2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2017 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
46 namespace Tablebases {
55 namespace TB = Tablebases;
59 using namespace Search;
63 // Different node types, used as a template parameter
64 enum NodeType { NonPV, PV };
66 // Sizes and phases of the skip-blocks, used for distributing search depths across the threads
67 const int skipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
68 const int skipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
70 // Razoring and futility margin based on depth
71 const int razor_margin[4] = { 483, 570, 603, 554 };
72 Value futility_margin(Depth d) { return Value(150 * d / ONE_PLY); }
74 // Futility and reductions lookup tables, initialized at startup
75 int FutilityMoveCounts[2][16]; // [improving][depth]
76 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
78 // 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_cm_stats(Stack* ss, Piece pc, Square s, int bonus);
155 void update_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
161 /// Search::init() is called during startup to initialize various lookup tables
163 void Search::init() {
165 for (int imp = 0; imp <= 1; ++imp)
166 for (int d = 1; d < 64; ++d)
167 for (int mc = 1; mc < 64; ++mc)
169 double r = log(d) * log(mc) / 1.95;
171 Reductions[NonPV][imp][d][mc] = int(std::round(r));
172 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
174 // Increase reduction for non-PV nodes when eval is not improving
175 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
176 Reductions[NonPV][imp][d][mc]++;
179 for (int d = 0; d < 16; ++d)
181 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
182 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
187 /// Search::clear() resets search state to zero, to obtain reproducible results
189 void Search::clear() {
193 for (Thread* th : Threads)
195 th->counterMoves.clear();
197 th->counterMoveHistory.clear();
198 th->counterMoveHistory[NO_PIECE][0].fill(CounterMovePruneThreshold-1);
199 th->resetCalls = true;
202 Threads.main()->previousScore = VALUE_INFINITE;
206 /// Search::perft() is our utility to verify move generation. All the leaf nodes
207 /// up to the given depth are generated and counted, and the sum is returned.
209 uint64_t Search::perft(Position& pos, Depth depth) {
212 uint64_t cnt, nodes = 0;
213 const bool leaf = (depth == 2 * ONE_PLY);
215 for (const auto& m : MoveList<LEGAL>(pos))
217 if (Root && depth <= ONE_PLY)
222 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
227 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
232 template uint64_t Search::perft<true>(Position&, Depth);
235 /// MainThread::search() is called by the main thread when the program receives
236 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
238 void MainThread::search() {
240 Color us = rootPos.side_to_move();
241 Time.init(Limits, us, rootPos.game_ply());
243 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
244 DrawValue[ us] = VALUE_DRAW - Value(contempt);
245 DrawValue[~us] = VALUE_DRAW + Value(contempt);
247 if (rootMoves.empty())
249 rootMoves.push_back(RootMove(MOVE_NONE));
250 sync_cout << "info depth 0 score "
251 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
256 for (Thread* th : Threads)
258 th->start_searching();
260 Thread::search(); // Let's start searching!
263 // When playing in 'nodes as time' mode, subtract the searched nodes from
264 // the available ones before exiting.
266 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
268 // When we reach the maximum depth, we can arrive here without a raise of
269 // Signals.stop. However, if we are pondering or in an infinite search,
270 // the UCI protocol states that we shouldn't print the best move before the
271 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
272 // until the GUI sends one of those commands (which also raises Signals.stop).
273 if (!Signals.stop && (Limits.ponder || Limits.infinite))
275 Signals.stopOnPonderhit = true;
279 // Stop the threads if not already stopped
282 // Wait until all threads have finished
283 for (Thread* th : Threads)
285 th->wait_for_search_finished();
287 // Check if there are threads with a better score than main thread
288 Thread* bestThread = this;
289 if ( !this->easyMovePlayed
290 && Options["MultiPV"] == 1
292 && !Skill(Options["Skill Level"]).enabled()
293 && rootMoves[0].pv[0] != MOVE_NONE)
295 for (Thread* th : Threads)
297 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
298 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
300 if (scoreDiff > 0 && depthDiff >= 0)
305 previousScore = bestThread->rootMoves[0].score;
307 // Send new PV when needed
308 if (bestThread != this)
309 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
311 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
313 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
314 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
316 std::cout << sync_endl;
320 /// Thread::search() is the main iterative deepening loop. It calls search()
321 /// repeatedly with increasing depth until the allocated thinking time has been
322 /// consumed, the user stops the search, or the maximum search depth is reached.
324 void Thread::search() {
326 Stack stack[MAX_PLY+7], *ss = stack+4; // To allow referencing (ss-4) and (ss+2)
327 Value bestValue, alpha, beta, delta;
328 Move easyMove = MOVE_NONE;
329 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
331 std::memset(ss-4, 0, 7 * sizeof(Stack));
332 for(int i = 4; i > 0; i--)
333 (ss-i)->counterMoves = &this->counterMoveHistory[NO_PIECE][0]; // Use as sentinel
335 bestValue = delta = alpha = -VALUE_INFINITE;
336 beta = VALUE_INFINITE;
337 completedDepth = DEPTH_ZERO;
341 easyMove = EasyMove.get(rootPos.key());
343 mainThread->easyMovePlayed = mainThread->failedLow = false;
344 mainThread->bestMoveChanges = 0;
348 size_t multiPV = Options["MultiPV"];
349 Skill skill(Options["Skill Level"]);
351 // When playing with strength handicap enable MultiPV search that we will
352 // use behind the scenes to retrieve a set of possible moves.
354 multiPV = std::max(multiPV, (size_t)4);
356 multiPV = std::min(multiPV, rootMoves.size());
358 // Iterative deepening loop until requested to stop or the target depth is reached
359 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
361 && (!Limits.depth || Threads.main()->rootDepth / ONE_PLY <= Limits.depth))
363 // Distribute search depths across the threads
366 int i = (idx - 1) % 20;
367 if (((rootDepth / ONE_PLY + rootPos.game_ply() + skipPhase[i]) / skipSize[i]) % 2)
371 // Age out PV variability metric
373 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
375 // Save the last iteration's scores before first PV line is searched and
376 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
377 for (RootMove& rm : rootMoves)
378 rm.previousScore = rm.score;
380 // MultiPV loop. We perform a full root search for each PV line
381 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
383 // Reset aspiration window starting size
384 if (rootDepth >= 5 * ONE_PLY)
387 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
388 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
391 // Start with a small aspiration window and, in the case of a fail
392 // high/low, re-search with a bigger window until we're not failing
396 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
398 // Bring the best move to the front. It is critical that sorting
399 // is done with a stable algorithm because all the values but the
400 // first and eventually the new best one are set to -VALUE_INFINITE
401 // and we want to keep the same order for all the moves except the
402 // new PV that goes to the front. Note that in case of MultiPV
403 // search the already searched PV lines are preserved.
404 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
406 // If search has been stopped, we break immediately. Sorting and
407 // writing PV back to TT is safe because RootMoves is still
408 // valid, although it refers to the previous iteration.
412 // When failing high/low give some update (without cluttering
413 // the UI) before a re-search.
416 && (bestValue <= alpha || bestValue >= beta)
417 && Time.elapsed() > 3000)
418 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
420 // In case of failing low/high increase aspiration window and
421 // re-search, otherwise exit the loop.
422 if (bestValue <= alpha)
424 beta = (alpha + beta) / 2;
425 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
429 mainThread->failedLow = true;
430 Signals.stopOnPonderhit = false;
433 else if (bestValue >= beta)
435 alpha = (alpha + beta) / 2;
436 beta = std::min(bestValue + delta, VALUE_INFINITE);
441 delta += delta / 4 + 5;
443 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
446 // Sort the PV lines searched so far and update the GUI
447 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
452 if (Signals.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000)
453 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
457 completedDepth = rootDepth;
462 // If skill level is enabled and time is up, pick a sub-optimal best move
463 if (skill.enabled() && skill.time_to_pick(rootDepth))
464 skill.pick_best(multiPV);
466 // Have we found a "mate in x"?
468 && bestValue >= VALUE_MATE_IN_MAX_PLY
469 && VALUE_MATE - bestValue <= 2 * Limits.mate)
472 // Do we have time for the next iteration? Can we stop searching now?
473 if (Limits.use_time_management())
475 if (!Signals.stop && !Signals.stopOnPonderhit)
477 // Stop the search if only one legal move is available, or if all
478 // of the available time has been used, or if we matched an easyMove
479 // from the previous search and just did a fast verification.
480 const int F[] = { mainThread->failedLow,
481 bestValue - mainThread->previousScore };
483 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
484 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
486 bool doEasyMove = rootMoves[0].pv[0] == easyMove
487 && mainThread->bestMoveChanges < 0.03
488 && Time.elapsed() > Time.optimum() * 5 / 44;
490 if ( rootMoves.size() == 1
491 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628
492 || (mainThread->easyMovePlayed = doEasyMove, doEasyMove))
494 // If we are allowed to ponder do not stop the search now but
495 // keep pondering until the GUI sends "ponderhit" or "stop".
497 Signals.stopOnPonderhit = true;
503 if (rootMoves[0].pv.size() >= 3)
504 EasyMove.update(rootPos, rootMoves[0].pv);
513 // Clear any candidate easy move that wasn't stable for the last search
514 // iterations; the second condition prevents consecutive fast moves.
515 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
518 // If skill level is enabled, swap best PV line with the sub-optimal one
520 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
521 rootMoves.end(), skill.best_move(multiPV)));
527 // search<>() is the main search function for both PV and non-PV nodes
529 template <NodeType NT>
530 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
532 const bool PvNode = NT == PV;
533 const bool rootNode = PvNode && (ss-1)->ply == 0;
535 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
536 assert(PvNode || (alpha == beta - 1));
537 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
538 assert(!(PvNode && cutNode));
539 assert(depth / ONE_PLY * ONE_PLY == depth);
541 Move pv[MAX_PLY+1], quietsSearched[64];
545 Move ttMove, move, excludedMove, bestMove;
546 Depth extension, newDepth;
547 Value bestValue, value, ttValue, eval;
548 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
549 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets;
551 int moveCount, quietCount;
553 // Step 1. Initialize node
554 Thread* thisThread = pos.this_thread();
555 inCheck = pos.checkers();
556 moveCount = quietCount = ss->moveCount = 0;
558 bestValue = -VALUE_INFINITE;
559 ss->ply = (ss-1)->ply + 1;
561 // Check for the available remaining time
562 if (thisThread->resetCalls.load(std::memory_order_relaxed))
564 thisThread->resetCalls = false;
565 // At low node count increase the checking rate to about 0.1% of nodes
566 // otherwise use a default value.
567 thisThread->callsCnt = Limits.nodes ? std::min((int64_t)4096, Limits.nodes / 1024)
571 if (--thisThread->callsCnt <= 0)
573 for (Thread* th : Threads)
574 th->resetCalls = true;
579 // Used to send selDepth info to GUI
580 if (PvNode && thisThread->maxPly < ss->ply)
581 thisThread->maxPly = ss->ply;
585 // Step 2. Check for aborted search and immediate draw
586 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
587 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
588 : DrawValue[pos.side_to_move()];
590 // Step 3. Mate distance pruning. Even if we mate at the next move our score
591 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
592 // a shorter mate was found upward in the tree then there is no need to search
593 // because we will never beat the current alpha. Same logic but with reversed
594 // signs applies also in the opposite condition of being mated instead of giving
595 // mate. In this case return a fail-high score.
596 alpha = std::max(mated_in(ss->ply), alpha);
597 beta = std::min(mate_in(ss->ply+1), beta);
602 assert(0 <= ss->ply && ss->ply < MAX_PLY);
604 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
605 ss->counterMoves = &thisThread->counterMoveHistory[NO_PIECE][0];
606 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
607 Square prevSq = to_sq((ss-1)->currentMove);
609 // Step 4. Transposition table lookup. We don't want the score of a partial
610 // search to overwrite a previous full search TT value, so we use a different
611 // position key in case of an excluded move.
612 excludedMove = ss->excludedMove;
613 posKey = pos.key() ^ Key(excludedMove);
614 tte = TT.probe(posKey, ttHit);
615 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
616 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
617 : ttHit ? tte->move() : MOVE_NONE;
619 // At non-PV nodes we check for an early TT cutoff
622 && tte->depth() >= depth
623 && ttValue != VALUE_NONE // Possible in case of TT access race
624 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
625 : (tte->bound() & BOUND_UPPER)))
627 // If ttMove is quiet, update move sorting heuristics on TT hit
632 if (!pos.capture_or_promotion(ttMove))
633 update_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
635 // Extra penalty for a quiet TT move in previous ply when it gets refuted
636 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
637 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
639 // Penalty for a quiet ttMove that fails low
640 else if (!pos.capture_or_promotion(ttMove))
642 int penalty = -stat_bonus(depth);
643 thisThread->history.update(pos.side_to_move(), ttMove, penalty);
644 update_cm_stats(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
650 // Step 4a. Tablebase probe
651 if (!rootNode && TB::Cardinality)
653 int piecesCount = pos.count<ALL_PIECES>();
655 if ( piecesCount <= TB::Cardinality
656 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
657 && pos.rule50_count() == 0
658 && !pos.can_castle(ANY_CASTLING))
661 TB::WDLScore v = Tablebases::probe_wdl(pos, &err);
663 if (err != TB::ProbeState::FAIL)
665 thisThread->tbHits++;
667 int drawScore = TB::UseRule50 ? 1 : 0;
669 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
670 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
671 : VALUE_DRAW + 2 * v * drawScore;
673 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
674 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
675 MOVE_NONE, VALUE_NONE, TT.generation());
682 // Step 5. Evaluate the position statically
685 ss->staticEval = eval = VALUE_NONE;
691 // Never assume anything on values stored in TT
692 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
693 eval = ss->staticEval = evaluate(pos);
695 // Can ttValue be used as a better position evaluation?
696 if (ttValue != VALUE_NONE)
697 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
702 eval = ss->staticEval =
703 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
704 : -(ss-1)->staticEval + 2 * Eval::Tempo;
706 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
707 ss->staticEval, TT.generation());
710 if (skipEarlyPruning)
713 // Step 6. Razoring (skipped when in check)
715 && depth < 4 * ONE_PLY
716 && eval + razor_margin[depth / ONE_PLY] <= alpha)
718 if (depth <= ONE_PLY)
719 return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
721 Value ralpha = alpha - razor_margin[depth / ONE_PLY];
722 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1);
727 // Step 7. Futility pruning: child node (skipped when in check)
729 && depth < 7 * ONE_PLY
730 && eval - futility_margin(depth) >= beta
731 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
732 && pos.non_pawn_material(pos.side_to_move()))
735 // Step 8. Null move search with verification search (is omitted in PV nodes)
738 && (ss->staticEval >= beta - 35 * (depth / ONE_PLY - 6) || depth >= 13 * ONE_PLY)
739 && pos.non_pawn_material(pos.side_to_move()))
742 assert(eval - beta >= 0);
744 // Null move dynamic reduction based on depth and value
745 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
747 ss->currentMove = MOVE_NULL;
748 ss->counterMoves = &thisThread->counterMoveHistory[NO_PIECE][0];
750 pos.do_null_move(st);
751 Value nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1)
752 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
753 pos.undo_null_move();
755 if (nullValue >= beta)
757 // Do not return unproven mate scores
758 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
761 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
764 // Do verification search at high depths
765 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
766 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
773 // Step 9. ProbCut (skipped when in check)
774 // If we have a good enough capture and a reduced search returns a value
775 // much above beta, we can (almost) safely prune the previous move.
777 && depth >= 5 * ONE_PLY
778 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
780 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
781 Depth rdepth = depth - 4 * ONE_PLY;
783 assert(rdepth >= ONE_PLY);
784 assert(is_ok((ss-1)->currentMove));
786 MovePicker mp(pos, ttMove, rbeta - ss->staticEval);
788 while ((move = mp.next_move()) != MOVE_NONE)
791 ss->currentMove = move;
792 ss->counterMoves = &thisThread->counterMoveHistory[pos.moved_piece(move)][to_sq(move)];
794 pos.do_move(move, st);
795 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode, false);
802 // Step 10. Internal iterative deepening (skipped when in check)
803 if ( depth >= 6 * ONE_PLY
805 && (PvNode || ss->staticEval + 256 >= beta))
807 Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
808 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
810 tte = TT.probe(posKey, ttHit);
811 ttMove = ttHit ? tte->move() : MOVE_NONE;
814 moves_loop: // When in check search starts from here
816 const CounterMoveStats& cmh = *(ss-1)->counterMoves;
817 const CounterMoveStats& fmh = *(ss-2)->counterMoves;
818 const CounterMoveStats& fm2 = *(ss-4)->counterMoves;
820 MovePicker mp(pos, ttMove, depth, ss);
821 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
822 improving = ss->staticEval >= (ss-2)->staticEval
823 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
824 ||(ss-2)->staticEval == VALUE_NONE;
826 singularExtensionNode = !rootNode
827 && depth >= 8 * ONE_PLY
828 && ttMove != MOVE_NONE
829 && ttValue != VALUE_NONE
830 && !excludedMove // Recursive singular search is not allowed
831 && (tte->bound() & BOUND_LOWER)
832 && tte->depth() >= depth - 3 * ONE_PLY;
835 // Step 11. Loop through moves
836 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
837 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
841 if (move == excludedMove)
844 // At root obey the "searchmoves" option and skip moves not listed in Root
845 // Move List. As a consequence any illegal move is also skipped. In MultiPV
846 // mode we also skip PV moves which have been already searched.
847 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
848 thisThread->rootMoves.end(), move))
851 ss->moveCount = ++moveCount;
853 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
854 sync_cout << "info depth " << depth / ONE_PLY
855 << " currmove " << UCI::move(move, pos.is_chess960())
856 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
859 (ss+1)->pv = nullptr;
861 extension = DEPTH_ZERO;
862 captureOrPromotion = pos.capture_or_promotion(move);
863 moved_piece = pos.moved_piece(move);
865 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
866 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
867 : pos.gives_check(move);
869 moveCountPruning = depth < 16 * ONE_PLY
870 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
872 // Step 12. Singular and Gives Check Extensions
874 // Singular extension search. If all moves but one fail low on a search of
875 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
876 // is singular and should be extended. To verify this we do a reduced search
877 // on all the other moves but the ttMove and if the result is lower than
878 // ttValue minus a margin then we extend the ttMove.
879 if ( singularExtensionNode
883 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
884 Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
885 ss->excludedMove = move;
886 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode, true);
887 ss->excludedMove = MOVE_NONE;
894 && pos.see_ge(move, VALUE_ZERO))
897 // Calculate new depth for this move
898 newDepth = depth - ONE_PLY + extension;
900 // Step 13. Pruning at shallow depth
902 && pos.non_pawn_material(pos.side_to_move())
903 && bestValue > VALUE_MATED_IN_MAX_PLY)
905 if ( !captureOrPromotion
907 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= 5000))
909 // Move count based pruning
910 if (moveCountPruning) {
915 // Reduced depth of the next LMR search
916 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
918 // Countermoves based pruning
920 && (cmh[moved_piece][to_sq(move)] < CounterMovePruneThreshold)
921 && (fmh[moved_piece][to_sq(move)] < CounterMovePruneThreshold))
924 // Futility pruning: parent node
927 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
930 // Prune moves with negative SEE
932 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
935 else if ( depth < 7 * ONE_PLY
937 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
941 // Speculative prefetch as early as possible
942 prefetch(TT.first_entry(pos.key_after(move)));
944 // Check for legality just before making the move
945 if (!rootNode && !pos.legal(move))
947 ss->moveCount = --moveCount;
951 // Update the current move (this must be done after singular extension search)
952 ss->currentMove = move;
953 ss->counterMoves = &thisThread->counterMoveHistory[moved_piece][to_sq(move)];
955 // Step 14. Make the move
956 pos.do_move(move, st, givesCheck);
958 // Step 15. Reduced depth search (LMR). If the move fails high it will be
959 // re-searched at full depth.
960 if ( depth >= 3 * ONE_PLY
962 && (!captureOrPromotion || moveCountPruning))
964 Depth r = reduction<PvNode>(improving, depth, moveCount);
966 if (captureOrPromotion)
967 r -= r ? ONE_PLY : DEPTH_ZERO;
970 // Increase reduction for cut nodes
974 // Decrease reduction for moves that escape a capture. Filter out
975 // castling moves, because they are coded as "king captures rook" and
976 // hence break make_move().
977 else if ( type_of(move) == NORMAL
978 && !pos.see_ge(make_move(to_sq(move), from_sq(move)), VALUE_ZERO))
981 ss->history = cmh[moved_piece][to_sq(move)]
982 + fmh[moved_piece][to_sq(move)]
983 + fm2[moved_piece][to_sq(move)]
984 + thisThread->history.get(~pos.side_to_move(), move)
985 - 4000; // Correction factor
987 // Decrease/increase reduction by comparing opponent's stat score
988 if (ss->history > 0 && (ss-1)->history < 0)
991 else if (ss->history < 0 && (ss-1)->history > 0)
994 // Decrease/increase reduction for moves with a good/bad history
995 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->history / 20000) * ONE_PLY);
998 Depth d = std::max(newDepth - r, ONE_PLY);
1000 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
1002 doFullDepthSearch = (value > alpha && d != newDepth);
1005 doFullDepthSearch = !PvNode || moveCount > 1;
1007 // Step 16. Full depth search when LMR is skipped or fails high
1008 if (doFullDepthSearch)
1009 value = newDepth < ONE_PLY ?
1010 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha)
1011 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha)
1012 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
1014 // For PV nodes only, do a full PV search on the first move or after a fail
1015 // high (in the latter case search only if value < beta), otherwise let the
1016 // parent node fail low with value <= alpha and try another move.
1017 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1020 (ss+1)->pv[0] = MOVE_NONE;
1022 value = newDepth < ONE_PLY ?
1023 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha)
1024 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha)
1025 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1028 // Step 17. Undo move
1029 pos.undo_move(move);
1031 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1033 // Step 18. Check for a new best move
1034 // Finished searching the move. If a stop occurred, the return value of
1035 // the search cannot be trusted, and we return immediately without
1036 // updating best move, PV and TT.
1037 if (Signals.stop.load(std::memory_order_relaxed))
1042 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1043 thisThread->rootMoves.end(), move);
1045 // PV move or new best move ?
1046 if (moveCount == 1 || value > alpha)
1053 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1054 rm.pv.push_back(*m);
1056 // We record how often the best move has been changed in each
1057 // iteration. This information is used for time management: When
1058 // the best move changes frequently, we allocate some more time.
1059 if (moveCount > 1 && thisThread == Threads.main())
1060 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1063 // All other moves but the PV are set to the lowest value: this is
1064 // not a problem when sorting because the sort is stable and the
1065 // move position in the list is preserved - just the PV is pushed up.
1066 rm.score = -VALUE_INFINITE;
1069 if (value > bestValue)
1077 if (PvNode && !rootNode) // Update pv even in fail-high case
1078 update_pv(ss->pv, move, (ss+1)->pv);
1080 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1084 assert(value >= beta); // Fail high
1090 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1091 quietsSearched[quietCount++] = move;
1094 // The following condition would detect a stop only after move loop has been
1095 // completed. But in this case bestValue is valid because we have fully
1096 // searched our subtree, and we can anyhow save the result in TT.
1102 // Step 20. Check for mate and stalemate
1103 // All legal moves have been searched and if there are no legal moves, it
1104 // must be a mate or a stalemate. If we are in a singular extension search then
1105 // return a fail low score.
1107 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1110 bestValue = excludedMove ? alpha
1111 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1115 // Quiet best move: update move sorting heuristics
1116 if (!pos.capture_or_promotion(bestMove))
1117 update_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1119 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1120 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1121 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1123 // Bonus for prior countermove that caused the fail low
1124 else if ( depth >= 3 * ONE_PLY
1125 && !pos.captured_piece()
1126 && is_ok((ss-1)->currentMove))
1127 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1130 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1131 bestValue >= beta ? BOUND_LOWER :
1132 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1133 depth, bestMove, ss->staticEval, TT.generation());
1135 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1141 // qsearch() is the quiescence search function, which is called by the main
1142 // search function with depth zero, or recursively with depth less than ONE_PLY.
1144 template <NodeType NT, bool InCheck>
1145 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1147 const bool PvNode = NT == PV;
1149 assert(InCheck == !!pos.checkers());
1150 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1151 assert(PvNode || (alpha == beta - 1));
1152 assert(depth <= DEPTH_ZERO);
1153 assert(depth / ONE_PLY * ONE_PLY == depth);
1159 Move ttMove, move, bestMove;
1160 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1161 bool ttHit, givesCheck, evasionPrunable;
1166 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1168 ss->pv[0] = MOVE_NONE;
1171 ss->currentMove = bestMove = MOVE_NONE;
1172 ss->ply = (ss-1)->ply + 1;
1174 // Check for an instant draw or if the maximum ply has been reached
1175 if (pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
1176 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1177 : DrawValue[pos.side_to_move()];
1179 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1181 // Decide whether or not to include checks: this fixes also the type of
1182 // TT entry depth that we are going to use. Note that in qsearch we use
1183 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1184 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1185 : DEPTH_QS_NO_CHECKS;
1187 // Transposition table lookup
1189 tte = TT.probe(posKey, ttHit);
1190 ttMove = ttHit ? tte->move() : MOVE_NONE;
1191 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1195 && tte->depth() >= ttDepth
1196 && ttValue != VALUE_NONE // Only in case of TT access race
1197 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1198 : (tte->bound() & BOUND_UPPER)))
1201 // Evaluate the position statically
1204 ss->staticEval = VALUE_NONE;
1205 bestValue = futilityBase = -VALUE_INFINITE;
1211 // Never assume anything on values stored in TT
1212 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1213 ss->staticEval = bestValue = evaluate(pos);
1215 // Can ttValue be used as a better position evaluation?
1216 if (ttValue != VALUE_NONE)
1217 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1218 bestValue = ttValue;
1221 ss->staticEval = bestValue =
1222 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1223 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1225 // Stand pat. Return immediately if static value is at least beta
1226 if (bestValue >= beta)
1229 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1230 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1235 if (PvNode && bestValue > alpha)
1238 futilityBase = bestValue + 128;
1241 // Initialize a MovePicker object for the current position, and prepare
1242 // to search the moves. Because the depth is <= 0 here, only captures,
1243 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1245 MovePicker mp(pos, ttMove, depth, to_sq((ss-1)->currentMove));
1247 // Loop through the moves until no moves remain or a beta cutoff occurs
1248 while ((move = mp.next_move()) != MOVE_NONE)
1250 assert(is_ok(move));
1252 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
1253 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
1254 : pos.gives_check(move);
1259 && futilityBase > -VALUE_KNOWN_WIN
1260 && !pos.advanced_pawn_push(move))
1262 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1264 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1266 if (futilityValue <= alpha)
1268 bestValue = std::max(bestValue, futilityValue);
1272 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1274 bestValue = std::max(bestValue, futilityBase);
1279 // Detect non-capture evasions that are candidates to be pruned
1280 evasionPrunable = InCheck
1281 && bestValue > VALUE_MATED_IN_MAX_PLY
1282 && !pos.capture(move);
1284 // Don't search moves with negative SEE values
1285 if ( (!InCheck || evasionPrunable)
1286 && type_of(move) != PROMOTION
1287 && !pos.see_ge(move, VALUE_ZERO))
1290 // Speculative prefetch as early as possible
1291 prefetch(TT.first_entry(pos.key_after(move)));
1293 // Check for legality just before making the move
1294 if (!pos.legal(move))
1297 ss->currentMove = move;
1299 // Make and search the move
1300 pos.do_move(move, st, givesCheck);
1301 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1302 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1303 pos.undo_move(move);
1305 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1307 // Check for a new best move
1308 if (value > bestValue)
1314 if (PvNode) // Update pv even in fail-high case
1315 update_pv(ss->pv, move, (ss+1)->pv);
1317 if (PvNode && value < beta) // Update alpha here!
1324 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1325 ttDepth, move, ss->staticEval, TT.generation());
1333 // All legal moves have been searched. A special case: If we're in check
1334 // and no legal moves were found, it is checkmate.
1335 if (InCheck && bestValue == -VALUE_INFINITE)
1336 return mated_in(ss->ply); // Plies to mate from the root
1338 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1339 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1340 ttDepth, bestMove, ss->staticEval, TT.generation());
1342 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1348 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1349 // "plies to mate from the current position". Non-mate scores are unchanged.
1350 // The function is called before storing a value in the transposition table.
1352 Value value_to_tt(Value v, int ply) {
1354 assert(v != VALUE_NONE);
1356 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1357 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1361 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1362 // from the transposition table (which refers to the plies to mate/be mated
1363 // from current position) to "plies to mate/be mated from the root".
1365 Value value_from_tt(Value v, int ply) {
1367 return v == VALUE_NONE ? VALUE_NONE
1368 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1369 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1373 // update_pv() adds current move and appends child pv[]
1375 void update_pv(Move* pv, Move move, Move* childPv) {
1377 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1383 // update_cm_stats() updates countermove and follow-up move history
1385 void update_cm_stats(Stack* ss, Piece pc, Square s, int bonus) {
1387 for (int i : {1, 2, 4})
1388 if (is_ok((ss-i)->currentMove))
1389 (ss-i)->counterMoves->update(pc, s, bonus);
1393 // update_stats() updates move sorting heuristics when a new quiet best move is found
1395 void update_stats(const Position& pos, Stack* ss, Move move,
1396 Move* quiets, int quietsCnt, int bonus) {
1398 if (ss->killers[0] != move)
1400 ss->killers[1] = ss->killers[0];
1401 ss->killers[0] = move;
1404 Color c = pos.side_to_move();
1405 Thread* thisThread = pos.this_thread();
1406 thisThread->history.update(c, move, bonus);
1407 update_cm_stats(ss, pos.moved_piece(move), to_sq(move), bonus);
1409 if (is_ok((ss-1)->currentMove))
1411 Square prevSq = to_sq((ss-1)->currentMove);
1412 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1415 // Decrease all the other played quiet moves
1416 for (int i = 0; i < quietsCnt; ++i)
1418 thisThread->history.update(c, quiets[i], -bonus);
1419 update_cm_stats(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1424 // When playing with strength handicap, choose best move among a set of RootMoves
1425 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1427 Move Skill::pick_best(size_t multiPV) {
1429 const RootMoves& rootMoves = Threads.main()->rootMoves;
1430 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1432 // RootMoves are already sorted by score in descending order
1433 Value topScore = rootMoves[0].score;
1434 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1435 int weakness = 120 - 2 * level;
1436 int maxScore = -VALUE_INFINITE;
1438 // Choose best move. For each move score we add two terms, both dependent on
1439 // weakness. One is deterministic and bigger for weaker levels, and one is
1440 // random. Then we choose the move with the resulting highest score.
1441 for (size_t i = 0; i < multiPV; ++i)
1443 // This is our magic formula
1444 int push = ( weakness * int(topScore - rootMoves[i].score)
1445 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1447 if (rootMoves[i].score + push > maxScore)
1449 maxScore = rootMoves[i].score + push;
1450 best = rootMoves[i].pv[0];
1458 // check_time() is used to print debug info and, more importantly, to detect
1459 // when we are out of available time and thus stop the search.
1463 static TimePoint lastInfoTime = now();
1465 int elapsed = Time.elapsed();
1466 TimePoint tick = Limits.startTime + elapsed;
1468 if (tick - lastInfoTime >= 1000)
1470 lastInfoTime = tick;
1474 // An engine may not stop pondering until told so by the GUI
1478 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1479 || (Limits.movetime && elapsed >= Limits.movetime)
1480 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1481 Signals.stop = true;
1487 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1488 /// that all (if any) unsearched PV lines are sent using a previous search score.
1490 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1492 std::stringstream ss;
1493 int elapsed = Time.elapsed() + 1;
1494 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1495 size_t PVIdx = pos.this_thread()->PVIdx;
1496 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1497 uint64_t nodesSearched = Threads.nodes_searched();
1498 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1500 for (size_t i = 0; i < multiPV; ++i)
1502 bool updated = (i <= PVIdx);
1504 if (depth == ONE_PLY && !updated)
1507 Depth d = updated ? depth : depth - ONE_PLY;
1508 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1510 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1511 v = tb ? TB::Score : v;
1513 if (ss.rdbuf()->in_avail()) // Not at first line
1517 << " depth " << d / ONE_PLY
1518 << " seldepth " << pos.this_thread()->maxPly
1519 << " multipv " << i + 1
1520 << " score " << UCI::value(v);
1522 if (!tb && i == PVIdx)
1523 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1525 ss << " nodes " << nodesSearched
1526 << " nps " << nodesSearched * 1000 / elapsed;
1528 if (elapsed > 1000) // Earlier makes little sense
1529 ss << " hashfull " << TT.hashfull();
1531 ss << " tbhits " << tbHits
1532 << " time " << elapsed
1535 for (Move m : rootMoves[i].pv)
1536 ss << " " << UCI::move(m, pos.is_chess960());
1543 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1544 /// before exiting the search, for instance, in case we stop the search during a
1545 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1546 /// otherwise in case of 'ponder on' we have nothing to think on.
1548 bool RootMove::extract_ponder_from_tt(Position& pos) {
1553 assert(pv.size() == 1);
1558 pos.do_move(pv[0], st);
1559 TTEntry* tte = TT.probe(pos.key(), ttHit);
1563 Move m = tte->move(); // Local copy to be SMP safe
1564 if (MoveList<LEGAL>(pos).contains(m))
1568 pos.undo_move(pv[0]);
1569 return pv.size() > 1;
1572 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1575 UseRule50 = Options["Syzygy50MoveRule"];
1576 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1577 Cardinality = Options["SyzygyProbeLimit"];
1579 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1580 if (Cardinality > MaxCardinality)
1582 Cardinality = MaxCardinality;
1583 ProbeDepth = DEPTH_ZERO;
1586 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1589 // If the current root position is in the tablebases, then RootMoves
1590 // contains only moves that preserve the draw or the win.
1591 RootInTB = root_probe(pos, rootMoves, TB::Score);
1594 Cardinality = 0; // Do not probe tablebases during the search
1596 else // If DTZ tables are missing, use WDL tables as a fallback
1598 // Filter out moves that do not preserve the draw or the win.
1599 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1601 // Only probe during search if winning
1602 if (RootInTB && TB::Score <= VALUE_DRAW)
1606 if (RootInTB && !UseRule50)
1607 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1608 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1