2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2017 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
54 namespace TB = Tablebases;
58 using namespace Search;
62 // Different node types, used as a template parameter
63 enum NodeType { NonPV, PV };
65 // Sizes and phases of the skip-blocks, used for distributing search depths across the threads
66 const int skipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
67 const int skipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
69 // Razoring and futility margin based on depth
70 // razor_margin[0] is unused as long as depth >= ONE_PLY in search
71 const int razor_margin[] = { 0, 570, 603, 554 };
72 Value futility_margin(Depth d) { return Value(150 * d / ONE_PLY); }
74 // Futility and reductions lookup tables, initialized at startup
75 int FutilityMoveCounts[2][16]; // [improving][depth]
76 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
78 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
79 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
82 // History and stats update bonus, based on depth
83 int stat_bonus(Depth depth) {
84 int d = depth / ONE_PLY;
85 return d > 17 ? 0 : d * d + 2 * d - 2;
88 // Skill structure is used to implement strength limit
90 explicit Skill(int l) : level(l) {}
91 bool enabled() const { return level < 20; }
92 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
93 Move pick_best(size_t multiPV);
96 Move best = MOVE_NONE;
99 Value DrawValue[COLOR_NB];
101 template <NodeType NT>
102 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning);
104 template <NodeType NT, bool InCheck>
105 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
107 Value value_to_tt(Value v, int ply);
108 Value value_from_tt(Value v, int ply);
109 void update_pv(Move* pv, Move move, Move* childPv);
110 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
111 void update_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
112 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCnt, int bonus);
113 bool pv_is_draw(Position& pos);
115 // perft() is our utility to verify move generation. All the leaf nodes up
116 // to the given depth are generated and counted, and the sum is returned.
118 uint64_t perft(Position& pos, Depth depth) {
121 uint64_t cnt, nodes = 0;
122 const bool leaf = (depth == 2 * ONE_PLY);
124 for (const auto& m : MoveList<LEGAL>(pos))
126 if (Root && depth <= ONE_PLY)
131 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
136 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
144 /// Search::init() is called during startup to initialize various lookup tables
146 void Search::init() {
148 for (int imp = 0; imp <= 1; ++imp)
149 for (int d = 1; d < 64; ++d)
150 for (int mc = 1; mc < 64; ++mc)
152 double r = log(d) * log(mc) / 1.95;
154 Reductions[NonPV][imp][d][mc] = int(std::round(r));
155 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
157 // Increase reduction for non-PV nodes when eval is not improving
158 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
159 Reductions[NonPV][imp][d][mc]++;
162 for (int d = 0; d < 16; ++d)
164 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
165 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
170 /// Search::clear() resets search state to its initial value
172 void Search::clear() {
174 Threads.main()->wait_for_search_finished();
176 Time.availableNodes = 0;
179 for (Thread* th : Threads)
182 Threads.main()->callsCnt = 0;
183 Threads.main()->previousScore = VALUE_INFINITE;
184 Threads.main()->previousTimeReduction = 1;
188 /// MainThread::search() is called by the main thread when the program receives
189 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
191 void MainThread::search() {
195 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
196 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
200 Color us = rootPos.side_to_move();
201 Time.init(Limits, us, rootPos.game_ply());
204 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
205 DrawValue[ us] = VALUE_DRAW - Value(contempt);
206 DrawValue[~us] = VALUE_DRAW + Value(contempt);
208 if (rootMoves.empty())
210 rootMoves.emplace_back(MOVE_NONE);
211 sync_cout << "info depth 0 score "
212 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
217 for (Thread* th : Threads)
219 th->start_searching();
221 Thread::search(); // Let's start searching!
224 // When we reach the maximum depth, we can arrive here without a raise of
225 // Threads.stop. However, if we are pondering or in an infinite search,
226 // the UCI protocol states that we shouldn't print the best move before the
227 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
228 // until the GUI sends one of those commands (which also raises Threads.stop).
229 Threads.stopOnPonderhit = true;
231 while (!Threads.stop && (Threads.ponder || Limits.infinite))
232 {} // Busy wait for a stop or a ponder reset
234 // Stop the threads if not already stopped (also raise the stop if
235 // "ponderhit" just reset Threads.ponder).
238 // Wait until all threads have finished
239 for (Thread* th : Threads)
241 th->wait_for_search_finished();
243 // When playing in 'nodes as time' mode, subtract the searched nodes from
244 // the available ones before exiting.
246 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
248 // Check if there are threads with a better score than main thread
249 Thread* bestThread = this;
250 if ( Options["MultiPV"] == 1
252 && !Skill(Options["Skill Level"]).enabled()
253 && rootMoves[0].pv[0] != MOVE_NONE)
255 for (Thread* th : Threads)
257 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
258 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
260 // Select the thread with the best score, always if it is a mate
262 && (depthDiff >= 0 || th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY))
267 previousScore = bestThread->rootMoves[0].score;
269 // Send new PV when needed
270 if (bestThread != this)
271 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
273 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
275 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
276 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
278 std::cout << sync_endl;
282 /// Thread::search() is the main iterative deepening loop. It calls search()
283 /// repeatedly with increasing depth until the allocated thinking time has been
284 /// consumed, the user stops the search, or the maximum search depth is reached.
286 void Thread::search() {
288 Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
289 Value bestValue, alpha, beta, delta;
290 Move lastBestMove = MOVE_NONE;
291 Depth lastBestMoveDepth = DEPTH_ZERO;
292 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
293 double timeReduction = 1.0;
295 std::memset(ss-4, 0, 7 * sizeof(Stack));
296 for (int i = 4; i > 0; i--)
297 (ss-i)->contHistory = &this->contHistory[NO_PIECE][0]; // Use as sentinel
299 bestValue = delta = alpha = -VALUE_INFINITE;
300 beta = VALUE_INFINITE;
304 mainThread->failedLow = false;
305 mainThread->bestMoveChanges = 0;
308 size_t multiPV = Options["MultiPV"];
309 Skill skill(Options["Skill Level"]);
311 // When playing with strength handicap enable MultiPV search that we will
312 // use behind the scenes to retrieve a set of possible moves.
314 multiPV = std::max(multiPV, (size_t)4);
316 multiPV = std::min(multiPV, rootMoves.size());
318 // Iterative deepening loop until requested to stop or the target depth is reached
319 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
321 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
323 // Distribute search depths across the threads
326 int i = (idx - 1) % 20;
327 if (((rootDepth / ONE_PLY + rootPos.game_ply() + skipPhase[i]) / skipSize[i]) % 2)
331 // Age out PV variability metric
333 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
335 // Save the last iteration's scores before first PV line is searched and
336 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
337 for (RootMove& rm : rootMoves)
338 rm.previousScore = rm.score;
340 // MultiPV loop. We perform a full root search for each PV line
341 for (PVIdx = 0; PVIdx < multiPV && !Threads.stop; ++PVIdx)
343 // Reset UCI info selDepth for each depth and each PV line
346 // Reset aspiration window starting size
347 if (rootDepth >= 5 * ONE_PLY)
350 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
351 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
354 // Start with a small aspiration window and, in the case of a fail
355 // high/low, re-search with a bigger window until we're not failing
359 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
361 // Bring the best move to the front. It is critical that sorting
362 // is done with a stable algorithm because all the values but the
363 // first and eventually the new best one are set to -VALUE_INFINITE
364 // and we want to keep the same order for all the moves except the
365 // new PV that goes to the front. Note that in case of MultiPV
366 // search the already searched PV lines are preserved.
367 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
369 // If search has been stopped, we break immediately. Sorting and
370 // writing PV back to TT is safe because RootMoves is still
371 // valid, although it refers to the previous iteration.
375 // When failing high/low give some update (without cluttering
376 // the UI) before a re-search.
379 && (bestValue <= alpha || bestValue >= beta)
380 && Time.elapsed() > 3000)
381 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
383 // In case of failing low/high increase aspiration window and
384 // re-search, otherwise exit the loop.
385 if (bestValue <= alpha)
387 beta = (alpha + beta) / 2;
388 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
392 mainThread->failedLow = true;
393 Threads.stopOnPonderhit = false;
396 else if (bestValue >= beta)
397 beta = std::min(bestValue + delta, VALUE_INFINITE);
401 delta += delta / 4 + 5;
403 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
406 // Sort the PV lines searched so far and update the GUI
407 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
410 && (Threads.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000))
411 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
415 completedDepth = rootDepth;
417 if (rootMoves[0].pv[0] != lastBestMove) {
418 lastBestMove = rootMoves[0].pv[0];
419 lastBestMoveDepth = rootDepth;
422 // Have we found a "mate in x"?
424 && bestValue >= VALUE_MATE_IN_MAX_PLY
425 && VALUE_MATE - bestValue <= 2 * Limits.mate)
431 // If skill level is enabled and time is up, pick a sub-optimal best move
432 if (skill.enabled() && skill.time_to_pick(rootDepth))
433 skill.pick_best(multiPV);
435 // Do we have time for the next iteration? Can we stop searching now?
436 if (Limits.use_time_management())
438 if (!Threads.stop && !Threads.stopOnPonderhit)
440 // Stop the search if only one legal move is available, or if all
441 // of the available time has been used
442 const int F[] = { mainThread->failedLow,
443 bestValue - mainThread->previousScore };
444 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
446 Color us = rootPos.side_to_move();
447 bool thinkHard = DrawValue[us] == bestValue
448 && Limits.time[us] - Time.elapsed() > Limits.time[~us]
449 && ::pv_is_draw(rootPos);
451 double unstablePvFactor = 1 + mainThread->bestMoveChanges + thinkHard;
453 // if the bestMove is stable over several iterations, reduce time for this move,
454 // the longer the move has been stable, the more.
455 // Use part of the gained time from a previous stable move for the current move.
457 for (int i : {3, 4, 5})
458 if (lastBestMoveDepth * i < completedDepth && !thinkHard)
459 timeReduction *= 1.3;
460 unstablePvFactor *= std::pow(mainThread->previousTimeReduction, 0.51) / timeReduction;
462 if ( rootMoves.size() == 1
463 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628)
465 // If we are allowed to ponder do not stop the search now but
466 // keep pondering until the GUI sends "ponderhit" or "stop".
468 Threads.stopOnPonderhit = true;
479 mainThread->previousTimeReduction = timeReduction;
481 // If skill level is enabled, swap best PV line with the sub-optimal one
483 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
484 skill.best ? skill.best : skill.pick_best(multiPV)));
490 // search<>() is the main search function for both PV and non-PV nodes
492 template <NodeType NT>
493 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
495 const bool PvNode = NT == PV;
496 const bool rootNode = PvNode && ss->ply == 0;
498 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
499 assert(PvNode || (alpha == beta - 1));
500 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
501 assert(!(PvNode && cutNode));
502 assert(depth / ONE_PLY * ONE_PLY == depth);
504 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
508 Move ttMove, move, excludedMove, bestMove;
509 Depth extension, newDepth;
510 Value bestValue, value, ttValue, eval;
511 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
512 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
514 int moveCount, captureCount, quietCount;
516 // Step 1. Initialize node
517 Thread* thisThread = pos.this_thread();
518 inCheck = pos.checkers();
519 moveCount = captureCount = quietCount = ss->moveCount = 0;
521 bestValue = -VALUE_INFINITE;
523 // Check for the available remaining time
524 if (thisThread == Threads.main())
525 static_cast<MainThread*>(thisThread)->check_time();
527 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
528 if (PvNode && thisThread->selDepth < ss->ply + 1)
529 thisThread->selDepth = ss->ply + 1;
533 // Step 2. Check for aborted search and immediate draw
534 if (Threads.stop.load(std::memory_order_relaxed) || pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
535 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
536 : DrawValue[pos.side_to_move()];
538 // Step 3. Mate distance pruning. Even if we mate at the next move our score
539 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
540 // a shorter mate was found upward in the tree then there is no need to search
541 // because we will never beat the current alpha. Same logic but with reversed
542 // signs applies also in the opposite condition of being mated instead of giving
543 // mate. In this case return a fail-high score.
544 alpha = std::max(mated_in(ss->ply), alpha);
545 beta = std::min(mate_in(ss->ply+1), beta);
550 assert(0 <= ss->ply && ss->ply < MAX_PLY);
552 (ss+1)->ply = ss->ply + 1;
553 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
554 ss->contHistory = &thisThread->contHistory[NO_PIECE][0];
555 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
556 Square prevSq = to_sq((ss-1)->currentMove);
558 // Step 4. Transposition table lookup. We don't want the score of a partial
559 // search to overwrite a previous full search TT value, so we use a different
560 // position key in case of an excluded move.
561 excludedMove = ss->excludedMove;
562 posKey = pos.key() ^ Key(excludedMove);
563 tte = TT.probe(posKey, ttHit);
564 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
565 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
566 : ttHit ? tte->move() : MOVE_NONE;
568 // At non-PV nodes we check for an early TT cutoff
571 && tte->depth() >= depth
572 && ttValue != VALUE_NONE // Possible in case of TT access race
573 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
574 : (tte->bound() & BOUND_UPPER)))
576 // If ttMove is quiet, update move sorting heuristics on TT hit
581 if (!pos.capture_or_promotion(ttMove))
582 update_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
584 // Extra penalty for a quiet TT move in previous ply when it gets refuted
585 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
586 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
588 // Penalty for a quiet ttMove that fails low
589 else if (!pos.capture_or_promotion(ttMove))
591 int penalty = -stat_bonus(depth);
592 thisThread->mainHistory.update(pos.side_to_move(), ttMove, penalty);
593 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
599 // Step 4a. Tablebase probe
600 if (!rootNode && TB::Cardinality)
602 int piecesCount = pos.count<ALL_PIECES>();
604 if ( piecesCount <= TB::Cardinality
605 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
606 && pos.rule50_count() == 0
607 && !pos.can_castle(ANY_CASTLING))
610 TB::WDLScore v = Tablebases::probe_wdl(pos, &err);
612 if (err != TB::ProbeState::FAIL)
614 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
616 int drawScore = TB::UseRule50 ? 1 : 0;
618 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
619 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
620 : VALUE_DRAW + 2 * v * drawScore;
622 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
623 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
624 MOVE_NONE, VALUE_NONE, TT.generation());
631 // Step 5. Evaluate the position statically
634 ss->staticEval = eval = VALUE_NONE;
640 // Never assume anything on values stored in TT
641 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
642 eval = ss->staticEval = evaluate(pos);
644 // Can ttValue be used as a better position evaluation?
645 if ( ttValue != VALUE_NONE
646 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
651 eval = ss->staticEval =
652 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
653 : -(ss-1)->staticEval + 2 * Eval::Tempo;
655 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
656 ss->staticEval, TT.generation());
659 if (skipEarlyPruning)
662 // Step 6. Razoring (skipped when in check)
664 && depth < 4 * ONE_PLY
665 && eval + razor_margin[depth / ONE_PLY] <= alpha)
667 if (depth <= ONE_PLY)
668 return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
670 Value ralpha = alpha - razor_margin[depth / ONE_PLY];
671 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1);
676 // Step 7. Futility pruning: child node (skipped when in check)
678 && depth < 7 * ONE_PLY
679 && eval - futility_margin(depth) >= beta
680 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
681 && pos.non_pawn_material(pos.side_to_move()))
684 // Step 8. Null move search with verification search (is omitted in PV nodes)
687 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
688 && pos.non_pawn_material(pos.side_to_move()))
691 assert(eval - beta >= 0);
693 // Null move dynamic reduction based on depth and value
694 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
696 ss->currentMove = MOVE_NULL;
697 ss->contHistory = &thisThread->contHistory[NO_PIECE][0];
699 pos.do_null_move(st);
700 Value nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1)
701 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
702 pos.undo_null_move();
704 if (nullValue >= beta)
706 // Do not return unproven mate scores
707 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
710 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
713 // Do verification search at high depths
714 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
715 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
722 // Step 9. ProbCut (skipped when in check)
723 // If we have a good enough capture and a reduced search returns a value
724 // much above beta, we can (almost) safely prune the previous move.
726 && depth >= 5 * ONE_PLY
727 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
729 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
731 assert(is_ok((ss-1)->currentMove));
733 MovePicker mp(pos, ttMove, rbeta - ss->staticEval, &thisThread->captureHistory);
735 while ((move = mp.next_move()) != MOVE_NONE)
738 ss->currentMove = move;
739 ss->contHistory = &thisThread->contHistory[pos.moved_piece(move)][to_sq(move)];
741 assert(depth >= 5 * ONE_PLY);
742 pos.do_move(move, st);
743 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode, false);
750 // Step 10. Internal iterative deepening (skipped when in check)
751 if ( depth >= 6 * ONE_PLY
753 && (PvNode || ss->staticEval + 256 >= beta))
755 Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
756 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
758 tte = TT.probe(posKey, ttHit);
759 ttMove = ttHit ? tte->move() : MOVE_NONE;
762 moves_loop: // When in check search starts from here
764 const PieceToHistory* contHist[] = { (ss-1)->contHistory, (ss-2)->contHistory, nullptr, (ss-4)->contHistory };
765 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
767 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory, contHist, countermove, ss->killers);
768 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
769 improving = ss->staticEval >= (ss-2)->staticEval
770 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
771 ||(ss-2)->staticEval == VALUE_NONE;
773 singularExtensionNode = !rootNode
774 && depth >= 8 * ONE_PLY
775 && ttMove != MOVE_NONE
776 && ttValue != VALUE_NONE
777 && !excludedMove // Recursive singular search is not allowed
778 && (tte->bound() & BOUND_LOWER)
779 && tte->depth() >= depth - 3 * ONE_PLY;
782 pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
784 // Step 11. Loop through moves
785 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
786 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
790 if (move == excludedMove)
793 // At root obey the "searchmoves" option and skip moves not listed in Root
794 // Move List. As a consequence any illegal move is also skipped. In MultiPV
795 // mode we also skip PV moves which have been already searched.
796 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
797 thisThread->rootMoves.end(), move))
800 ss->moveCount = ++moveCount;
802 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
803 sync_cout << "info depth " << depth / ONE_PLY
804 << " currmove " << UCI::move(move, pos.is_chess960())
805 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
808 (ss+1)->pv = nullptr;
810 extension = DEPTH_ZERO;
811 captureOrPromotion = pos.capture_or_promotion(move);
812 movedPiece = pos.moved_piece(move);
814 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
815 ? pos.check_squares(type_of(movedPiece)) & to_sq(move)
816 : pos.gives_check(move);
818 moveCountPruning = depth < 16 * ONE_PLY
819 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
821 // Step 12. Singular and Gives Check Extensions
823 // Singular extension search. If all moves but one fail low on a search of
824 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
825 // is singular and should be extended. To verify this we do a reduced search
826 // on all the other moves but the ttMove and if the result is lower than
827 // ttValue minus a margin then we will extend the ttMove.
828 if ( singularExtensionNode
832 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
833 Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
834 ss->excludedMove = move;
835 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode, true);
836 ss->excludedMove = MOVE_NONE;
846 // Calculate new depth for this move
847 newDepth = depth - ONE_PLY + extension;
849 // Step 13. Pruning at shallow depth
851 && pos.non_pawn_material(pos.side_to_move())
852 && bestValue > VALUE_MATED_IN_MAX_PLY)
854 if ( !captureOrPromotion
856 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
858 // Move count based pruning
859 if (moveCountPruning)
865 // Reduced depth of the next LMR search
866 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
868 // Countermoves based pruning
870 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
871 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
874 // Futility pruning: parent node
877 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
880 // Prune moves with negative SEE
882 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
885 else if ( depth < 7 * ONE_PLY
887 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
891 // Speculative prefetch as early as possible
892 prefetch(TT.first_entry(pos.key_after(move)));
894 // Check for legality just before making the move
895 if (!rootNode && !pos.legal(move))
897 ss->moveCount = --moveCount;
901 if (move == ttMove && captureOrPromotion)
904 // Update the current move (this must be done after singular extension search)
905 ss->currentMove = move;
906 ss->contHistory = &thisThread->contHistory[movedPiece][to_sq(move)];
908 // Step 14. Make the move
909 pos.do_move(move, st, givesCheck);
911 // Step 15. Reduced depth search (LMR). If the move fails high it will be
912 // re-searched at full depth.
913 if ( depth >= 3 * ONE_PLY
915 && (!captureOrPromotion || moveCountPruning))
917 Depth r = reduction<PvNode>(improving, depth, moveCount);
919 if (captureOrPromotion)
920 r -= r ? ONE_PLY : DEPTH_ZERO;
923 // Decrease reduction if opponent's move count is high
924 if ((ss-1)->moveCount > 15)
927 // Decrease reduction for exact PV nodes
931 // Increase reduction if ttMove is a capture
935 // Increase reduction for cut nodes
939 // Decrease reduction for moves that escape a capture. Filter out
940 // castling moves, because they are coded as "king captures rook" and
941 // hence break make_move().
942 else if ( type_of(move) == NORMAL
943 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
946 ss->statScore = thisThread->mainHistory[~pos.side_to_move()][from_to(move)]
947 + (*contHist[0])[movedPiece][to_sq(move)]
948 + (*contHist[1])[movedPiece][to_sq(move)]
949 + (*contHist[3])[movedPiece][to_sq(move)]
952 // Decrease/increase reduction by comparing opponent's stat score
953 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
956 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
959 // Decrease/increase reduction for moves with a good/bad history
960 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->statScore / 20000) * ONE_PLY);
963 Depth d = std::max(newDepth - r, ONE_PLY);
965 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
967 doFullDepthSearch = (value > alpha && d != newDepth);
970 doFullDepthSearch = !PvNode || moveCount > 1;
972 // Step 16. Full depth search when LMR is skipped or fails high
973 if (doFullDepthSearch)
974 value = newDepth < ONE_PLY ?
975 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha)
976 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha)
977 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
979 // For PV nodes only, do a full PV search on the first move or after a fail
980 // high (in the latter case search only if value < beta), otherwise let the
981 // parent node fail low with value <= alpha and try another move.
982 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
985 (ss+1)->pv[0] = MOVE_NONE;
987 value = newDepth < ONE_PLY ?
988 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha)
989 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha)
990 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
993 // Step 17. Undo move
996 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
998 // Step 18. Check for a new best move
999 // Finished searching the move. If a stop occurred, the return value of
1000 // the search cannot be trusted, and we return immediately without
1001 // updating best move, PV and TT.
1002 if (Threads.stop.load(std::memory_order_relaxed))
1007 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1008 thisThread->rootMoves.end(), move);
1010 // PV move or new best move ?
1011 if (moveCount == 1 || value > alpha)
1014 rm.selDepth = thisThread->selDepth;
1019 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1020 rm.pv.push_back(*m);
1022 // We record how often the best move has been changed in each
1023 // iteration. This information is used for time management: When
1024 // the best move changes frequently, we allocate some more time.
1025 if (moveCount > 1 && thisThread == Threads.main())
1026 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1029 // All other moves but the PV are set to the lowest value: this
1030 // is not a problem when sorting because the sort is stable and the
1031 // move position in the list is preserved - just the PV is pushed up.
1032 rm.score = -VALUE_INFINITE;
1035 if (value > bestValue)
1043 if (PvNode && !rootNode) // Update pv even in fail-high case
1044 update_pv(ss->pv, move, (ss+1)->pv);
1046 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1050 assert(value >= beta); // Fail high
1056 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1057 quietsSearched[quietCount++] = move;
1058 else if (captureOrPromotion && move != bestMove && captureCount < 32)
1059 capturesSearched[captureCount++] = move;
1062 // The following condition would detect a stop only after move loop has been
1063 // completed. But in this case bestValue is valid because we have fully
1064 // searched our subtree, and we can anyhow save the result in TT.
1070 // Step 20. Check for mate and stalemate
1071 // All legal moves have been searched and if there are no legal moves, it
1072 // must be a mate or a stalemate. If we are in a singular extension search then
1073 // return a fail low score.
1075 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1078 bestValue = excludedMove ? alpha
1079 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1082 // Quiet best move: update move sorting heuristics
1083 if (!pos.capture_or_promotion(bestMove))
1084 update_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1086 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth));
1088 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1089 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1090 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1092 // Bonus for prior countermove that caused the fail low
1093 else if ( depth >= 3 * ONE_PLY
1094 && !pos.captured_piece()
1095 && is_ok((ss-1)->currentMove))
1096 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1099 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1100 bestValue >= beta ? BOUND_LOWER :
1101 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1102 depth, bestMove, ss->staticEval, TT.generation());
1104 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1110 // qsearch() is the quiescence search function, which is called by the main
1111 // search function with depth zero, or recursively with depth less than ONE_PLY.
1113 template <NodeType NT, bool InCheck>
1114 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1116 const bool PvNode = NT == PV;
1118 assert(InCheck == bool(pos.checkers()));
1119 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1120 assert(PvNode || (alpha == beta - 1));
1121 assert(depth <= DEPTH_ZERO);
1122 assert(depth / ONE_PLY * ONE_PLY == depth);
1128 Move ttMove, move, bestMove;
1129 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1130 bool ttHit, givesCheck, evasionPrunable;
1136 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1138 ss->pv[0] = MOVE_NONE;
1141 ss->currentMove = bestMove = MOVE_NONE;
1142 (ss+1)->ply = ss->ply + 1;
1145 // Check for an instant draw or if the maximum ply has been reached
1146 if (pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
1147 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1148 : DrawValue[pos.side_to_move()];
1150 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1152 // Decide whether or not to include checks: this fixes also the type of
1153 // TT entry depth that we are going to use. Note that in qsearch we use
1154 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1155 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1156 : DEPTH_QS_NO_CHECKS;
1157 // Transposition table lookup
1159 tte = TT.probe(posKey, ttHit);
1160 ttMove = ttHit ? tte->move() : MOVE_NONE;
1161 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1165 && tte->depth() >= ttDepth
1166 && ttValue != VALUE_NONE // Only in case of TT access race
1167 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1168 : (tte->bound() & BOUND_UPPER)))
1171 // Evaluate the position statically
1174 ss->staticEval = VALUE_NONE;
1175 bestValue = futilityBase = -VALUE_INFINITE;
1181 // Never assume anything on values stored in TT
1182 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1183 ss->staticEval = bestValue = evaluate(pos);
1185 // Can ttValue be used as a better position evaluation?
1186 if ( ttValue != VALUE_NONE
1187 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1188 bestValue = ttValue;
1191 ss->staticEval = bestValue =
1192 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1193 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1195 // Stand pat. Return immediately if static value is at least beta
1196 if (bestValue >= beta)
1199 tte->save(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1200 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1205 if (PvNode && bestValue > alpha)
1208 futilityBase = bestValue + 128;
1211 // Initialize a MovePicker object for the current position, and prepare
1212 // to search the moves. Because the depth is <= 0 here, only captures,
1213 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1215 MovePicker mp(pos, ttMove, depth, &pos.this_thread()->mainHistory, &pos.this_thread()->captureHistory, to_sq((ss-1)->currentMove));
1217 // Loop through the moves until no moves remain or a beta cutoff occurs
1218 while ((move = mp.next_move()) != MOVE_NONE)
1220 assert(is_ok(move));
1222 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
1223 ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
1224 : pos.gives_check(move);
1231 && futilityBase > -VALUE_KNOWN_WIN
1232 && !pos.advanced_pawn_push(move))
1234 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1236 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1238 if (futilityValue <= alpha)
1240 bestValue = std::max(bestValue, futilityValue);
1244 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1246 bestValue = std::max(bestValue, futilityBase);
1251 // Detect non-capture evasions that are candidates to be pruned
1252 evasionPrunable = InCheck
1253 && (depth != DEPTH_ZERO || moveCount > 2)
1254 && bestValue > VALUE_MATED_IN_MAX_PLY
1255 && !pos.capture(move);
1257 // Don't search moves with negative SEE values
1258 if ( (!InCheck || evasionPrunable)
1259 && type_of(move) != PROMOTION
1260 && !pos.see_ge(move))
1263 // Speculative prefetch as early as possible
1264 prefetch(TT.first_entry(pos.key_after(move)));
1266 // Check for legality just before making the move
1267 if (!pos.legal(move))
1273 ss->currentMove = move;
1275 // Make and search the move
1276 pos.do_move(move, st, givesCheck);
1277 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1278 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1279 pos.undo_move(move);
1281 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1283 // Check for a new best move
1284 if (value > bestValue)
1290 if (PvNode) // Update pv even in fail-high case
1291 update_pv(ss->pv, move, (ss+1)->pv);
1293 if (PvNode && value < beta) // Update alpha here!
1300 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1301 ttDepth, move, ss->staticEval, TT.generation());
1309 // All legal moves have been searched. A special case: If we're in check
1310 // and no legal moves were found, it is checkmate.
1311 if (InCheck && bestValue == -VALUE_INFINITE)
1312 return mated_in(ss->ply); // Plies to mate from the root
1314 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1315 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1316 ttDepth, bestMove, ss->staticEval, TT.generation());
1318 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1324 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1325 // "plies to mate from the current position". Non-mate scores are unchanged.
1326 // The function is called before storing a value in the transposition table.
1328 Value value_to_tt(Value v, int ply) {
1330 assert(v != VALUE_NONE);
1332 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1333 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1337 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1338 // from the transposition table (which refers to the plies to mate/be mated
1339 // from current position) to "plies to mate/be mated from the root".
1341 Value value_from_tt(Value v, int ply) {
1343 return v == VALUE_NONE ? VALUE_NONE
1344 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1345 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1349 // update_pv() adds current move and appends child pv[]
1351 void update_pv(Move* pv, Move move, Move* childPv) {
1353 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1359 // update_continuation_histories() updates histories of the move pairs formed
1360 // by moves at ply -1, -2, and -4 with current move.
1362 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1364 for (int i : {1, 2, 4})
1365 if (is_ok((ss-i)->currentMove))
1366 (ss-i)->contHistory->update(pc, to, bonus);
1370 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1372 void update_capture_stats(const Position& pos, Move move,
1373 Move* captures, int captureCnt, int bonus) {
1375 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1376 Piece moved_piece = pos.moved_piece(move);
1377 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1378 captureHistory.update(moved_piece, to_sq(move), captured, bonus);
1380 // Decrease all the other played capture moves
1381 for (int i = 0; i < captureCnt; ++i)
1383 moved_piece = pos.moved_piece(captures[i]);
1384 captured = type_of(pos.piece_on(to_sq(captures[i])));
1385 captureHistory.update(moved_piece, to_sq(captures[i]), captured, -bonus);
1390 // update_stats() updates move sorting heuristics when a new quiet best move is found
1392 void update_stats(const Position& pos, Stack* ss, Move move,
1393 Move* quiets, int quietsCnt, int bonus) {
1395 if (ss->killers[0] != move)
1397 ss->killers[1] = ss->killers[0];
1398 ss->killers[0] = move;
1401 Color c = pos.side_to_move();
1402 Thread* thisThread = pos.this_thread();
1403 thisThread->mainHistory.update(c, move, bonus);
1404 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1406 if (is_ok((ss-1)->currentMove))
1408 Square prevSq = to_sq((ss-1)->currentMove);
1409 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1412 // Decrease all the other played quiet moves
1413 for (int i = 0; i < quietsCnt; ++i)
1415 thisThread->mainHistory.update(c, quiets[i], -bonus);
1416 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1421 // Is the PV leading to a draw position? Assumes all pv moves are legal
1422 bool pv_is_draw(Position& pos) {
1424 StateInfo st[MAX_PLY];
1425 auto& pv = pos.this_thread()->rootMoves[0].pv;
1427 for (size_t i = 0; i < pv.size(); ++i)
1428 pos.do_move(pv[i], st[i]);
1430 bool isDraw = pos.is_draw(pv.size());
1432 for (size_t i = pv.size(); i > 0; --i)
1433 pos.undo_move(pv[i-1]);
1439 // When playing with strength handicap, choose best move among a set of RootMoves
1440 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1442 Move Skill::pick_best(size_t multiPV) {
1444 const RootMoves& rootMoves = Threads.main()->rootMoves;
1445 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1447 // RootMoves are already sorted by score in descending order
1448 Value topScore = rootMoves[0].score;
1449 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1450 int weakness = 120 - 2 * level;
1451 int maxScore = -VALUE_INFINITE;
1453 // Choose best move. For each move score we add two terms, both dependent on
1454 // weakness. One is deterministic and bigger for weaker levels, and one is
1455 // random. Then we choose the move with the resulting highest score.
1456 for (size_t i = 0; i < multiPV; ++i)
1458 // This is our magic formula
1459 int push = ( weakness * int(topScore - rootMoves[i].score)
1460 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1462 if (rootMoves[i].score + push >= maxScore)
1464 maxScore = rootMoves[i].score + push;
1465 best = rootMoves[i].pv[0];
1474 // check_time() is used to print debug info and, more importantly, to detect
1475 // when we are out of available time and thus stop the search.
1477 void MainThread::check_time() {
1482 // At low node count increase the checking rate to about 0.1% of nodes
1483 // otherwise use a default value.
1484 callsCnt = Limits.nodes ? std::min(4096, int(Limits.nodes / 1024)) : 4096;
1486 static TimePoint lastInfoTime = now();
1488 int elapsed = Time.elapsed();
1489 TimePoint tick = Limits.startTime + elapsed;
1491 if (tick - lastInfoTime >= 1000)
1493 lastInfoTime = tick;
1497 // An engine may not stop pondering until told so by the GUI
1501 if ( (Limits.use_time_management() && elapsed > Time.maximum())
1502 || (Limits.movetime && elapsed >= Limits.movetime)
1503 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1504 Threads.stop = true;
1508 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1509 /// that all (if any) unsearched PV lines are sent using a previous search score.
1511 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1513 std::stringstream ss;
1514 int elapsed = Time.elapsed() + 1;
1515 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1516 size_t PVIdx = pos.this_thread()->PVIdx;
1517 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1518 uint64_t nodesSearched = Threads.nodes_searched();
1519 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1521 for (size_t i = 0; i < multiPV; ++i)
1523 bool updated = (i <= PVIdx && rootMoves[i].score != -VALUE_INFINITE);
1525 if (depth == ONE_PLY && !updated)
1528 Depth d = updated ? depth : depth - ONE_PLY;
1529 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1531 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1532 v = tb ? TB::Score : v;
1534 if (ss.rdbuf()->in_avail()) // Not at first line
1538 << " depth " << d / ONE_PLY
1539 << " seldepth " << rootMoves[i].selDepth
1540 << " multipv " << i + 1
1541 << " score " << UCI::value(v);
1543 if (!tb && i == PVIdx)
1544 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1546 ss << " nodes " << nodesSearched
1547 << " nps " << nodesSearched * 1000 / elapsed;
1549 if (elapsed > 1000) // Earlier makes little sense
1550 ss << " hashfull " << TT.hashfull();
1552 ss << " tbhits " << tbHits
1553 << " time " << elapsed
1556 for (Move m : rootMoves[i].pv)
1557 ss << " " << UCI::move(m, pos.is_chess960());
1564 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1565 /// before exiting the search, for instance, in case we stop the search during a
1566 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1567 /// otherwise in case of 'ponder on' we have nothing to think on.
1569 bool RootMove::extract_ponder_from_tt(Position& pos) {
1574 assert(pv.size() == 1);
1579 pos.do_move(pv[0], st);
1580 TTEntry* tte = TT.probe(pos.key(), ttHit);
1584 Move m = tte->move(); // Local copy to be SMP safe
1585 if (MoveList<LEGAL>(pos).contains(m))
1589 pos.undo_move(pv[0]);
1590 return pv.size() > 1;
1593 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1596 UseRule50 = Options["Syzygy50MoveRule"];
1597 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1598 Cardinality = Options["SyzygyProbeLimit"];
1600 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1601 if (Cardinality > MaxCardinality)
1603 Cardinality = MaxCardinality;
1604 ProbeDepth = DEPTH_ZERO;
1607 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1610 // Don't filter any moves if the user requested analysis on multiple
1611 if (Options["MultiPV"] != 1)
1614 // If the current root position is in the tablebases, then RootMoves
1615 // contains only moves that preserve the draw or the win.
1616 RootInTB = root_probe(pos, rootMoves, TB::Score);
1619 Cardinality = 0; // Do not probe tablebases during the search
1621 else // If DTZ tables are missing, use WDL tables as a fallback
1623 // Filter out moves that do not preserve the draw or the win.
1624 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1626 // Only probe during search if winning
1627 if (RootInTB && TB::Score <= VALUE_DRAW)
1631 if (RootInTB && !UseRule50)
1632 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1633 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1