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 // Threshold used for countermoves based pruning
79 const int CounterMovePruneThreshold = 0;
81 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
82 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
85 // History and stats update bonus, based on depth
86 int stat_bonus(Depth depth) {
87 int d = depth / ONE_PLY;
88 return d > 17 ? 0 : d * d + 2 * d - 2;
91 // Skill structure is used to implement strength limit
93 Skill(int l) : level(l) {}
94 bool enabled() const { return level < 20; }
95 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
96 Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
97 Move pick_best(size_t multiPV);
100 Move best = MOVE_NONE;
103 // EasyMoveManager structure is used to detect an 'easy move'. When the PV is stable
104 // across multiple search iterations, we can quickly return the best move.
105 struct EasyMoveManager {
110 pv[0] = pv[1] = pv[2] = MOVE_NONE;
113 Move get(Key key) const {
114 return expectedPosKey == key ? pv[2] : MOVE_NONE;
117 void update(Position& pos, const std::vector<Move>& newPv) {
119 assert(newPv.size() >= 3);
121 // Keep track of how many times in a row the 3rd ply remains stable
122 stableCnt = (newPv[2] == pv[2]) ? stableCnt + 1 : 0;
124 if (!std::equal(newPv.begin(), newPv.begin() + 3, pv))
126 std::copy(newPv.begin(), newPv.begin() + 3, pv);
129 pos.do_move(newPv[0], st[0]);
130 pos.do_move(newPv[1], st[1]);
131 expectedPosKey = pos.key();
132 pos.undo_move(newPv[1]);
133 pos.undo_move(newPv[0]);
142 EasyMoveManager EasyMove;
143 Value DrawValue[COLOR_NB];
145 template <NodeType NT>
146 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning);
148 template <NodeType NT, bool InCheck>
149 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
151 Value value_to_tt(Value v, int ply);
152 Value value_from_tt(Value v, int ply);
153 void update_pv(Move* pv, Move move, Move* childPv);
154 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
155 void update_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
160 /// Search::init() is called during startup to initialize various lookup tables
162 void Search::init() {
164 for (int imp = 0; imp <= 1; ++imp)
165 for (int d = 1; d < 64; ++d)
166 for (int mc = 1; mc < 64; ++mc)
168 double r = log(d) * log(mc) / 1.95;
170 Reductions[NonPV][imp][d][mc] = int(std::round(r));
171 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
173 // Increase reduction for non-PV nodes when eval is not improving
174 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
175 Reductions[NonPV][imp][d][mc]++;
178 for (int d = 0; d < 16; ++d)
180 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
181 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
186 /// Search::clear() resets search state to its initial value, to obtain reproducible results
188 void Search::clear() {
192 for (Thread* th : Threads)
194 th->counterMoves.fill(MOVE_NONE);
195 th->mainHistory.fill(0);
197 for (auto& to : th->contHistory)
201 th->contHistory[NO_PIECE][0].fill(CounterMovePruneThreshold - 1);
204 Threads.main()->callsCnt = 0;
205 Threads.main()->previousScore = VALUE_INFINITE;
209 /// Search::perft() is our utility to verify move generation. All the leaf nodes
210 /// up to the given depth are generated and counted, and the sum is returned.
212 uint64_t Search::perft(Position& pos, Depth depth) {
215 uint64_t cnt, nodes = 0;
216 const bool leaf = (depth == 2 * ONE_PLY);
218 for (const auto& m : MoveList<LEGAL>(pos))
220 if (Root && depth <= ONE_PLY)
225 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
230 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
235 template uint64_t Search::perft<true>(Position&, Depth);
238 /// MainThread::search() is called by the main thread when the program receives
239 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
241 void MainThread::search() {
243 Color us = rootPos.side_to_move();
244 Time.init(Limits, us, rootPos.game_ply());
247 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
248 DrawValue[ us] = VALUE_DRAW - Value(contempt);
249 DrawValue[~us] = VALUE_DRAW + Value(contempt);
251 if (rootMoves.empty())
253 rootMoves.push_back(RootMove(MOVE_NONE));
254 sync_cout << "info depth 0 score "
255 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
260 for (Thread* th : Threads)
262 th->start_searching();
264 Thread::search(); // Let's start searching!
267 // When playing in 'nodes as time' mode, subtract the searched nodes from
268 // the available ones before exiting.
270 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
272 // When we reach the maximum depth, we can arrive here without a raise of
273 // Threads.stop. However, if we are pondering or in an infinite search,
274 // the UCI protocol states that we shouldn't print the best move before the
275 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
276 // until the GUI sends one of those commands (which also raises Threads.stop).
277 if (!Threads.stop && (Limits.ponder || Limits.infinite))
279 Threads.stopOnPonderhit = true;
283 // Stop the threads if not already stopped
286 // Wait until all threads have finished
287 for (Thread* th : Threads)
289 th->wait_for_search_finished();
291 // Check if there are threads with a better score than main thread
292 Thread* bestThread = this;
293 if ( !this->easyMovePlayed
294 && Options["MultiPV"] == 1
296 && !Skill(Options["Skill Level"]).enabled()
297 && rootMoves[0].pv[0] != MOVE_NONE)
299 for (Thread* th : Threads)
301 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
302 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
304 if (scoreDiff > 0 && depthDiff >= 0)
309 previousScore = bestThread->rootMoves[0].score;
311 // Send new PV when needed
312 if (bestThread != this)
313 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
315 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
317 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
318 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
320 std::cout << sync_endl;
324 /// Thread::search() is the main iterative deepening loop. It calls search()
325 /// repeatedly with increasing depth until the allocated thinking time has been
326 /// consumed, the user stops the search, or the maximum search depth is reached.
328 void Thread::search() {
330 Stack stack[MAX_PLY+7], *ss = stack+4; // To allow referencing (ss-4) and (ss+2)
331 Value bestValue, alpha, beta, delta;
332 Move easyMove = MOVE_NONE;
333 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
335 std::memset(ss-4, 0, 7 * sizeof(Stack));
336 for (int i = 4; i > 0; i--)
337 (ss-i)->contHistory = &this->contHistory[NO_PIECE][0]; // Use as sentinel
339 bestValue = delta = alpha = -VALUE_INFINITE;
340 beta = VALUE_INFINITE;
341 completedDepth = DEPTH_ZERO;
345 easyMove = EasyMove.get(rootPos.key());
347 mainThread->easyMovePlayed = mainThread->failedLow = false;
348 mainThread->bestMoveChanges = 0;
351 size_t multiPV = Options["MultiPV"];
352 Skill skill(Options["Skill Level"]);
354 // When playing with strength handicap enable MultiPV search that we will
355 // use behind the scenes to retrieve a set of possible moves.
357 multiPV = std::max(multiPV, (size_t)4);
359 multiPV = std::min(multiPV, rootMoves.size());
361 // Iterative deepening loop until requested to stop or the target depth is reached
362 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
364 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
366 // Distribute search depths across the threads
369 int i = (idx - 1) % 20;
370 if (((rootDepth / ONE_PLY + rootPos.game_ply() + skipPhase[i]) / skipSize[i]) % 2)
374 // Age out PV variability metric
376 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
378 // Save the last iteration's scores before first PV line is searched and
379 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
380 for (RootMove& rm : rootMoves)
381 rm.previousScore = rm.score;
383 // MultiPV loop. We perform a full root search for each PV line
384 for (PVIdx = 0; PVIdx < multiPV && !Threads.stop; ++PVIdx)
386 // Reset UCI info selDepth for each depth and each PV line
389 // Reset aspiration window starting size
390 if (rootDepth >= 5 * ONE_PLY)
393 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
394 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
397 // Start with a small aspiration window and, in the case of a fail
398 // high/low, re-search with a bigger window until we're not failing
402 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
404 // Bring the best move to the front. It is critical that sorting
405 // is done with a stable algorithm because all the values but the
406 // first and eventually the new best one are set to -VALUE_INFINITE
407 // and we want to keep the same order for all the moves except the
408 // new PV that goes to the front. Note that in case of MultiPV
409 // search the already searched PV lines are preserved.
410 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
412 // If search has been stopped, we break immediately. Sorting and
413 // writing PV back to TT is safe because RootMoves is still
414 // valid, although it refers to the previous iteration.
418 // When failing high/low give some update (without cluttering
419 // the UI) before a re-search.
422 && (bestValue <= alpha || bestValue >= beta)
423 && Time.elapsed() > 3000)
424 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
426 // In case of failing low/high increase aspiration window and
427 // re-search, otherwise exit the loop.
428 if (bestValue <= alpha)
430 beta = (alpha + beta) / 2;
431 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
435 mainThread->failedLow = true;
436 Threads.stopOnPonderhit = false;
439 else if (bestValue >= beta)
440 beta = std::min(bestValue + delta, VALUE_INFINITE);
444 delta += delta / 4 + 5;
446 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
449 // Sort the PV lines searched so far and update the GUI
450 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
455 if (Threads.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000)
456 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
460 completedDepth = rootDepth;
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 // Have we found a "mate in x"?
471 && bestValue >= VALUE_MATE_IN_MAX_PLY
472 && VALUE_MATE - bestValue <= 2 * Limits.mate)
475 // Do we have time for the next iteration? Can we stop searching now?
476 if (Limits.use_time_management())
478 if (!Threads.stop && !Threads.stopOnPonderhit)
480 // Stop the search if only one legal move is available, or if all
481 // of the available time has been used, or if we matched an easyMove
482 // from the previous search and just did a fast verification.
483 const int F[] = { mainThread->failedLow,
484 bestValue - mainThread->previousScore };
486 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
487 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
489 bool doEasyMove = rootMoves[0].pv[0] == easyMove
490 && mainThread->bestMoveChanges < 0.03
491 && Time.elapsed() > Time.optimum() * 5 / 44;
493 if ( rootMoves.size() == 1
494 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628
495 || (mainThread->easyMovePlayed = doEasyMove, doEasyMove))
497 // If we are allowed to ponder do not stop the search now but
498 // keep pondering until the GUI sends "ponderhit" or "stop".
500 Threads.stopOnPonderhit = true;
506 if (rootMoves[0].pv.size() >= 3)
507 EasyMove.update(rootPos, rootMoves[0].pv);
516 // Clear any candidate easy move that wasn't stable for the last search
517 // iterations; the second condition prevents consecutive fast moves.
518 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
521 // If skill level is enabled, swap best PV line with the sub-optimal one
523 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
524 rootMoves.end(), skill.best_move(multiPV)));
530 // search<>() is the main search function for both PV and non-PV nodes
532 template <NodeType NT>
533 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
535 const bool PvNode = NT == PV;
536 const bool rootNode = PvNode && (ss-1)->ply == 0;
538 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
539 assert(PvNode || (alpha == beta - 1));
540 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
541 assert(!(PvNode && cutNode));
542 assert(depth / ONE_PLY * ONE_PLY == depth);
544 Move pv[MAX_PLY+1], quietsSearched[64];
548 Move ttMove, move, excludedMove, bestMove;
549 Depth extension, newDepth;
550 Value bestValue, value, ttValue, eval;
551 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
552 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture;
554 int moveCount, quietCount;
556 // Step 1. Initialize node
557 Thread* thisThread = pos.this_thread();
558 inCheck = pos.checkers();
559 moveCount = quietCount = ss->moveCount = 0;
561 bestValue = -VALUE_INFINITE;
562 ss->ply = (ss-1)->ply + 1;
564 // Check for the available remaining time
565 if (thisThread == Threads.main())
566 static_cast<MainThread*>(thisThread)->check_time();
568 // Used to send selDepth info to GUI
569 if (PvNode && thisThread->selDepth < ss->ply)
570 thisThread->selDepth = ss->ply;
574 // Step 2. Check for aborted search and immediate draw
575 if (Threads.stop.load(std::memory_order_relaxed) || pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
576 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
577 : DrawValue[pos.side_to_move()];
579 // Step 3. Mate distance pruning. Even if we mate at the next move our score
580 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
581 // a shorter mate was found upward in the tree then there is no need to search
582 // because we will never beat the current alpha. Same logic but with reversed
583 // signs applies also in the opposite condition of being mated instead of giving
584 // mate. In this case return a fail-high score.
585 alpha = std::max(mated_in(ss->ply), alpha);
586 beta = std::min(mate_in(ss->ply+1), beta);
591 assert(0 <= ss->ply && ss->ply < MAX_PLY);
593 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
594 ss->contHistory = &thisThread->contHistory[NO_PIECE][0];
595 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
596 Square prevSq = to_sq((ss-1)->currentMove);
598 // Step 4. Transposition table lookup. We don't want the score of a partial
599 // search to overwrite a previous full search TT value, so we use a different
600 // position key in case of an excluded move.
601 excludedMove = ss->excludedMove;
602 posKey = pos.key() ^ Key(excludedMove);
603 tte = TT.probe(posKey, ttHit);
604 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
605 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
606 : ttHit ? tte->move() : MOVE_NONE;
608 // At non-PV nodes we check for an early TT cutoff
611 && tte->depth() >= depth
612 && ttValue != VALUE_NONE // Possible in case of TT access race
613 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
614 : (tte->bound() & BOUND_UPPER)))
616 // If ttMove is quiet, update move sorting heuristics on TT hit
621 if (!pos.capture_or_promotion(ttMove))
622 update_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
624 // Extra penalty for a quiet TT move in previous ply when it gets refuted
625 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
626 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
628 // Penalty for a quiet ttMove that fails low
629 else if (!pos.capture_or_promotion(ttMove))
631 int penalty = -stat_bonus(depth);
632 thisThread->mainHistory.update(pos.side_to_move(), ttMove, penalty);
633 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
639 // Step 4a. Tablebase probe
640 if (!rootNode && TB::Cardinality)
642 int piecesCount = pos.count<ALL_PIECES>();
644 if ( piecesCount <= TB::Cardinality
645 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
646 && pos.rule50_count() == 0
647 && !pos.can_castle(ANY_CASTLING))
650 TB::WDLScore v = Tablebases::probe_wdl(pos, &err);
652 if (err != TB::ProbeState::FAIL)
654 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
656 int drawScore = TB::UseRule50 ? 1 : 0;
658 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
659 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
660 : VALUE_DRAW + 2 * v * drawScore;
662 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
663 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
664 MOVE_NONE, VALUE_NONE, TT.generation());
671 // Step 5. Evaluate the position statically
674 ss->staticEval = eval = VALUE_NONE;
680 // Never assume anything on values stored in TT
681 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
682 eval = ss->staticEval = evaluate(pos);
684 // Can ttValue be used as a better position evaluation?
685 if ( ttValue != VALUE_NONE
686 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
691 eval = ss->staticEval =
692 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
693 : -(ss-1)->staticEval + 2 * Eval::Tempo;
695 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
696 ss->staticEval, TT.generation());
699 if (skipEarlyPruning)
702 // Step 6. Razoring (skipped when in check)
704 && depth < 4 * ONE_PLY
705 && eval + razor_margin[depth / ONE_PLY] <= alpha)
707 if (depth <= ONE_PLY)
708 return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
710 Value ralpha = alpha - razor_margin[depth / ONE_PLY];
711 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1);
716 // Step 7. Futility pruning: child node (skipped when in check)
718 && depth < 7 * ONE_PLY
719 && eval - futility_margin(depth) >= beta
720 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
721 && pos.non_pawn_material(pos.side_to_move()))
724 // Step 8. Null move search with verification search (is omitted in PV nodes)
727 && (ss->staticEval >= beta - 35 * (depth / ONE_PLY - 6) || depth >= 13 * ONE_PLY)
728 && pos.non_pawn_material(pos.side_to_move()))
731 assert(eval - beta >= 0);
733 // Null move dynamic reduction based on depth and value
734 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
736 ss->currentMove = MOVE_NULL;
737 ss->contHistory = &thisThread->contHistory[NO_PIECE][0];
739 pos.do_null_move(st);
740 Value nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1)
741 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
742 pos.undo_null_move();
744 if (nullValue >= beta)
746 // Do not return unproven mate scores
747 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
750 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
753 // Do verification search at high depths
754 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
755 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
762 // Step 9. ProbCut (skipped when in check)
763 // If we have a good enough capture and a reduced search returns a value
764 // much above beta, we can (almost) safely prune the previous move.
766 && depth >= 5 * ONE_PLY
767 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
769 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
771 assert(is_ok((ss-1)->currentMove));
773 MovePicker mp(pos, ttMove, rbeta - ss->staticEval);
775 while ((move = mp.next_move()) != MOVE_NONE)
778 ss->currentMove = move;
779 ss->contHistory = &thisThread->contHistory[pos.moved_piece(move)][to_sq(move)];
781 assert(depth >= 5 * ONE_PLY);
782 pos.do_move(move, st);
783 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode, false);
790 // Step 10. Internal iterative deepening (skipped when in check)
791 if ( depth >= 6 * ONE_PLY
793 && (PvNode || ss->staticEval + 256 >= beta))
795 Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
796 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
798 tte = TT.probe(posKey, ttHit);
799 ttMove = ttHit ? tte->move() : MOVE_NONE;
802 moves_loop: // When in check search starts from here
804 const PieceToHistory* contHist[] = { (ss-1)->contHistory, (ss-2)->contHistory, nullptr, (ss-4)->contHistory };
805 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
807 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, contHist, countermove, ss->killers);
808 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
809 improving = ss->staticEval >= (ss-2)->staticEval
810 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
811 ||(ss-2)->staticEval == VALUE_NONE;
813 singularExtensionNode = !rootNode
814 && depth >= 8 * ONE_PLY
815 && ttMove != MOVE_NONE
816 && ttValue != VALUE_NONE
817 && !excludedMove // Recursive singular search is not allowed
818 && (tte->bound() & BOUND_LOWER)
819 && tte->depth() >= depth - 3 * ONE_PLY;
823 // Step 11. Loop through moves
824 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
825 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
829 if (move == excludedMove)
832 // At root obey the "searchmoves" option and skip moves not listed in Root
833 // Move List. As a consequence any illegal move is also skipped. In MultiPV
834 // mode we also skip PV moves which have been already searched.
835 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
836 thisThread->rootMoves.end(), move))
839 ss->moveCount = ++moveCount;
841 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
842 sync_cout << "info depth " << depth / ONE_PLY
843 << " currmove " << UCI::move(move, pos.is_chess960())
844 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
847 (ss+1)->pv = nullptr;
849 extension = DEPTH_ZERO;
850 captureOrPromotion = pos.capture_or_promotion(move);
851 movedPiece = pos.moved_piece(move);
853 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
854 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
855 : pos.gives_check(move);
857 moveCountPruning = depth < 16 * ONE_PLY
858 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
860 // Step 12. Singular and Gives Check Extensions
862 // Singular extension search. If all moves but one fail low on a search of
863 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
864 // is singular and should be extended. To verify this we do a reduced search
865 // on all the other moves but the ttMove and if the result is lower than
866 // ttValue minus a margin then we will extend the ttMove.
867 if ( singularExtensionNode
871 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
872 Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
873 ss->excludedMove = move;
874 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode, true);
875 ss->excludedMove = MOVE_NONE;
885 // Calculate new depth for this move
886 newDepth = depth - ONE_PLY + extension;
888 // Step 13. Pruning at shallow depth
890 && pos.non_pawn_material(pos.side_to_move())
891 && bestValue > VALUE_MATED_IN_MAX_PLY)
893 if ( !captureOrPromotion
895 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
897 // Move count based pruning
898 if (moveCountPruning)
904 // Reduced depth of the next LMR search
905 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
907 // Countermoves based pruning
909 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
910 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
913 // Futility pruning: parent node
916 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
919 // Prune moves with negative SEE
921 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
924 else if ( depth < 7 * ONE_PLY
926 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
930 // Speculative prefetch as early as possible
931 prefetch(TT.first_entry(pos.key_after(move)));
933 // Check for legality just before making the move
934 if (!rootNode && !pos.legal(move))
936 ss->moveCount = --moveCount;
940 if (move == ttMove && captureOrPromotion)
943 // Update the current move (this must be done after singular extension search)
944 ss->currentMove = move;
945 ss->contHistory = &thisThread->contHistory[movedPiece][to_sq(move)];
947 // Step 14. Make the move
948 pos.do_move(move, st, givesCheck);
950 // Step 15. Reduced depth search (LMR). If the move fails high it will be
951 // re-searched at full depth.
952 if ( depth >= 3 * ONE_PLY
954 && (!captureOrPromotion || moveCountPruning))
956 Depth r = reduction<PvNode>(improving, depth, moveCount);
958 if (captureOrPromotion)
959 r -= r ? ONE_PLY : DEPTH_ZERO;
962 // Increase reduction if ttMove is a capture
966 // Increase reduction for cut nodes
970 // Decrease reduction for moves that escape a capture. Filter out
971 // castling moves, because they are coded as "king captures rook" and
972 // hence break make_move().
973 else if ( type_of(move) == NORMAL
974 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
977 ss->statScore = thisThread->mainHistory[~pos.side_to_move()][from_to(move)]
978 + (*contHist[0])[movedPiece][to_sq(move)]
979 + (*contHist[1])[movedPiece][to_sq(move)]
980 + (*contHist[3])[movedPiece][to_sq(move)]
983 // Decrease/increase reduction by comparing opponent's stat score
984 if (ss->statScore > 0 && (ss-1)->statScore < 0)
987 else if (ss->statScore < 0 && (ss-1)->statScore > 0)
990 // Decrease/increase reduction for moves with a good/bad history
991 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->statScore / 20000) * ONE_PLY);
994 Depth d = std::max(newDepth - r, ONE_PLY);
996 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
998 doFullDepthSearch = (value > alpha && d != newDepth);
1001 doFullDepthSearch = !PvNode || moveCount > 1;
1003 // Step 16. Full depth search when LMR is skipped or fails high
1004 if (doFullDepthSearch)
1005 value = newDepth < ONE_PLY ?
1006 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha)
1007 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha)
1008 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
1010 // For PV nodes only, do a full PV search on the first move or after a fail
1011 // high (in the latter case search only if value < beta), otherwise let the
1012 // parent node fail low with value <= alpha and try another move.
1013 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1016 (ss+1)->pv[0] = MOVE_NONE;
1018 value = newDepth < ONE_PLY ?
1019 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha)
1020 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha)
1021 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1024 // Step 17. Undo move
1025 pos.undo_move(move);
1027 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1029 // Step 18. Check for a new best move
1030 // Finished searching the move. If a stop occurred, the return value of
1031 // the search cannot be trusted, and we return immediately without
1032 // updating best move, PV and TT.
1033 if (Threads.stop.load(std::memory_order_relaxed))
1038 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1039 thisThread->rootMoves.end(), move);
1041 // PV move or new best move ?
1042 if (moveCount == 1 || value > alpha)
1045 rm.selDepth = thisThread->selDepth;
1050 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1051 rm.pv.push_back(*m);
1053 // We record how often the best move has been changed in each
1054 // iteration. This information is used for time management: When
1055 // the best move changes frequently, we allocate some more time.
1056 if (moveCount > 1 && thisThread == Threads.main())
1057 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1060 // All other moves but the PV are set to the lowest value: this
1061 // is not a problem when sorting because the sort is stable and the
1062 // move position in the list is preserved - just the PV is pushed up.
1063 rm.score = -VALUE_INFINITE;
1066 if (value > bestValue)
1074 if (PvNode && !rootNode) // Update pv even in fail-high case
1075 update_pv(ss->pv, move, (ss+1)->pv);
1077 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1081 assert(value >= beta); // Fail high
1087 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1088 quietsSearched[quietCount++] = move;
1091 // The following condition would detect a stop only after move loop has been
1092 // completed. But in this case bestValue is valid because we have fully
1093 // searched our subtree, and we can anyhow save the result in TT.
1099 // Step 20. Check for mate and stalemate
1100 // All legal moves have been searched and if there are no legal moves, it
1101 // must be a mate or a stalemate. If we are in a singular extension search then
1102 // return a fail low score.
1104 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1107 bestValue = excludedMove ? alpha
1108 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1111 // Quiet best move: update move sorting heuristics
1112 if (!pos.capture_or_promotion(bestMove))
1113 update_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1115 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1116 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1117 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1119 // Bonus for prior countermove that caused the fail low
1120 else if ( depth >= 3 * ONE_PLY
1121 && !pos.captured_piece()
1122 && is_ok((ss-1)->currentMove))
1123 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1126 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1127 bestValue >= beta ? BOUND_LOWER :
1128 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1129 depth, bestMove, ss->staticEval, TT.generation());
1131 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1137 // qsearch() is the quiescence search function, which is called by the main
1138 // search function with depth zero, or recursively with depth less than ONE_PLY.
1140 template <NodeType NT, bool InCheck>
1141 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1143 const bool PvNode = NT == PV;
1145 assert(InCheck == !!pos.checkers());
1146 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1147 assert(PvNode || (alpha == beta - 1));
1148 assert(depth <= DEPTH_ZERO);
1149 assert(depth / ONE_PLY * ONE_PLY == depth);
1155 Move ttMove, move, bestMove;
1156 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1157 bool ttHit, givesCheck, evasionPrunable;
1163 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1165 ss->pv[0] = MOVE_NONE;
1168 ss->currentMove = bestMove = MOVE_NONE;
1169 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 && (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 const PieceToHistory* contHist[4] = {};
1244 MovePicker mp(pos, ttMove, depth, &pos.this_thread()->mainHistory, contHist, to_sq((ss-1)->currentMove));
1246 // Loop through the moves until no moves remain or a beta cutoff occurs
1247 while ((move = mp.next_move()) != MOVE_NONE)
1249 assert(is_ok(move));
1251 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
1252 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
1253 : pos.gives_check(move);
1260 && futilityBase > -VALUE_KNOWN_WIN
1261 && !pos.advanced_pawn_push(move))
1263 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1265 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1267 if (futilityValue <= alpha)
1269 bestValue = std::max(bestValue, futilityValue);
1273 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1275 bestValue = std::max(bestValue, futilityBase);
1280 // Detect non-capture evasions that are candidates to be pruned
1281 evasionPrunable = InCheck
1282 && (depth != DEPTH_ZERO || moveCount > 2)
1283 && bestValue > VALUE_MATED_IN_MAX_PLY
1284 && !pos.capture(move);
1286 // Don't search moves with negative SEE values
1287 if ( (!InCheck || evasionPrunable)
1288 && type_of(move) != PROMOTION
1289 && !pos.see_ge(move))
1292 // Speculative prefetch as early as possible
1293 prefetch(TT.first_entry(pos.key_after(move)));
1295 // Check for legality just before making the move
1296 if (!pos.legal(move))
1302 ss->currentMove = move;
1304 // Make and search the move
1305 pos.do_move(move, st, givesCheck);
1306 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1307 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1308 pos.undo_move(move);
1310 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1312 // Check for a new best move
1313 if (value > bestValue)
1319 if (PvNode) // Update pv even in fail-high case
1320 update_pv(ss->pv, move, (ss+1)->pv);
1322 if (PvNode && value < beta) // Update alpha here!
1329 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1330 ttDepth, move, ss->staticEval, TT.generation());
1338 // All legal moves have been searched. A special case: If we're in check
1339 // and no legal moves were found, it is checkmate.
1340 if (InCheck && bestValue == -VALUE_INFINITE)
1341 return mated_in(ss->ply); // Plies to mate from the root
1343 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1344 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1345 ttDepth, bestMove, ss->staticEval, TT.generation());
1347 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1353 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1354 // "plies to mate from the current position". Non-mate scores are unchanged.
1355 // The function is called before storing a value in the transposition table.
1357 Value value_to_tt(Value v, int ply) {
1359 assert(v != VALUE_NONE);
1361 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1362 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1366 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1367 // from the transposition table (which refers to the plies to mate/be mated
1368 // from current position) to "plies to mate/be mated from the root".
1370 Value value_from_tt(Value v, int ply) {
1372 return v == VALUE_NONE ? VALUE_NONE
1373 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1374 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1378 // update_pv() adds current move and appends child pv[]
1380 void update_pv(Move* pv, Move move, Move* childPv) {
1382 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1388 // update_continuation_histories() updates histories of the move pairs formed
1389 // by moves at ply -1, -2, and -4 with current move.
1391 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1393 for (int i : {1, 2, 4})
1394 if (is_ok((ss-i)->currentMove))
1395 (ss-i)->contHistory->update(pc, to, bonus);
1399 // update_stats() updates move sorting heuristics when a new quiet best move is found
1401 void update_stats(const Position& pos, Stack* ss, Move move,
1402 Move* quiets, int quietsCnt, int bonus) {
1404 if (ss->killers[0] != move)
1406 ss->killers[1] = ss->killers[0];
1407 ss->killers[0] = move;
1410 Color c = pos.side_to_move();
1411 Thread* thisThread = pos.this_thread();
1412 thisThread->mainHistory.update(c, move, bonus);
1413 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1415 if (is_ok((ss-1)->currentMove))
1417 Square prevSq = to_sq((ss-1)->currentMove);
1418 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1421 // Decrease all the other played quiet moves
1422 for (int i = 0; i < quietsCnt; ++i)
1424 thisThread->mainHistory.update(c, quiets[i], -bonus);
1425 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1430 // When playing with strength handicap, choose best move among a set of RootMoves
1431 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1433 Move Skill::pick_best(size_t multiPV) {
1435 const RootMoves& rootMoves = Threads.main()->rootMoves;
1436 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1438 // RootMoves are already sorted by score in descending order
1439 Value topScore = rootMoves[0].score;
1440 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1441 int weakness = 120 - 2 * level;
1442 int maxScore = -VALUE_INFINITE;
1444 // Choose best move. For each move score we add two terms, both dependent on
1445 // weakness. One is deterministic and bigger for weaker levels, and one is
1446 // random. Then we choose the move with the resulting highest score.
1447 for (size_t i = 0; i < multiPV; ++i)
1449 // This is our magic formula
1450 int push = ( weakness * int(topScore - rootMoves[i].score)
1451 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1453 if (rootMoves[i].score + push > maxScore)
1455 maxScore = rootMoves[i].score + push;
1456 best = rootMoves[i].pv[0];
1465 // check_time() is used to print debug info and, more importantly, to detect
1466 // when we are out of available time and thus stop the search.
1468 void MainThread::check_time() {
1473 // At low node count increase the checking rate to about 0.1% of nodes
1474 // otherwise use a default value.
1475 callsCnt = Limits.nodes ? std::min(4096, int(Limits.nodes / 1024)) : 4096;
1477 static TimePoint lastInfoTime = now();
1479 int elapsed = Time.elapsed();
1480 TimePoint tick = Limits.startTime + elapsed;
1482 if (tick - lastInfoTime >= 1000)
1484 lastInfoTime = tick;
1488 // An engine may not stop pondering until told so by the GUI
1492 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1493 || (Limits.movetime && elapsed >= Limits.movetime)
1494 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1495 Threads.stop = true;
1499 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1500 /// that all (if any) unsearched PV lines are sent using a previous search score.
1502 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1504 std::stringstream ss;
1505 int elapsed = Time.elapsed() + 1;
1506 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1507 size_t PVIdx = pos.this_thread()->PVIdx;
1508 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1509 uint64_t nodesSearched = Threads.nodes_searched();
1510 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1512 for (size_t i = 0; i < multiPV; ++i)
1514 bool updated = (i <= PVIdx && rootMoves[i].score != -VALUE_INFINITE);
1516 if (depth == ONE_PLY && !updated)
1519 Depth d = updated ? depth : depth - ONE_PLY;
1520 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1522 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1523 v = tb ? TB::Score : v;
1525 if (ss.rdbuf()->in_avail()) // Not at first line
1529 << " depth " << d / ONE_PLY
1530 << " seldepth " << rootMoves[i].selDepth
1531 << " multipv " << i + 1
1532 << " score " << UCI::value(v);
1534 if (!tb && i == PVIdx)
1535 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1537 ss << " nodes " << nodesSearched
1538 << " nps " << nodesSearched * 1000 / elapsed;
1540 if (elapsed > 1000) // Earlier makes little sense
1541 ss << " hashfull " << TT.hashfull();
1543 ss << " tbhits " << tbHits
1544 << " time " << elapsed
1547 for (Move m : rootMoves[i].pv)
1548 ss << " " << UCI::move(m, pos.is_chess960());
1555 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1556 /// before exiting the search, for instance, in case we stop the search during a
1557 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1558 /// otherwise in case of 'ponder on' we have nothing to think on.
1560 bool RootMove::extract_ponder_from_tt(Position& pos) {
1565 assert(pv.size() == 1);
1570 pos.do_move(pv[0], st);
1571 TTEntry* tte = TT.probe(pos.key(), ttHit);
1575 Move m = tte->move(); // Local copy to be SMP safe
1576 if (MoveList<LEGAL>(pos).contains(m))
1580 pos.undo_move(pv[0]);
1581 return pv.size() > 1;
1584 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1587 UseRule50 = Options["Syzygy50MoveRule"];
1588 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1589 Cardinality = Options["SyzygyProbeLimit"];
1591 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1592 if (Cardinality > MaxCardinality)
1594 Cardinality = MaxCardinality;
1595 ProbeDepth = DEPTH_ZERO;
1598 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1601 // If the current root position is in the tablebases, then RootMoves
1602 // contains only moves that preserve the draw or the win.
1603 RootInTB = root_probe(pos, rootMoves, TB::Score);
1606 Cardinality = 0; // Do not probe tablebases during the search
1608 else // If DTZ tables are missing, use WDL tables as a fallback
1610 // Filter out moves that do not preserve the draw or the win.
1611 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1613 // Only probe during search if winning
1614 if (RootInTB && TB::Score <= VALUE_DRAW)
1618 if (RootInTB && !UseRule50)
1619 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1620 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1