pos.is_chess960())
839 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
842 (ss+1)->pv = nullptr;
844 extension = DEPTH_ZERO;
845 captureOrPromotion = pos.capture_or_promotion(move);
846 movedPiece = pos.moved_piece(move);
848 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
849 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
850 : pos.gives_check(move);
852 moveCountPruning = depth < 16 * ONE_PLY
853 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
855 // Step 12. Singular and Gives Check Extensions
857 // Singular extension search. If all moves but one fail low on a search of
858 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
859 // is singular and should be extended. To verify this we do a reduced search
860 // on all the other moves but the ttMove and if the result is lower than
861 // ttValue minus a margin then we will extend the ttMove.
862 if ( singularExtensionNode
866 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
867 Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
868 ss->excludedMove = move;
869 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode, true);
870 ss->excludedMove = MOVE_NONE;
880 // Calculate new depth for this move
881 newDepth = depth - ONE_PLY + extension;
883 // Step 13. Pruning at shallow depth
885 && pos.non_pawn_material(pos.side_to_move())
886 && bestValue > VALUE_MATED_IN_MAX_PLY)
888 if ( !captureOrPromotion
890 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
892 // Move count based pruning
893 if (moveCountPruning)
899 // Reduced depth of the next LMR search
900 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
902 // Countermoves based pruning
904 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
905 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
908 // Futility pruning: parent node
911 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
914 // Prune moves with negative SEE
916 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
919 else if ( depth < 7 * ONE_PLY
921 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
925 // Speculative prefetch as early as possible
926 prefetch(TT.first_entry(pos.key_after(move)));
928 // Check for legality just before making the move
929 if (!rootNode && !pos.legal(move))
931 ss->moveCount = --moveCount;
935 if (move == ttMove && captureOrPromotion)
938 // Update the current move (this must be done after singular extension search)
939 ss->currentMove = move;
940 ss->contHistory = &thisThread->contHistory[movedPiece][to_sq(move)];
942 // Step 14. Make the move
943 pos.do_move(move, st, givesCheck);
945 // Step 15. Reduced depth search (LMR). If the move fails high it will be
946 // re-searched at full depth.
947 if ( depth >= 3 * ONE_PLY
949 && (!captureOrPromotion || moveCountPruning))
951 Depth r = reduction<PvNode>(improving, depth, moveCount);
953 if (captureOrPromotion)
954 r -= r ? ONE_PLY : DEPTH_ZERO;
957 // Decrease reduction if opponent's move count is high
958 if ((ss-1)->moveCount > 15)
961 // Decrease reduction for exact PV nodes
965 // Increase reduction if ttMove is a capture
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))))
980 ss->statScore = thisThread->mainHistory[~pos.side_to_move()][from_to(move)]
981 + (*contHist[0])[movedPiece][to_sq(move)]
982 + (*contHist[1])[movedPiece][to_sq(move)]
983 + (*contHist[3])[movedPiece][to_sq(move)]
986 // Decrease/increase reduction by comparing opponent's stat score
987 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
990 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
993 // Decrease/increase reduction for moves with a good/bad history
994 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->statScore / 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 (Threads.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)
1048 rm.selDepth = thisThread->selDepth;
1053 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1054 rm.pv.push_back(*m);
1056 // We record how often the best move has been changed in each
1057 // iteration. This information is used for time management: When
1058 // the best move changes frequently, we allocate some more time.
1059 if (moveCount > 1 && thisThread == Threads.main())
1060 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1063 // All other moves but the PV are set to the lowest value: this
1064 // is not a problem when sorting because the sort is stable and the
1065 // move position in the list is preserved - just the PV is pushed up.
1066 rm.score = -VALUE_INFINITE;
1069 if (value > bestValue)
1077 if (PvNode && !rootNode) // Update pv even in fail-high case
1078 update_pv(ss->pv, move, (ss+1)->pv);
1080 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1084 assert(value >= beta); // Fail high
1090 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1091 quietsSearched[quietCount++] = move;
1094 // The following condition would detect a stop only after move loop has been
1095 // completed. But in this case bestValue is valid because we have fully
1096 // searched our subtree, and we can anyhow save the result in TT.
1102 // Step 20. Check for mate and stalemate
1103 // All legal moves have been searched and if there are no legal moves, it
1104 // must be a mate or a stalemate. If we are in a singular extension search then
1105 // return a fail low score.
1107 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1110 bestValue = excludedMove ? alpha
1111 : 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_continuation_histories(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()
1125 && is_ok((ss-1)->currentMove))
1126 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1129 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1130 bestValue >= beta ? BOUND_LOWER :
1131 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1132 depth, bestMove, ss->staticEval, TT.generation());
1134 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1140 // qsearch() is the quiescence search function, which is called by the main
1141 // search function with depth zero, or recursively with depth less than ONE_PLY.
1143 template <NodeType NT, bool InCheck>
1144 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1146 const bool PvNode = NT == PV;
1148 assert(InCheck == !!pos.checkers());
1149 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1150 assert(PvNode || (alpha == beta - 1));
1151 assert(depth <= DEPTH_ZERO);
1152 assert(depth / ONE_PLY * ONE_PLY == depth);
1158 Move ttMove, move, bestMove;
1159 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1160 bool ttHit, givesCheck, evasionPrunable;
1166 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1168 ss->pv[0] = MOVE_NONE;
1171 ss->currentMove = bestMove = MOVE_NONE;
1172 (ss+1)->ply = ss->ply + 1;
1175 // Check for an instant draw or if the maximum ply has been reached
1176 if (pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
1177 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1178 : DrawValue[pos.side_to_move()];
1180 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1182 // Decide whether or not to include checks: this fixes also the type of
1183 // TT entry depth that we are going to use. Note that in qsearch we use
1184 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1185 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1186 : DEPTH_QS_NO_CHECKS;
1187 // Transposition table lookup
1189 tte = TT.probe(posKey, ttHit);
1190 ttMove = ttHit ? tte->move() : MOVE_NONE;
1191 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1195 && tte->depth() >= ttDepth
1196 && ttValue != VALUE_NONE // Only in case of TT access race
1197 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1198 : (tte->bound() & BOUND_UPPER)))
1201 // Evaluate the position statically
1204 ss->staticEval = VALUE_NONE;
1205 bestValue = futilityBase = -VALUE_INFINITE;
1211 // Never assume anything on values stored in TT
1212 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1213 ss->staticEval = bestValue = evaluate(pos);
1215 // Can ttValue be used as a better position evaluation?
1216 if ( ttValue != VALUE_NONE
1217 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1218 bestValue = ttValue;
1221 ss->staticEval = bestValue =
1222 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1223 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1225 // Stand pat. Return immediately if static value is at least beta
1226 if (bestValue >= beta)
1229 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1230 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1235 if (PvNode && bestValue > alpha)
1238 futilityBase = bestValue + 128;
1241 // Initialize a MovePicker object for the current position, and prepare
1242 // to search the moves. Because the depth is <= 0 here, only captures,
1243 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1245 MovePicker mp(pos, ttMove, depth, &pos.this_thread()->mainHistory, to_sq((ss-1)->currentMove));
1247 // Loop through the moves until no moves remain or a beta cutoff occurs
1248 while ((move = mp.next_move()) != MOVE_NONE)
1250 assert(is_ok(move));
1252 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
1253 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
1254 : pos.gives_check(move);
1261 && futilityBase > -VALUE_KNOWN_WIN
1262 && !pos.advanced_pawn_push(move))
1264 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1266 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1268 if (futilityValue <= alpha)
1270 bestValue = std::max(bestValue, futilityValue);
1274 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1276 bestValue = std::max(bestValue, futilityBase);
1281 // Detect non-capture evasions that are candidates to be pruned
1282 evasionPrunable = InCheck
1283 && (depth != DEPTH_ZERO || moveCount > 2)
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))
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))
1303 ss->currentMove = move;
1305 // Make and search the move
1306 pos.do_move(move, st, givesCheck);
1307 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1308 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1309 pos.undo_move(move);
1311 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1313 // Check for a new best move
1314 if (value > bestValue)
1320 if (PvNode) // Update pv even in fail-high case
1321 update_pv(ss->pv, move, (ss+1)->pv);
1323 if (PvNode && value < beta) // Update alpha here!
1330 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1331 ttDepth, move, ss->staticEval, TT.generation());
1339 // All legal moves have been searched. A special case: If we're in check
1340 // and no legal moves were found, it is checkmate.
1341 if (InCheck && bestValue == -VALUE_INFINITE)
1342 return mated_in(ss->ply); // Plies to mate from the root
1344 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1345 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1346 ttDepth, bestMove, ss->staticEval, TT.generation());
1348 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1354 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1355 // "plies to mate from the current position". Non-mate scores are unchanged.
1356 // The function is called before storing a value in the transposition table.
1358 Value value_to_tt(Value v, int ply) {
1360 assert(v != VALUE_NONE);
1362 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1363 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1367 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1368 // from the transposition table (which refers to the plies to mate/be mated
1369 // from current position) to "plies to mate/be mated from the root".
1371 Value value_from_tt(Value v, int ply) {
1373 return v == VALUE_NONE ? VALUE_NONE
1374 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1375 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1379 // update_pv() adds current move and appends child pv[]
1381 void update_pv(Move* pv, Move move, Move* childPv) {
1383 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1389 // update_continuation_histories() updates histories of the move pairs formed
1390 // by moves at ply -1, -2, and -4 with current move.
1392 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1394 for (int i : {1, 2, 4})
1395 if (is_ok((ss-i)->currentMove))
1396 (ss-i)->contHistory->update(pc, to, bonus);
1400 // update_stats() updates move sorting heuristics when a new quiet best move is found
1402 void update_stats(const Position& pos, Stack* ss, Move move,
1403 Move* quiets, int quietsCnt, int bonus) {
1405 if (ss->killers[0] != move)
1407 ss->killers[1] = ss->killers[0];
1408 ss->killers[0] = move;
1411 Color c = pos.side_to_move();
1412 Thread* thisThread = pos.this_thread();
1413 thisThread->mainHistory.update(c, move, bonus);
1414 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1416 if (is_ok((ss-1)->currentMove))
1418 Square prevSq = to_sq((ss-1)->currentMove);
1419 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1422 // Decrease all the other played quiet moves
1423 for (int i = 0; i < quietsCnt; ++i)
1425 thisThread->mainHistory.update(c, quiets[i], -bonus);
1426 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1431 // When playing with strength handicap, choose best move among a set of RootMoves
1432 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1434 Move Skill::pick_best(size_t multiPV) {
1436 const RootMoves& rootMoves = Threads.main()->rootMoves;
1437 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1439 // RootMoves are already sorted by score in descending order
1440 Value topScore = rootMoves[0].score;
1441 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1442 int weakness = 120 - 2 * level;
1443 int maxScore = -VALUE_INFINITE;
1445 // Choose best move. For each move score we add two terms, both dependent on
1446 // weakness. One is deterministic and bigger for weaker levels, and one is
1447 // random. Then we choose the move with the resulting highest score.
1448 for (size_t i = 0; i < multiPV; ++i)
1450 // This is our magic formula
1451 int push = ( weakness * int(topScore - rootMoves[i].score)
1452 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1454 if (rootMoves[i].score + push > maxScore)
1456 maxScore = rootMoves[i].score + push;
1457 best = rootMoves[i].pv[0];
1466 // check_time() is used to print debug info and, more importantly, to detect
1467 // when we are out of available time and thus stop the search.
1469 void MainThread::check_time() {
1474 // At low node count increase the checking rate to about 0.1% of nodes
1475 // otherwise use a default value.
1476 callsCnt = Limits.nodes ? std::min(4096, int(Limits.nodes / 1024)) : 4096;
1478 static TimePoint lastInfoTime = now();
1480 int elapsed = Time.elapsed();
1481 TimePoint tick = Limits.startTime + elapsed;
1483 if (tick - lastInfoTime >= 1000)
1485 lastInfoTime = tick;
1489 // An engine may not stop pondering until told so by the GUI
1493 if ( (Limits.use_time_management() && elapsed > Time.maximum())
1494 || (Limits.movetime && elapsed >= Limits.movetime)
1495 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1496 Threads.stop = true;
1500 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1501 /// that all (if any) unsearched PV lines are sent using a previous search score.
1503 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1505 std::stringstream ss;
1506 int elapsed = Time.elapsed() + 1;
1507 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1508 size_t PVIdx = pos.this_thread()->PVIdx;
1509 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1510 uint64_t nodesSearched = Threads.nodes_searched();
1511 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1513 for (size_t i = 0; i < multiPV; ++i)
1515 bool updated = (i <= PVIdx && rootMoves[i].score != -VALUE_INFINITE);
1517 if (depth == ONE_PLY && !updated)
1520 Depth d = updated ? depth : depth - ONE_PLY;
1521 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1523 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1524 v = tb ? TB::Score : v;
1526 if (ss.rdbuf()->in_avail()) // Not at first line
1530 << " depth " << d / ONE_PLY
1531 << " seldepth " << rootMoves[i].selDepth
1532 << " multipv " << i + 1
1533 << " score " << UCI::value(v);
1535 if (!tb && i == PVIdx)
1536 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1538 ss << " nodes " << nodesSearched
1539 << " nps " << nodesSearched * 1000 / elapsed;
1541 if (elapsed > 1000) // Earlier makes little sense
1542 ss << " hashfull " << TT.hashfull();
1544 ss << " tbhits " << tbHits
1545 << " time " << elapsed
1548 for (Move m : rootMoves[i].pv)
1549 ss << " " << UCI::move(m, pos.is_chess960());
1556 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1557 /// before exiting the search, for instance, in case we stop the search during a
1558 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1559 /// otherwise in case of 'ponder on' we have nothing to think on.
1561 bool RootMove::extract_ponder_from_tt(Position& pos) {
1566 assert(pv.size() == 1);
1571 pos.do_move(pv[0], st);
1572 TTEntry* tte = TT.probe(pos.key(), ttHit);
1576 Move m = tte->move(); // Local copy to be SMP safe
1577 if (MoveList<LEGAL>(pos).contains(m))
1581 pos.undo_move(pv[0]);
1582 return pv.size() > 1;
1585 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1588 UseRule50 = Options["Syzygy50MoveRule"];
1589 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1590 Cardinality = Options["SyzygyProbeLimit"];
1592 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1593 if (Cardinality > MaxCardinality)
1595 Cardinality = MaxCardinality;
1596 ProbeDepth = DEPTH_ZERO;
1599 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1602 // If the current root position is in the tablebases, then RootMoves
1603 // contains only moves that preserve the draw or the win.
1604 RootInTB = root_probe(pos, rootMoves, TB::Score);
1607 Cardinality = 0; // Do not probe tablebases during the search
1609 else // If DTZ tables are missing, use WDL tables as a fallback
1611 // Filter out moves that do not preserve the draw or the win.
1612 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1614 // Only probe during search if winning
1615 if (RootInTB && TB::Score <= VALUE_DRAW)
1619 if (RootInTB && !UseRule50)
1620 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1621 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1