2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2017 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
46 namespace Tablebases {
55 namespace TB = Tablebases;
59 using namespace Search;
63 // Different node types, used as a template parameter
64 enum NodeType { NonPV, PV };
66 // Razoring and futility margin based on depth
67 const int razor_margin[4] = { 483, 570, 603, 554 };
68 Value futility_margin(Depth d) { return Value(150 * d / ONE_PLY); }
70 // Futility and reductions lookup tables, initialized at startup
71 int FutilityMoveCounts[2][16]; // [improving][depth]
72 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
74 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
75 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
78 // History and stats update bonus, based on depth
79 Value stat_bonus(Depth depth) {
80 int d = depth / ONE_PLY ;
81 return Value(d * d + 2 * d - 2);
84 // Skill structure is used to implement strength limit
86 Skill(int l) : level(l) {}
87 bool enabled() const { return level < 20; }
88 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
89 Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
90 Move pick_best(size_t multiPV);
93 Move best = MOVE_NONE;
96 // EasyMoveManager structure is used to detect an 'easy move'. When the PV is
97 // stable across multiple search iterations, we can quickly return the best move.
98 struct EasyMoveManager {
103 pv[0] = pv[1] = pv[2] = MOVE_NONE;
106 Move get(Key key) const {
107 return expectedPosKey == key ? pv[2] : MOVE_NONE;
110 void update(Position& pos, const std::vector<Move>& newPv) {
112 assert(newPv.size() >= 3);
114 // Keep track of how many times in a row the 3rd ply remains stable
115 stableCnt = (newPv[2] == pv[2]) ? stableCnt + 1 : 0;
117 if (!std::equal(newPv.begin(), newPv.begin() + 3, pv))
119 std::copy(newPv.begin(), newPv.begin() + 3, pv);
122 pos.do_move(newPv[0], st[0]);
123 pos.do_move(newPv[1], st[1]);
124 expectedPosKey = pos.key();
125 pos.undo_move(newPv[1]);
126 pos.undo_move(newPv[0]);
135 // skip half of the plies in blocks depending on the helper thread idx.
136 bool skip_ply(int idx, int ply) {
138 idx = (idx - 1) % 20 + 1; // cycle after 20 threads.
140 // number of successive plies to skip, depending on idx.
142 while (ones * (ones + 1) < idx)
145 return ((ply + idx - 1) / ones - ones) % 2 == 0;
148 EasyMoveManager EasyMove;
149 Value DrawValue[COLOR_NB];
151 template <NodeType NT>
152 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning);
154 template <NodeType NT, bool InCheck>
155 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
157 Value value_to_tt(Value v, int ply);
158 Value value_from_tt(Value v, int ply);
159 void update_pv(Move* pv, Move move, Move* childPv);
160 void update_cm_stats(Stack* ss, Piece pc, Square s, Value bonus);
161 void update_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, Value bonus);
167 /// Search::init() is called during startup to initialize various lookup tables
169 void Search::init() {
171 for (int imp = 0; imp <= 1; ++imp)
172 for (int d = 1; d < 64; ++d)
173 for (int mc = 1; mc < 64; ++mc)
175 double r = log(d) * log(mc) / 2;
177 Reductions[NonPV][imp][d][mc] = int(std::round(r));
178 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
180 // Increase reduction for non-PV nodes when eval is not improving
181 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
182 Reductions[NonPV][imp][d][mc]++;
185 for (int d = 0; d < 16; ++d)
187 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
188 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
193 /// Search::clear() resets search state to zero, to obtain reproducible results
195 void Search::clear() {
199 for (Thread* th : Threads)
201 th->counterMoves.clear();
203 th->counterMoveHistory.clear();
204 th->resetCalls = true;
207 Threads.main()->previousScore = VALUE_INFINITE;
211 /// Search::perft() is our utility to verify move generation. All the leaf nodes
212 /// up to the given depth are generated and counted, and the sum is returned.
214 uint64_t Search::perft(Position& pos, Depth depth) {
217 uint64_t cnt, nodes = 0;
218 const bool leaf = (depth == 2 * ONE_PLY);
220 for (const auto& m : MoveList<LEGAL>(pos))
222 if (Root && depth <= ONE_PLY)
227 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
232 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
237 template uint64_t Search::perft<true>(Position&, Depth);
240 /// MainThread::search() is called by the main thread when the program receives
241 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
243 void MainThread::search() {
245 Color us = rootPos.side_to_move();
246 Time.init(Limits, us, rootPos.game_ply());
248 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
249 DrawValue[ us] = VALUE_DRAW - Value(contempt);
250 DrawValue[~us] = VALUE_DRAW + Value(contempt);
252 if (rootMoves.empty())
254 rootMoves.push_back(RootMove(MOVE_NONE));
255 sync_cout << "info depth 0 score "
256 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
261 for (Thread* th : Threads)
263 th->start_searching();
265 Thread::search(); // Let's start searching!
268 // When playing in 'nodes as time' mode, subtract the searched nodes from
269 // the available ones before exiting.
271 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
273 // When we reach the maximum depth, we can arrive here without a raise of
274 // Signals.stop. However, if we are pondering or in an infinite search,
275 // the UCI protocol states that we shouldn't print the best move before the
276 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
277 // until the GUI sends one of those commands (which also raises Signals.stop).
278 if (!Signals.stop && (Limits.ponder || Limits.infinite))
280 Signals.stopOnPonderhit = true;
284 // Stop the threads if not already stopped
287 // Wait until all threads have finished
288 for (Thread* th : Threads)
290 th->wait_for_search_finished();
292 // Check if there are threads with a better score than main thread
293 Thread* bestThread = this;
294 if ( !this->easyMovePlayed
295 && Options["MultiPV"] == 1
297 && !Skill(Options["Skill Level"]).enabled()
298 && rootMoves[0].pv[0] != MOVE_NONE)
300 for (Thread* th : Threads)
302 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
303 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
305 if (scoreDiff > 0 && depthDiff >= 0)
310 previousScore = bestThread->rootMoves[0].score;
312 // Send new PV when needed
313 if (bestThread != this)
314 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
316 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
318 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
319 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
321 std::cout << sync_endl;
325 // Thread::search() is the main iterative deepening loop. It calls search()
326 // repeatedly with increasing depth until the allocated thinking time has been
327 // consumed, the user stops the search, or the maximum search depth is reached.
329 void Thread::search() {
331 Stack stack[MAX_PLY+7], *ss = stack+4; // To allow referencing (ss-4) and (ss+2)
332 Value bestValue, alpha, beta, delta;
333 Move easyMove = MOVE_NONE;
334 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
336 std::memset(ss-4, 0, 7 * sizeof(Stack));
338 bestValue = delta = alpha = -VALUE_INFINITE;
339 beta = VALUE_INFINITE;
340 completedDepth = DEPTH_ZERO;
344 easyMove = EasyMove.get(rootPos.key());
346 mainThread->easyMovePlayed = mainThread->failedLow = false;
347 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 || Threads.main()->rootDepth / ONE_PLY <= Limits.depth))
366 // skip plies for helper threads
367 if (idx && skip_ply(idx, rootDepth / ONE_PLY + rootPos.game_ply()))
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 && !Signals.stop; ++PVIdx)
382 // Reset aspiration window starting size
383 if (rootDepth >= 5 * ONE_PLY)
386 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
387 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
390 // Start with a small aspiration window and, in the case of a fail
391 // high/low, re-search with a bigger window until we're not failing
395 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
397 // Bring the best move to the front. It is critical that sorting
398 // is done with a stable algorithm because all the values but the
399 // first and eventually the new best one are set to -VALUE_INFINITE
400 // and we want to keep the same order for all the moves except the
401 // new PV that goes to the front. Note that in case of MultiPV
402 // search the already searched PV lines are preserved.
403 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
405 // If search has been stopped, we break immediately. Sorting and
406 // writing PV back to TT is safe because RootMoves is still
407 // valid, although it refers to the previous iteration.
411 // When failing high/low give some update (without cluttering
412 // the UI) before a re-search.
415 && (bestValue <= alpha || bestValue >= beta)
416 && Time.elapsed() > 3000)
417 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
419 // In case of failing low/high increase aspiration window and
420 // re-search, otherwise exit the loop.
421 if (bestValue <= alpha)
423 beta = (alpha + beta) / 2;
424 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
428 mainThread->failedLow = true;
429 Signals.stopOnPonderhit = false;
432 else if (bestValue >= beta)
434 alpha = (alpha + beta) / 2;
435 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);
451 if (Signals.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000)
452 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
456 completedDepth = rootDepth;
461 // If skill level is enabled and time is up, pick a sub-optimal best move
462 if (skill.enabled() && skill.time_to_pick(rootDepth))
463 skill.pick_best(multiPV);
465 // Have we found a "mate in x"?
467 && bestValue >= VALUE_MATE_IN_MAX_PLY
468 && VALUE_MATE - bestValue <= 2 * Limits.mate)
471 // Do we have time for the next iteration? Can we stop searching now?
472 if (Limits.use_time_management())
474 if (!Signals.stop && !Signals.stopOnPonderhit)
476 // Stop the search if only one legal move is available, or if all
477 // of the available time has been used, or if we matched an easyMove
478 // from the previous search and just did a fast verification.
479 const int F[] = { mainThread->failedLow,
480 bestValue - mainThread->previousScore };
482 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
483 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
485 bool doEasyMove = rootMoves[0].pv[0] == easyMove
486 && mainThread->bestMoveChanges < 0.03
487 && Time.elapsed() > Time.optimum() * 5 / 44;
489 if ( rootMoves.size() == 1
490 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628
491 || (mainThread->easyMovePlayed = doEasyMove, doEasyMove))
493 // If we are allowed to ponder do not stop the search now but
494 // keep pondering until the GUI sends "ponderhit" or "stop".
496 Signals.stopOnPonderhit = true;
502 if (rootMoves[0].pv.size() >= 3)
503 EasyMove.update(rootPos, rootMoves[0].pv);
512 // Clear any candidate easy move that wasn't stable for the last search
513 // iterations; the second condition prevents consecutive fast moves.
514 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
517 // If skill level is enabled, swap best PV line with the sub-optimal one
519 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
520 rootMoves.end(), skill.best_move(multiPV)));
526 // search<>() is the main search function for both PV and non-PV nodes
528 template <NodeType NT>
529 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
531 const bool PvNode = NT == PV;
532 const bool rootNode = PvNode && (ss-1)->ply == 0;
534 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
535 assert(PvNode || (alpha == beta - 1));
536 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
537 assert(!(PvNode && cutNode));
538 assert(depth / ONE_PLY * ONE_PLY == depth);
540 Move pv[MAX_PLY+1], quietsSearched[64];
544 Move ttMove, move, excludedMove, bestMove;
545 Depth extension, newDepth;
546 Value bestValue, value, ttValue, eval;
547 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
548 bool captureOrPromotion, doFullDepthSearch, moveCountPruning;
550 int moveCount, quietCount;
552 // Step 1. Initialize node
553 Thread* thisThread = pos.this_thread();
554 inCheck = pos.checkers();
555 moveCount = quietCount = ss->moveCount = 0;
556 ss->history = VALUE_ZERO;
557 bestValue = -VALUE_INFINITE;
558 ss->ply = (ss-1)->ply + 1;
560 // Check for the available remaining time
561 if (thisThread->resetCalls.load(std::memory_order_relaxed))
563 thisThread->resetCalls = false;
564 // At low node count increase the checking rate to about 0.1% of nodes
565 // otherwise use a default value.
566 thisThread->callsCnt = Limits.nodes ? std::min((int64_t)4096, Limits.nodes / 1024)
570 if (--thisThread->callsCnt <= 0)
572 for (Thread* th : Threads)
573 th->resetCalls = true;
578 // Used to send selDepth info to GUI
579 if (PvNode && thisThread->maxPly < ss->ply)
580 thisThread->maxPly = ss->ply;
584 // Step 2. Check for aborted search and immediate draw
585 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
586 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
587 : DrawValue[pos.side_to_move()];
589 // Step 3. Mate distance pruning. Even if we mate at the next move our score
590 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
591 // a shorter mate was found upward in the tree then there is no need to search
592 // because we will never beat the current alpha. Same logic but with reversed
593 // signs applies also in the opposite condition of being mated instead of giving
594 // mate. In this case return a fail-high score.
595 alpha = std::max(mated_in(ss->ply), alpha);
596 beta = std::min(mate_in(ss->ply+1), beta);
601 assert(0 <= ss->ply && ss->ply < MAX_PLY);
603 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
604 ss->counterMoves = nullptr;
605 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
606 Square prevSq = to_sq((ss-1)->currentMove);
608 // Step 4. Transposition table lookup. We don't want the score of a partial
609 // search to overwrite a previous full search TT value, so we use a different
610 // position key in case of an excluded move.
611 excludedMove = ss->excludedMove;
612 posKey = pos.key() ^ Key(excludedMove);
613 tte = TT.probe(posKey, ttHit);
614 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
615 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
616 : ttHit ? tte->move() : MOVE_NONE;
618 // At non-PV nodes we check for an early TT cutoff
621 && tte->depth() >= depth
622 && ttValue != VALUE_NONE // Possible in case of TT access race
623 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
624 : (tte->bound() & BOUND_UPPER)))
626 // If ttMove is quiet, update move sorting heuristics on TT hit
631 if (!pos.capture_or_promotion(ttMove))
632 update_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
634 // Extra penalty for a quiet TT move in previous ply when it gets refuted
635 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
636 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
638 // Penalty for a quiet ttMove that fails low
639 else if (!pos.capture_or_promotion(ttMove))
641 Value penalty = -stat_bonus(depth + ONE_PLY);
642 thisThread->history.update(pos.side_to_move(), ttMove, penalty);
643 update_cm_stats(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
649 // Step 4a. Tablebase probe
650 if (!rootNode && TB::Cardinality)
652 int piecesCount = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
654 if ( piecesCount <= TB::Cardinality
655 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
656 && pos.rule50_count() == 0
657 && !pos.can_castle(ANY_CASTLING))
660 TB::WDLScore v = Tablebases::probe_wdl(pos, &err);
662 if (err != TB::ProbeState::FAIL)
664 thisThread->tbHits++;
666 int drawScore = TB::UseRule50 ? 1 : 0;
668 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
669 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
670 : VALUE_DRAW + 2 * v * drawScore;
672 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
673 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
674 MOVE_NONE, VALUE_NONE, TT.generation());
681 // Step 5. Evaluate the position statically
684 ss->staticEval = eval = VALUE_NONE;
690 // Never assume anything on values stored in TT
691 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
692 eval = ss->staticEval = evaluate(pos);
694 // Can ttValue be used as a better position evaluation?
695 if (ttValue != VALUE_NONE)
696 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
701 eval = ss->staticEval =
702 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
703 : -(ss-1)->staticEval + 2 * Eval::Tempo;
705 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
706 ss->staticEval, TT.generation());
709 if (skipEarlyPruning)
712 // Step 6. Razoring (skipped when in check)
714 && depth < 4 * ONE_PLY
715 && eval + razor_margin[depth / ONE_PLY] <= alpha)
717 if (depth <= ONE_PLY)
718 return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
720 Value ralpha = alpha - razor_margin[depth / ONE_PLY];
721 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1);
726 // Step 7. Futility pruning: child node (skipped when in check)
728 && depth < 7 * ONE_PLY
729 && eval - futility_margin(depth) >= beta
730 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
731 && pos.non_pawn_material(pos.side_to_move()))
734 // Step 8. Null move search with verification search (is omitted in PV nodes)
737 && (ss->staticEval >= beta - 35 * (depth / ONE_PLY - 6) || depth >= 13 * ONE_PLY)
738 && pos.non_pawn_material(pos.side_to_move()))
740 ss->currentMove = MOVE_NULL;
741 ss->counterMoves = nullptr;
743 assert(eval - beta >= 0);
745 // Null move dynamic reduction based on depth and value
746 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
748 pos.do_null_move(st);
749 Value nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1)
750 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
751 pos.undo_null_move();
753 if (nullValue >= beta)
755 // Do not return unproven mate scores
756 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
759 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
762 // Do verification search at high depths
763 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
764 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
771 // Step 9. ProbCut (skipped when in check)
772 // If we have a good enough capture and a reduced search returns a value
773 // much above beta, we can (almost) safely prune the previous move.
775 && depth >= 5 * ONE_PLY
776 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
778 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
779 Depth rdepth = depth - 4 * ONE_PLY;
781 assert(rdepth >= ONE_PLY);
782 assert((ss-1)->currentMove != MOVE_NONE);
783 assert((ss-1)->currentMove != MOVE_NULL);
785 MovePicker mp(pos, ttMove, rbeta - ss->staticEval);
787 while ((move = mp.next_move()) != MOVE_NONE)
790 ss->currentMove = move;
791 ss->counterMoves = &thisThread->counterMoveHistory[pos.moved_piece(move)][to_sq(move)];
792 pos.do_move(move, st);
793 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode, false);
800 // Step 10. Internal iterative deepening (skipped when in check)
801 if ( depth >= 6 * ONE_PLY
803 && (PvNode || ss->staticEval + 256 >= beta))
805 Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
806 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
808 tte = TT.probe(posKey, ttHit);
809 ttMove = ttHit ? tte->move() : MOVE_NONE;
812 moves_loop: // When in check search starts from here
814 const CounterMoveStats* cmh = (ss-1)->counterMoves;
815 const CounterMoveStats* fmh = (ss-2)->counterMoves;
816 const CounterMoveStats* fmh2 = (ss-4)->counterMoves;
818 MovePicker mp(pos, ttMove, depth, ss);
819 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
820 improving = ss->staticEval >= (ss-2)->staticEval
821 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
822 ||(ss-2)->staticEval == VALUE_NONE;
824 singularExtensionNode = !rootNode
825 && depth >= 8 * ONE_PLY
826 && ttMove != MOVE_NONE
827 && ttValue != VALUE_NONE
828 && !excludedMove // Recursive singular search is not allowed
829 && (tte->bound() & BOUND_LOWER)
830 && tte->depth() >= depth - 3 * ONE_PLY;
832 // Step 11. Loop through moves
833 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
834 while ((move = mp.next_move()) != MOVE_NONE)
838 if (move == excludedMove)
841 // At root obey the "searchmoves" option and skip moves not listed in Root
842 // Move List. As a consequence any illegal move is also skipped. In MultiPV
843 // mode we also skip PV moves which have been already searched.
844 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
845 thisThread->rootMoves.end(), move))
848 ss->moveCount = ++moveCount;
850 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
851 sync_cout << "info depth " << depth / ONE_PLY
852 << " currmove " << UCI::move(move, pos.is_chess960())
853 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
856 (ss+1)->pv = nullptr;
858 extension = DEPTH_ZERO;
859 captureOrPromotion = pos.capture_or_promotion(move);
860 moved_piece = pos.moved_piece(move);
862 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
863 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
864 : pos.gives_check(move);
866 moveCountPruning = depth < 16 * ONE_PLY
867 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
869 // Step 12. Extensions
873 && pos.see_ge(move, VALUE_ZERO))
876 // Singular extension search. If all moves but one fail low on a search of
877 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
878 // is singular and should be extended. To verify this we do a reduced search
879 // on all the other moves but the ttMove and if the result is lower than
880 // ttValue minus a margin then we extend the ttMove.
881 if ( singularExtensionNode
886 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
887 Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
888 ss->excludedMove = move;
889 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode, true);
890 ss->excludedMove = MOVE_NONE;
896 // Calculate new depth for this move
897 newDepth = depth - ONE_PLY + extension;
899 // Step 13. Pruning at shallow depth
901 && bestValue > VALUE_MATED_IN_MAX_PLY)
903 if ( !captureOrPromotion
905 && !pos.advanced_pawn_push(move))
907 // Move count based pruning
908 if (moveCountPruning)
911 // Reduced depth of the next LMR search
912 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
914 // Countermoves based pruning
916 && (!cmh || (*cmh )[moved_piece][to_sq(move)] < VALUE_ZERO)
917 && (!fmh || (*fmh )[moved_piece][to_sq(move)] < VALUE_ZERO)
918 && (!fmh2 || (*fmh2)[moved_piece][to_sq(move)] < VALUE_ZERO || (cmh && fmh)))
921 // Futility pruning: parent node
924 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
927 // Prune moves with negative SEE
929 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
932 else if ( depth < 7 * ONE_PLY
934 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
938 // Speculative prefetch as early as possible
939 prefetch(TT.first_entry(pos.key_after(move)));
941 // Check for legality just before making the move
942 if (!rootNode && !pos.legal(move))
944 ss->moveCount = --moveCount;
948 // Update the current move (this must be done after singular extension search)
949 ss->currentMove = move;
950 ss->counterMoves = &thisThread->counterMoveHistory[moved_piece][to_sq(move)];
952 // Step 14. Make the move
953 pos.do_move(move, st, givesCheck);
955 // Step 15. Reduced depth search (LMR). If the move fails high it will be
956 // re-searched at full depth.
957 if ( depth >= 3 * ONE_PLY
959 && (!captureOrPromotion || moveCountPruning))
961 Depth r = reduction<PvNode>(improving, depth, moveCount);
963 if (captureOrPromotion)
964 r -= r ? ONE_PLY : DEPTH_ZERO;
967 // Increase reduction for cut nodes
971 // Decrease reduction for moves that escape a capture. Filter out
972 // castling moves, because they are coded as "king captures rook" and
973 // hence break make_move().
974 else if ( type_of(move) == NORMAL
975 && !pos.see_ge(make_move(to_sq(move), from_sq(move)), VALUE_ZERO))
978 ss->history = (cmh ? (*cmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
979 + (fmh ? (*fmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
980 + (fmh2 ? (*fmh2)[moved_piece][to_sq(move)] : VALUE_ZERO)
981 + thisThread->history.get(~pos.side_to_move(), move)
982 - 4000; // Correction factor
984 // Decrease/increase reduction by comparing opponent's stat score
985 if (ss->history > VALUE_ZERO && (ss-1)->history < VALUE_ZERO)
988 else if (ss->history < VALUE_ZERO && (ss-1)->history > VALUE_ZERO)
991 // Decrease/increase reduction for moves with a good/bad history
992 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->history / 20000) * ONE_PLY);
995 Depth d = std::max(newDepth - r, ONE_PLY);
997 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
999 doFullDepthSearch = (value > alpha && d != newDepth);
1002 doFullDepthSearch = !PvNode || moveCount > 1;
1004 // Step 16. Full depth search when LMR is skipped or fails high
1005 if (doFullDepthSearch)
1006 value = newDepth < ONE_PLY ?
1007 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha)
1008 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha)
1009 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
1011 // For PV nodes only, do a full PV search on the first move or after a fail
1012 // high (in the latter case search only if value < beta), otherwise let the
1013 // parent node fail low with value <= alpha and try another move.
1014 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1017 (ss+1)->pv[0] = MOVE_NONE;
1019 value = newDepth < ONE_PLY ?
1020 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha)
1021 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha)
1022 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1025 // Step 17. Undo move
1026 pos.undo_move(move);
1028 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1030 // Step 18. Check for a new best move
1031 // Finished searching the move. If a stop occurred, the return value of
1032 // the search cannot be trusted, and we return immediately without
1033 // updating best move, PV and TT.
1034 if (Signals.stop.load(std::memory_order_relaxed))
1039 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1040 thisThread->rootMoves.end(), move);
1042 // PV move or new best move ?
1043 if (moveCount == 1 || value > alpha)
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 is
1061 // 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()];
1112 // Quiet best move: update move sorting heuristics
1113 if (!pos.capture_or_promotion(bestMove))
1114 update_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1116 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1117 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1118 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1120 // Bonus for prior countermove that caused the fail low
1121 else if ( depth >= 3 * ONE_PLY
1122 && !pos.captured_piece()
1123 && is_ok((ss-1)->currentMove))
1124 update_cm_stats(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;
1162 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1164 ss->pv[0] = MOVE_NONE;
1167 ss->currentMove = bestMove = MOVE_NONE;
1168 ss->ply = (ss-1)->ply + 1;
1170 // Check for an instant draw or if the maximum ply has been reached
1171 if (pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
1172 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1173 : DrawValue[pos.side_to_move()];
1175 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1177 // Decide whether or not to include checks: this fixes also the type of
1178 // TT entry depth that we are going to use. Note that in qsearch we use
1179 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1180 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1181 : DEPTH_QS_NO_CHECKS;
1183 // Transposition table lookup
1185 tte = TT.probe(posKey, ttHit);
1186 ttMove = ttHit ? tte->move() : MOVE_NONE;
1187 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1191 && tte->depth() >= ttDepth
1192 && ttValue != VALUE_NONE // Only in case of TT access race
1193 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1194 : (tte->bound() & BOUND_UPPER)))
1197 // Evaluate the position statically
1200 ss->staticEval = VALUE_NONE;
1201 bestValue = futilityBase = -VALUE_INFINITE;
1207 // Never assume anything on values stored in TT
1208 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1209 ss->staticEval = bestValue = evaluate(pos);
1211 // Can ttValue be used as a better position evaluation?
1212 if (ttValue != VALUE_NONE)
1213 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1214 bestValue = ttValue;
1217 ss->staticEval = bestValue =
1218 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1219 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1221 // Stand pat. Return immediately if static value is at least beta
1222 if (bestValue >= beta)
1225 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1226 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1231 if (PvNode && bestValue > alpha)
1234 futilityBase = bestValue + 128;
1237 // Initialize a MovePicker object for the current position, and prepare
1238 // to search the moves. Because the depth is <= 0 here, only captures,
1239 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1241 MovePicker mp(pos, ttMove, depth, to_sq((ss-1)->currentMove));
1243 // Loop through the moves until no moves remain or a beta cutoff occurs
1244 while ((move = mp.next_move()) != MOVE_NONE)
1246 assert(is_ok(move));
1248 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
1249 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
1250 : pos.gives_check(move);
1255 && futilityBase > -VALUE_KNOWN_WIN
1256 && !pos.advanced_pawn_push(move))
1258 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1260 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1262 if (futilityValue <= alpha)
1264 bestValue = std::max(bestValue, futilityValue);
1268 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1270 bestValue = std::max(bestValue, futilityBase);
1275 // Detect non-capture evasions that are candidates to be pruned
1276 evasionPrunable = InCheck
1277 && bestValue > VALUE_MATED_IN_MAX_PLY
1278 && !pos.capture(move);
1280 // Don't search moves with negative SEE values
1281 if ( (!InCheck || evasionPrunable)
1282 && type_of(move) != PROMOTION
1283 && !pos.see_ge(move, VALUE_ZERO))
1286 // Speculative prefetch as early as possible
1287 prefetch(TT.first_entry(pos.key_after(move)));
1289 // Check for legality just before making the move
1290 if (!pos.legal(move))
1293 ss->currentMove = move;
1295 // Make and search the move
1296 pos.do_move(move, st, givesCheck);
1297 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1298 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1299 pos.undo_move(move);
1301 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1303 // Check for a new best move
1304 if (value > bestValue)
1310 if (PvNode) // Update pv even in fail-high case
1311 update_pv(ss->pv, move, (ss+1)->pv);
1313 if (PvNode && value < beta) // Update alpha here!
1320 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1321 ttDepth, move, ss->staticEval, TT.generation());
1329 // All legal moves have been searched. A special case: If we're in check
1330 // and no legal moves were found, it is checkmate.
1331 if (InCheck && bestValue == -VALUE_INFINITE)
1332 return mated_in(ss->ply); // Plies to mate from the root
1334 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1335 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1336 ttDepth, bestMove, ss->staticEval, TT.generation());
1338 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1344 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1345 // "plies to mate from the current position". Non-mate scores are unchanged.
1346 // The function is called before storing a value in the transposition table.
1348 Value value_to_tt(Value v, int ply) {
1350 assert(v != VALUE_NONE);
1352 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1353 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1357 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1358 // from the transposition table (which refers to the plies to mate/be mated
1359 // from current position) to "plies to mate/be mated from the root".
1361 Value value_from_tt(Value v, int ply) {
1363 return v == VALUE_NONE ? VALUE_NONE
1364 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1365 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1369 // update_pv() adds current move and appends child pv[]
1371 void update_pv(Move* pv, Move move, Move* childPv) {
1373 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1379 // update_cm_stats() updates countermove and follow-up move history
1381 void update_cm_stats(Stack* ss, Piece pc, Square s, Value bonus) {
1383 CounterMoveStats* cmh = (ss-1)->counterMoves;
1384 CounterMoveStats* fmh1 = (ss-2)->counterMoves;
1385 CounterMoveStats* fmh2 = (ss-4)->counterMoves;
1388 cmh->update(pc, s, bonus);
1391 fmh1->update(pc, s, bonus);
1394 fmh2->update(pc, s, bonus);
1398 // update_stats() updates move sorting heuristics when a new quiet best move is found
1400 void update_stats(const Position& pos, Stack* ss, Move move,
1401 Move* quiets, int quietsCnt, Value bonus) {
1403 if (ss->killers[0] != move)
1405 ss->killers[1] = ss->killers[0];
1406 ss->killers[0] = move;
1409 Color c = pos.side_to_move();
1410 Thread* thisThread = pos.this_thread();
1411 thisThread->history.update(c, move, bonus);
1412 update_cm_stats(ss, pos.moved_piece(move), to_sq(move), bonus);
1414 if ((ss-1)->counterMoves)
1416 Square prevSq = to_sq((ss-1)->currentMove);
1417 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1420 // Decrease all the other played quiet moves
1421 for (int i = 0; i < quietsCnt; ++i)
1423 thisThread->history.update(c, quiets[i], -bonus);
1424 update_cm_stats(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1429 // When playing with strength handicap, choose best move among a set of RootMoves
1430 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1432 Move Skill::pick_best(size_t multiPV) {
1434 const RootMoves& rootMoves = Threads.main()->rootMoves;
1435 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1437 // RootMoves are already sorted by score in descending order
1438 Value topScore = rootMoves[0].score;
1439 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1440 int weakness = 120 - 2 * level;
1441 int maxScore = -VALUE_INFINITE;
1443 // Choose best move. For each move score we add two terms, both dependent on
1444 // weakness. One is deterministic and bigger for weaker levels, and one is
1445 // random. Then we choose the move with the resulting highest score.
1446 for (size_t i = 0; i < multiPV; ++i)
1448 // This is our magic formula
1449 int push = ( weakness * int(topScore - rootMoves[i].score)
1450 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1452 if (rootMoves[i].score + push > maxScore)
1454 maxScore = rootMoves[i].score + push;
1455 best = rootMoves[i].pv[0];
1463 // check_time() is used to print debug info and, more importantly, to detect
1464 // when we are out of available time and thus stop the search.
1468 static TimePoint lastInfoTime = now();
1470 int elapsed = Time.elapsed();
1471 TimePoint tick = Limits.startTime + elapsed;
1473 if (tick - lastInfoTime >= 1000)
1475 lastInfoTime = tick;
1479 // An engine may not stop pondering until told so by the GUI
1483 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1484 || (Limits.movetime && elapsed >= Limits.movetime)
1485 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1486 Signals.stop = true;
1492 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1493 /// that all (if any) unsearched PV lines are sent using a previous search score.
1495 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1497 std::stringstream ss;
1498 int elapsed = Time.elapsed() + 1;
1499 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1500 size_t PVIdx = pos.this_thread()->PVIdx;
1501 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1502 uint64_t nodesSearched = Threads.nodes_searched();
1503 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1505 for (size_t i = 0; i < multiPV; ++i)
1507 bool updated = (i <= PVIdx);
1509 if (depth == ONE_PLY && !updated)
1512 Depth d = updated ? depth : depth - ONE_PLY;
1513 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1515 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1516 v = tb ? TB::Score : v;
1518 if (ss.rdbuf()->in_avail()) // Not at first line
1522 << " depth " << d / ONE_PLY
1523 << " seldepth " << pos.this_thread()->maxPly
1524 << " multipv " << i + 1
1525 << " score " << UCI::value(v);
1527 if (!tb && i == PVIdx)
1528 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1530 ss << " nodes " << nodesSearched
1531 << " nps " << nodesSearched * 1000 / elapsed;
1533 if (elapsed > 1000) // Earlier makes little sense
1534 ss << " hashfull " << TT.hashfull();
1536 ss << " tbhits " << tbHits
1537 << " time " << elapsed
1540 for (Move m : rootMoves[i].pv)
1541 ss << " " << UCI::move(m, pos.is_chess960());
1548 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1549 /// before exiting the search, for instance, in case we stop the search during a
1550 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1551 /// otherwise in case of 'ponder on' we have nothing to think on.
1553 bool RootMove::extract_ponder_from_tt(Position& pos) {
1558 assert(pv.size() == 1);
1563 pos.do_move(pv[0], st);
1564 TTEntry* tte = TT.probe(pos.key(), ttHit);
1568 Move m = tte->move(); // Local copy to be SMP safe
1569 if (MoveList<LEGAL>(pos).contains(m))
1573 pos.undo_move(pv[0]);
1574 return pv.size() > 1;
1577 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1580 UseRule50 = Options["Syzygy50MoveRule"];
1581 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1582 Cardinality = Options["SyzygyProbeLimit"];
1584 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1585 if (Cardinality > MaxCardinality)
1587 Cardinality = MaxCardinality;
1588 ProbeDepth = DEPTH_ZERO;
1591 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1594 // If the current root position is in the tablebases, then RootMoves
1595 // contains only moves that preserve the draw or the win.
1596 RootInTB = root_probe(pos, rootMoves, TB::Score);
1599 Cardinality = 0; // Do not probe tablebases during the search
1601 else // If DTZ tables are missing, use WDL tables as a fallback
1603 // Filter out moves that do not preserve the draw or the win.
1604 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1606 // Only probe during search if winning
1607 if (RootInTB && TB::Score <= VALUE_DRAW)
1611 if (RootInTB && !UseRule50)
1612 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1613 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1