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) 2015-2018 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 margins
70 const int RazorMargin = 590;
71 Value futility_margin(Depth d) { return Value(150 * d / ONE_PLY); }
73 // Futility and reductions lookup tables, initialized at startup
74 int FutilityMoveCounts[2][16]; // [improving][depth]
75 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
77 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
78 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
81 // History and stats update bonus, based on depth
82 int stat_bonus(Depth depth) {
83 int d = depth / ONE_PLY;
84 return d > 17 ? 0 : d * d + 2 * d - 2;
87 // Skill structure is used to implement strength limit
89 explicit Skill(int l) : level(l) {}
90 bool enabled() const { return level < 20; }
91 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
92 Move pick_best(size_t multiPV);
95 Move best = MOVE_NONE;
98 template <NodeType NT>
99 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning);
101 template <NodeType NT, bool InCheck>
102 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
104 Value value_to_tt(Value v, int ply);
105 Value value_from_tt(Value v, int ply);
106 void update_pv(Move* pv, Move move, Move* childPv);
107 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
108 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
109 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCnt, int bonus);
111 // perft() is our utility to verify move generation. All the leaf nodes up
112 // to the given depth are generated and counted, and the sum is returned.
114 uint64_t perft(Position& pos, Depth depth) {
117 uint64_t cnt, nodes = 0;
118 const bool leaf = (depth == 2 * ONE_PLY);
120 for (const auto& m : MoveList<LEGAL>(pos))
122 if (Root && depth <= ONE_PLY)
127 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
132 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
140 /// Search::init() is called at startup to initialize various lookup tables
142 void Search::init() {
144 for (int imp = 0; imp <= 1; ++imp)
145 for (int d = 1; d < 64; ++d)
146 for (int mc = 1; mc < 64; ++mc)
148 double r = log(d) * log(mc) / 1.95;
150 Reductions[NonPV][imp][d][mc] = int(std::round(r));
151 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
153 // Increase reduction for non-PV nodes when eval is not improving
154 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
155 Reductions[NonPV][imp][d][mc]++;
158 for (int d = 0; d < 16; ++d)
160 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
161 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
166 /// Search::clear() resets search state to its initial value
168 void Search::clear() {
170 Threads.main()->wait_for_search_finished();
172 Time.availableNodes = 0;
178 /// MainThread::search() is called by the main thread when the program receives
179 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
181 void MainThread::search() {
185 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
186 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
190 Color us = rootPos.side_to_move();
191 Time.init(Limits, us, rootPos.game_ply());
194 if (rootMoves.empty())
196 rootMoves.emplace_back(MOVE_NONE);
197 sync_cout << "info depth 0 score "
198 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
203 for (Thread* th : Threads)
205 th->start_searching();
207 Thread::search(); // Let's start searching!
210 // When we reach the maximum depth, we can arrive here without a raise of
211 // Threads.stop. However, if we are pondering or in an infinite search,
212 // the UCI protocol states that we shouldn't print the best move before the
213 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
214 // until the GUI sends one of those commands (which also raises Threads.stop).
215 Threads.stopOnPonderhit = true;
217 while (!Threads.stop && (Threads.ponder || Limits.infinite))
218 {} // Busy wait for a stop or a ponder reset
220 // Stop the threads if not already stopped (also raise the stop if
221 // "ponderhit" just reset Threads.ponder).
224 // Wait until all threads have finished
225 for (Thread* th : Threads)
227 th->wait_for_search_finished();
229 // When playing in 'nodes as time' mode, subtract the searched nodes from
230 // the available ones before exiting.
232 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
234 // Check if there are threads with a better score than main thread
235 Thread* bestThread = this;
236 if ( Options["MultiPV"] == 1
238 && !Skill(Options["Skill Level"]).enabled()
239 && rootMoves[0].pv[0] != MOVE_NONE)
241 for (Thread* th : Threads)
243 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
244 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
246 // Select the thread with the best score, always if it is a mate
248 && (depthDiff >= 0 || th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY))
253 previousScore = bestThread->rootMoves[0].score;
255 // Send again PV info if we have a new best thread
256 if (bestThread != this)
257 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
259 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
261 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
262 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
264 std::cout << sync_endl;
268 /// Thread::search() is the main iterative deepening loop. It calls search()
269 /// repeatedly with increasing depth until the allocated thinking time has been
270 /// consumed, the user stops the search, or the maximum search depth is reached.
272 void Thread::search() {
274 Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
275 Value bestValue, alpha, beta, delta;
276 Move lastBestMove = MOVE_NONE;
277 Depth lastBestMoveDepth = DEPTH_ZERO;
278 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
279 double timeReduction = 1.0;
280 Color us = rootPos.side_to_move();
282 std::memset(ss-4, 0, 7 * sizeof(Stack));
283 for (int i = 4; i > 0; i--)
284 (ss-i)->contHistory = &this->contHistory[NO_PIECE][0]; // Use as sentinel
286 bestValue = delta = alpha = -VALUE_INFINITE;
287 beta = VALUE_INFINITE;
290 mainThread->bestMoveChanges = 0, mainThread->failedLow = false;
292 size_t multiPV = Options["MultiPV"];
293 Skill skill(Options["Skill Level"]);
295 // When playing with strength handicap enable MultiPV search that we will
296 // use behind the scenes to retrieve a set of possible moves.
298 multiPV = std::max(multiPV, (size_t)4);
300 multiPV = std::min(multiPV, rootMoves.size());
302 int ct = Options["Contempt"] * PawnValueEg / 100; // From centipawns
303 Eval::Contempt = (us == WHITE ? make_score(ct, ct / 2)
304 : -make_score(ct, ct / 2));
306 // Iterative deepening loop until requested to stop or the target depth is reached
307 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
309 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
311 // Distribute search depths across the helper threads
314 int i = (idx - 1) % 20;
315 if (((rootDepth / ONE_PLY + rootPos.game_ply() + SkipPhase[i]) / SkipSize[i]) % 2)
316 continue; // Retry with an incremented rootDepth
319 // Age out PV variability metric
321 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
323 // Save the last iteration's scores before first PV line is searched and
324 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
325 for (RootMove& rm : rootMoves)
326 rm.previousScore = rm.score;
328 // MultiPV loop. We perform a full root search for each PV line
329 for (PVIdx = 0; PVIdx < multiPV && !Threads.stop; ++PVIdx)
331 // Reset UCI info selDepth for each depth and each PV line
334 // Reset aspiration window starting size
335 if (rootDepth >= 5 * ONE_PLY)
338 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
339 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
341 // Adjust contempt based on current bestValue
342 ct = Options["Contempt"] * PawnValueEg / 100 // From centipawns
343 + (bestValue > 500 ? 50: // Dynamic contempt
344 bestValue < -500 ? -50:
347 Eval::Contempt = (us == WHITE ? make_score(ct, ct / 2)
348 : -make_score(ct, ct / 2));
351 // Start with a small aspiration window and, in the case of a fail
352 // high/low, re-search with a bigger window until we don't fail
356 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
358 // Bring the best move to the front. It is critical that sorting
359 // is done with a stable algorithm because all the values but the
360 // first and eventually the new best one are set to -VALUE_INFINITE
361 // and we want to keep the same order for all the moves except the
362 // new PV that goes to the front. Note that in case of MultiPV
363 // search the already searched PV lines are preserved.
364 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
366 // If search has been stopped, we break immediately. Sorting is
367 // safe because RootMoves is still valid, although it refers to
368 // the previous iteration.
372 // When failing high/low give some update (without cluttering
373 // the UI) before a re-search.
376 && (bestValue <= alpha || bestValue >= beta)
377 && Time.elapsed() > 3000)
378 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
380 // In case of failing low/high increase aspiration window and
381 // re-search, otherwise exit the loop.
382 if (bestValue <= alpha)
384 beta = (alpha + beta) / 2;
385 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
389 mainThread->failedLow = true;
390 Threads.stopOnPonderhit = false;
393 else if (bestValue >= beta)
394 beta = std::min(bestValue + delta, VALUE_INFINITE);
398 delta += delta / 4 + 5;
400 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
403 // Sort the PV lines searched so far and update the GUI
404 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
407 && (Threads.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000))
408 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
412 completedDepth = rootDepth;
414 if (rootMoves[0].pv[0] != lastBestMove) {
415 lastBestMove = rootMoves[0].pv[0];
416 lastBestMoveDepth = rootDepth;
419 // Have we found a "mate in x"?
421 && bestValue >= VALUE_MATE_IN_MAX_PLY
422 && VALUE_MATE - bestValue <= 2 * Limits.mate)
428 // If skill level is enabled and time is up, pick a sub-optimal best move
429 if (skill.enabled() && skill.time_to_pick(rootDepth))
430 skill.pick_best(multiPV);
432 // Do we have time for the next iteration? Can we stop searching now?
433 if ( Limits.use_time_management()
435 && !Threads.stopOnPonderhit)
437 const int F[] = { mainThread->failedLow,
438 bestValue - mainThread->previousScore };
440 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
442 // If the bestMove is stable over several iterations, reduce time accordingly
444 for (int i : {3, 4, 5})
445 if (lastBestMoveDepth * i < completedDepth)
446 timeReduction *= 1.3;
448 // Use part of the gained time from a previous stable move for the current move
449 double unstablePvFactor = 1.0 + mainThread->bestMoveChanges;
450 unstablePvFactor *= std::pow(mainThread->previousTimeReduction, 0.51) / timeReduction;
452 // Stop the search if we have only one legal move, or if available time elapsed
453 if ( rootMoves.size() == 1
454 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 605)
456 // If we are allowed to ponder do not stop the search now but
457 // keep pondering until the GUI sends "ponderhit" or "stop".
459 Threads.stopOnPonderhit = true;
469 mainThread->previousTimeReduction = timeReduction;
471 // If skill level is enabled, swap best PV line with the sub-optimal one
473 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
474 skill.best ? skill.best : skill.pick_best(multiPV)));
480 // search<>() is the main search function for both PV and non-PV nodes
482 template <NodeType NT>
483 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
485 const bool PvNode = NT == PV;
486 const bool rootNode = PvNode && ss->ply == 0;
488 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
489 assert(PvNode || (alpha == beta - 1));
490 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
491 assert(!(PvNode && cutNode));
492 assert(depth / ONE_PLY * ONE_PLY == depth);
494 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
498 Move ttMove, move, excludedMove, bestMove;
499 Depth extension, newDepth;
500 Value bestValue, value, ttValue, eval, maxValue;
501 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
502 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
504 int moveCount, captureCount, quietCount;
506 // Step 1. Initialize node
507 Thread* thisThread = pos.this_thread();
508 inCheck = pos.checkers();
509 moveCount = captureCount = quietCount = ss->moveCount = 0;
511 bestValue = -VALUE_INFINITE;
512 maxValue = VALUE_INFINITE;
514 // Check for the available remaining time
515 if (thisThread == Threads.main())
516 static_cast<MainThread*>(thisThread)->check_time();
518 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
519 if (PvNode && thisThread->selDepth < ss->ply + 1)
520 thisThread->selDepth = ss->ply + 1;
524 // Step 2. Check for aborted search and immediate draw
525 if ( Threads.stop.load(std::memory_order_relaxed)
526 || pos.is_draw(ss->ply)
527 || ss->ply >= MAX_PLY)
528 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
530 // Step 3. Mate distance pruning. Even if we mate at the next move our score
531 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
532 // a shorter mate was found upward in the tree then there is no need to search
533 // because we will never beat the current alpha. Same logic but with reversed
534 // signs applies also in the opposite condition of being mated instead of giving
535 // mate. In this case return a fail-high score.
536 alpha = std::max(mated_in(ss->ply), alpha);
537 beta = std::min(mate_in(ss->ply+1), beta);
542 assert(0 <= ss->ply && ss->ply < MAX_PLY);
544 (ss+1)->ply = ss->ply + 1;
545 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
546 ss->contHistory = &thisThread->contHistory[NO_PIECE][0];
547 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
548 Square prevSq = to_sq((ss-1)->currentMove);
550 // Step 4. Transposition table lookup. We don't want the score of a partial
551 // search to overwrite a previous full search TT value, so we use a different
552 // position key in case of an excluded move.
553 excludedMove = ss->excludedMove;
554 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
555 tte = TT.probe(posKey, ttHit);
556 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
557 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
558 : ttHit ? tte->move() : MOVE_NONE;
560 // At non-PV nodes we check for an early TT cutoff
563 && tte->depth() >= depth
564 && ttValue != VALUE_NONE // Possible in case of TT access race
565 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
566 : (tte->bound() & BOUND_UPPER)))
568 // If ttMove is quiet, update move sorting heuristics on TT hit
573 if (!pos.capture_or_promotion(ttMove))
574 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
576 // Extra penalty for a quiet TT move in previous ply when it gets refuted
577 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
578 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
580 // Penalty for a quiet ttMove that fails low
581 else if (!pos.capture_or_promotion(ttMove))
583 int penalty = -stat_bonus(depth);
584 thisThread->mainHistory.update(pos.side_to_move(), ttMove, penalty);
585 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
591 // Step 5. Tablebases probe
592 if (!rootNode && TB::Cardinality)
594 int piecesCount = pos.count<ALL_PIECES>();
596 if ( piecesCount <= TB::Cardinality
597 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
598 && pos.rule50_count() == 0
599 && !pos.can_castle(ANY_CASTLING))
602 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
604 if (err != TB::ProbeState::FAIL)
606 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
608 int drawScore = TB::UseRule50 ? 1 : 0;
610 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
611 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
612 : VALUE_DRAW + 2 * wdl * drawScore;
614 Bound b = wdl < -drawScore ? BOUND_UPPER
615 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
617 if ( b == BOUND_EXACT
618 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
620 tte->save(posKey, value_to_tt(value, ss->ply), b,
621 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
622 MOVE_NONE, VALUE_NONE, TT.generation());
629 if (b == BOUND_LOWER)
630 bestValue = value, alpha = std::max(alpha, bestValue);
638 // Step 6. Evaluate the position statically
641 ss->staticEval = eval = VALUE_NONE;
646 // Never assume anything on values stored in TT
647 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
648 eval = ss->staticEval = evaluate(pos);
650 // Can ttValue be used as a better position evaluation?
651 if ( ttValue != VALUE_NONE
652 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
657 ss->staticEval = eval =
658 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
659 : -(ss-1)->staticEval + 2 * Eval::Tempo;
661 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
662 ss->staticEval, TT.generation());
665 if (skipEarlyPruning || !pos.non_pawn_material(pos.side_to_move()))
668 // Step 7. Razoring (skipped when in check)
671 && eval + RazorMargin <= alpha)
672 return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
674 // Step 8. Futility pruning: child node (skipped when in check)
676 && depth < 7 * ONE_PLY
677 && eval - futility_margin(depth) >= beta
678 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
681 // Step 9. Null move search with verification search
684 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
685 && (ss->ply >= thisThread->nmp_ply || ss->ply % 2 != thisThread->nmp_odd))
687 assert(eval - beta >= 0);
689 // Null move dynamic reduction based on depth and value
690 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
692 ss->currentMove = MOVE_NULL;
693 ss->contHistory = &thisThread->contHistory[NO_PIECE][0];
695 pos.do_null_move(st);
696 Value nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1)
697 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
698 pos.undo_null_move();
700 if (nullValue >= beta)
702 // Do not return unproven mate scores
703 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
706 if (abs(beta) < VALUE_KNOWN_WIN && (depth < 12 * ONE_PLY || thisThread->nmp_ply))
709 // Do verification search at high depths. Disable null move pruning
710 // for side to move for the first part of the remaining search tree.
711 thisThread->nmp_ply = ss->ply + 3 * (depth-R) / 4;
712 thisThread->nmp_odd = ss->ply % 2;
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);
717 thisThread->nmp_odd = thisThread->nmp_ply = 0;
724 // Step 10. ProbCut (skipped when in check)
725 // If we have a good enough capture and a reduced search returns a value
726 // much above beta, we can (almost) safely prune the previous move.
728 && depth >= 5 * ONE_PLY
729 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
731 assert(is_ok((ss-1)->currentMove));
733 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
734 MovePicker mp(pos, ttMove, rbeta - ss->staticEval, &thisThread->captureHistory);
736 while ((move = mp.next_move()) != MOVE_NONE)
739 ss->currentMove = move;
740 ss->contHistory = &thisThread->contHistory[pos.moved_piece(move)][to_sq(move)];
742 assert(depth >= 5 * ONE_PLY);
744 pos.do_move(move, st);
746 // Perform a preliminary search at depth 1 to verify that the move holds.
747 // We will only do this search if the depth is not 5, thus avoiding two
748 // searches at depth 1 in a row.
749 if (depth != 5 * ONE_PLY)
750 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, ONE_PLY, !cutNode, true);
752 // If the first search was skipped or was performed and held, perform
753 // the regular search.
754 if (depth == 5 * ONE_PLY || value >= rbeta)
755 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode, false);
763 // Step 11. Internal iterative deepening (skipped when in check)
764 if ( depth >= 6 * ONE_PLY
766 && (PvNode || ss->staticEval + 256 >= beta))
768 Depth d = 3 * depth / 4 - 2 * ONE_PLY;
769 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
771 tte = TT.probe(posKey, ttHit);
772 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
773 ttMove = ttHit ? tte->move() : MOVE_NONE;
776 moves_loop: // When in check, search starts from here
778 const PieceToHistory* contHist[] = { (ss-1)->contHistory, (ss-2)->contHistory, nullptr, (ss-4)->contHistory };
779 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
781 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory, contHist, countermove, ss->killers);
782 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
783 improving = ss->staticEval >= (ss-2)->staticEval
784 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
785 ||(ss-2)->staticEval == VALUE_NONE;
787 singularExtensionNode = !rootNode
788 && depth >= 8 * ONE_PLY
789 && ttMove != MOVE_NONE
790 && ttValue != VALUE_NONE
791 && !excludedMove // Recursive singular search is not allowed
792 && (tte->bound() & BOUND_LOWER)
793 && tte->depth() >= depth - 3 * ONE_PLY;
796 pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
798 // Step 12. Loop through all pseudo-legal moves until no moves remain
799 // or a beta cutoff occurs.
800 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
804 if (move == excludedMove)
807 // At root obey the "searchmoves" option and skip moves not listed in Root
808 // Move List. As a consequence any illegal move is also skipped. In MultiPV
809 // mode we also skip PV moves which have been already searched.
810 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
811 thisThread->rootMoves.end(), move))
814 ss->moveCount = ++moveCount;
816 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
817 sync_cout << "info depth " << depth / ONE_PLY
818 << " currmove " << UCI::move(move, pos.is_chess960())
819 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
821 (ss+1)->pv = nullptr;
823 extension = DEPTH_ZERO;
824 captureOrPromotion = pos.capture_or_promotion(move);
825 movedPiece = pos.moved_piece(move);
827 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
828 ? pos.check_squares(type_of(movedPiece)) & to_sq(move)
829 : pos.gives_check(move);
831 moveCountPruning = depth < 16 * ONE_PLY
832 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
834 // Step 13. Extensions
836 // Singular extension search. If all moves but one fail low on a search
837 // of (alpha-s, beta-s), and just one fails high on (alpha, beta), then
838 // that move is singular and should be extended. To verify this we do a
839 // reduced search on on all the other moves but the ttMove and if the
840 // result is lower than ttValue minus a margin then we will extend the ttMove.
841 if ( singularExtensionNode
845 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
846 ss->excludedMove = move;
847 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode, true);
848 ss->excludedMove = MOVE_NONE;
853 else if ( givesCheck // Check extension
858 // Calculate new depth for this move
859 newDepth = depth - ONE_PLY + extension;
861 // Step 14. Pruning at shallow depth
863 && pos.non_pawn_material(pos.side_to_move())
864 && bestValue > VALUE_MATED_IN_MAX_PLY)
866 if ( !captureOrPromotion
868 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
870 // Move count based pruning
871 if (moveCountPruning)
877 // Reduced depth of the next LMR search
878 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
880 // Countermoves based pruning
882 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
883 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
886 // Futility pruning: parent node
889 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
892 // Prune moves with negative SEE
894 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
897 else if ( depth < 7 * ONE_PLY
899 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
903 // Speculative prefetch as early as possible
904 prefetch(TT.first_entry(pos.key_after(move)));
906 // Check for legality just before making the move
907 if (!rootNode && !pos.legal(move))
909 ss->moveCount = --moveCount;
913 if (move == ttMove && captureOrPromotion)
916 // Update the current move (this must be done after singular extension search)
917 ss->currentMove = move;
918 ss->contHistory = &thisThread->contHistory[movedPiece][to_sq(move)];
920 // Step 15. Make the move
921 pos.do_move(move, st, givesCheck);
923 // Step 16. Reduced depth search (LMR). If the move fails high it will be
924 // re-searched at full depth.
925 if ( depth >= 3 * ONE_PLY
927 && (!captureOrPromotion || moveCountPruning))
929 Depth r = reduction<PvNode>(improving, depth, moveCount);
931 if (captureOrPromotion)
932 r -= r ? ONE_PLY : DEPTH_ZERO;
935 // Decrease reduction if opponent's move count is high
936 if ((ss-1)->moveCount > 15)
939 // Decrease reduction for exact PV nodes
943 // Increase reduction if ttMove is a capture
947 // Increase reduction for cut nodes
951 // Decrease reduction for moves that escape a capture. Filter out
952 // castling moves, because they are coded as "king captures rook" and
953 // hence break make_move().
954 else if ( type_of(move) == NORMAL
955 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
958 ss->statScore = thisThread->mainHistory[~pos.side_to_move()][from_to(move)]
959 + (*contHist[0])[movedPiece][to_sq(move)]
960 + (*contHist[1])[movedPiece][to_sq(move)]
961 + (*contHist[3])[movedPiece][to_sq(move)]
964 // Decrease/increase reduction by comparing opponent's stat score
965 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
968 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
971 // Decrease/increase reduction for moves with a good/bad history
972 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->statScore / 20000) * ONE_PLY);
975 Depth d = std::max(newDepth - r, ONE_PLY);
977 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
979 doFullDepthSearch = (value > alpha && d != newDepth);
982 doFullDepthSearch = !PvNode || moveCount > 1;
984 // Step 17. Full depth search when LMR is skipped or fails high
985 if (doFullDepthSearch)
986 value = newDepth < ONE_PLY ?
987 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha)
988 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha)
989 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
991 // For PV nodes only, do a full PV search on the first move or after a fail
992 // high (in the latter case search only if value < beta), otherwise let the
993 // parent node fail low with value <= alpha and try another move.
994 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
997 (ss+1)->pv[0] = MOVE_NONE;
999 value = newDepth < ONE_PLY ?
1000 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha)
1001 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha)
1002 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1005 // Step 18. Undo move
1006 pos.undo_move(move);
1008 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1010 // Step 19. Check for a new best move
1011 // Finished searching the move. If a stop occurred, the return value of
1012 // the search cannot be trusted, and we return immediately without
1013 // updating best move, PV and TT.
1014 if (Threads.stop.load(std::memory_order_relaxed))
1019 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1020 thisThread->rootMoves.end(), move);
1022 // PV move or new best move?
1023 if (moveCount == 1 || value > alpha)
1026 rm.selDepth = thisThread->selDepth;
1031 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1032 rm.pv.push_back(*m);
1034 // We record how often the best move has been changed in each
1035 // iteration. This information is used for time management: When
1036 // the best move changes frequently, we allocate some more time.
1037 if (moveCount > 1 && thisThread == Threads.main())
1038 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1041 // All other moves but the PV are set to the lowest value: this
1042 // is not a problem when sorting because the sort is stable and the
1043 // move position in the list is preserved - just the PV is pushed up.
1044 rm.score = -VALUE_INFINITE;
1047 if (value > bestValue)
1055 if (PvNode && !rootNode) // Update pv even in fail-high case
1056 update_pv(ss->pv, move, (ss+1)->pv);
1058 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1062 assert(value >= beta); // Fail high
1068 if (move != bestMove)
1070 if (captureOrPromotion && captureCount < 32)
1071 capturesSearched[captureCount++] = move;
1073 else if (!captureOrPromotion && quietCount < 64)
1074 quietsSearched[quietCount++] = move;
1078 // The following condition would detect a stop only after move loop has been
1079 // completed. But in this case bestValue is valid because we have fully
1080 // searched our subtree, and we can anyhow save the result in TT.
1086 // Step 20. Check for mate and stalemate
1087 // All legal moves have been searched and if there are no legal moves, it
1088 // must be a mate or a stalemate. If we are in a singular extension search then
1089 // return a fail low score.
1091 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1094 bestValue = excludedMove ? alpha
1095 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1098 // Quiet best move: update move sorting heuristics
1099 if (!pos.capture_or_promotion(bestMove))
1100 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1102 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth));
1104 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1105 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1106 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1108 // Bonus for prior countermove that caused the fail low
1109 else if ( depth >= 3 * ONE_PLY
1110 && !pos.captured_piece()
1111 && is_ok((ss-1)->currentMove))
1112 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1115 bestValue = std::min(bestValue, maxValue);
1118 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1119 bestValue >= beta ? BOUND_LOWER :
1120 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1121 depth, bestMove, ss->staticEval, TT.generation());
1123 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1129 // qsearch() is the quiescence search function, which is called by the main
1130 // search function with depth zero, or recursively with depth less than ONE_PLY.
1132 template <NodeType NT, bool InCheck>
1133 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1135 const bool PvNode = NT == PV;
1137 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1138 assert(PvNode || (alpha == beta - 1));
1139 assert(depth <= DEPTH_ZERO);
1140 assert(depth / ONE_PLY * ONE_PLY == depth);
1141 assert(InCheck == bool(pos.checkers()));
1147 Move ttMove, move, bestMove;
1149 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1150 bool ttHit, givesCheck, evasionPrunable;
1155 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1157 ss->pv[0] = MOVE_NONE;
1160 (ss+1)->ply = ss->ply + 1;
1161 ss->currentMove = bestMove = MOVE_NONE;
1164 // Check for an immediate draw or maximum ply reached
1165 if ( pos.is_draw(ss->ply)
1166 || ss->ply >= MAX_PLY)
1167 return (ss->ply >= MAX_PLY && !InCheck) ? evaluate(pos) : VALUE_DRAW;
1169 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1171 // Decide whether or not to include checks: this fixes also the type of
1172 // TT entry depth that we are going to use. Note that in qsearch we use
1173 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1174 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1175 : DEPTH_QS_NO_CHECKS;
1176 // Transposition table lookup
1178 tte = TT.probe(posKey, ttHit);
1179 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1180 ttMove = ttHit ? tte->move() : MOVE_NONE;
1184 && tte->depth() >= ttDepth
1185 && ttValue != VALUE_NONE // Only in case of TT access race
1186 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1187 : (tte->bound() & BOUND_UPPER)))
1190 // Evaluate the position statically
1193 ss->staticEval = VALUE_NONE;
1194 bestValue = futilityBase = -VALUE_INFINITE;
1200 // Never assume anything on values stored in TT
1201 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1202 ss->staticEval = bestValue = evaluate(pos);
1204 // Can ttValue be used as a better position evaluation?
1205 if ( ttValue != VALUE_NONE
1206 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1207 bestValue = ttValue;
1210 ss->staticEval = bestValue =
1211 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1212 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1214 // Stand pat. Return immediately if static value is at least beta
1215 if (bestValue >= beta)
1218 tte->save(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1219 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1224 if (PvNode && bestValue > alpha)
1227 futilityBase = bestValue + 128;
1230 // Initialize a MovePicker object for the current position, and prepare
1231 // to search the moves. Because the depth is <= 0 here, only captures,
1232 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1234 MovePicker mp(pos, ttMove, depth, &pos.this_thread()->mainHistory, &pos.this_thread()->captureHistory, to_sq((ss-1)->currentMove));
1236 // Loop through the moves until no moves remain or a beta cutoff occurs
1237 while ((move = mp.next_move()) != MOVE_NONE)
1239 assert(is_ok(move));
1241 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
1242 ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
1243 : pos.gives_check(move);
1250 && futilityBase > -VALUE_KNOWN_WIN
1251 && !pos.advanced_pawn_push(move))
1253 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1255 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1257 if (futilityValue <= alpha)
1259 bestValue = std::max(bestValue, futilityValue);
1263 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1265 bestValue = std::max(bestValue, futilityBase);
1270 // Detect non-capture evasions that are candidates to be pruned
1271 evasionPrunable = InCheck
1272 && (depth != DEPTH_ZERO || moveCount > 2)
1273 && bestValue > VALUE_MATED_IN_MAX_PLY
1274 && !pos.capture(move);
1276 // Don't search moves with negative SEE values
1277 if ( (!InCheck || evasionPrunable)
1278 && !pos.see_ge(move))
1281 // Speculative prefetch as early as possible
1282 prefetch(TT.first_entry(pos.key_after(move)));
1284 // Check for legality just before making the move
1285 if (!pos.legal(move))
1291 ss->currentMove = move;
1293 // Make and search the move
1294 pos.do_move(move, st, givesCheck);
1295 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1296 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1297 pos.undo_move(move);
1299 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1301 // Check for a new best move
1302 if (value > bestValue)
1308 if (PvNode) // Update pv even in fail-high case
1309 update_pv(ss->pv, move, (ss+1)->pv);
1311 if (PvNode && value < beta) // Update alpha here!
1318 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1319 ttDepth, move, ss->staticEval, TT.generation());
1327 // All legal moves have been searched. A special case: If we're in check
1328 // and no legal moves were found, it is checkmate.
1329 if (InCheck && bestValue == -VALUE_INFINITE)
1330 return mated_in(ss->ply); // Plies to mate from the root
1332 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1333 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1334 ttDepth, bestMove, ss->staticEval, TT.generation());
1336 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1342 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1343 // "plies to mate from the current position". Non-mate scores are unchanged.
1344 // The function is called before storing a value in the transposition table.
1346 Value value_to_tt(Value v, int ply) {
1348 assert(v != VALUE_NONE);
1350 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1351 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1355 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1356 // from the transposition table (which refers to the plies to mate/be mated
1357 // from current position) to "plies to mate/be mated from the root".
1359 Value value_from_tt(Value v, int ply) {
1361 return v == VALUE_NONE ? VALUE_NONE
1362 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1363 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1367 // update_pv() adds current move and appends child pv[]
1369 void update_pv(Move* pv, Move move, Move* childPv) {
1371 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1377 // update_continuation_histories() updates histories of the move pairs formed
1378 // by moves at ply -1, -2, and -4 with current move.
1380 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1382 for (int i : {1, 2, 4})
1383 if (is_ok((ss-i)->currentMove))
1384 (ss-i)->contHistory->update(pc, to, bonus);
1388 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1390 void update_capture_stats(const Position& pos, Move move,
1391 Move* captures, int captureCnt, int bonus) {
1393 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1394 Piece moved_piece = pos.moved_piece(move);
1395 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1396 captureHistory.update(moved_piece, to_sq(move), captured, bonus);
1398 // Decrease all the other played capture moves
1399 for (int i = 0; i < captureCnt; ++i)
1401 moved_piece = pos.moved_piece(captures[i]);
1402 captured = type_of(pos.piece_on(to_sq(captures[i])));
1403 captureHistory.update(moved_piece, to_sq(captures[i]), captured, -bonus);
1408 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1410 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1411 Move* quiets, int quietsCnt, int bonus) {
1413 if (ss->killers[0] != move)
1415 ss->killers[1] = ss->killers[0];
1416 ss->killers[0] = move;
1419 Color us = pos.side_to_move();
1420 Thread* thisThread = pos.this_thread();
1421 thisThread->mainHistory.update(us, move, bonus);
1422 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1424 if (is_ok((ss-1)->currentMove))
1426 Square prevSq = to_sq((ss-1)->currentMove);
1427 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1430 // Decrease all the other played quiet moves
1431 for (int i = 0; i < quietsCnt; ++i)
1433 thisThread->mainHistory.update(us, quiets[i], -bonus);
1434 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1438 // When playing with strength handicap, choose best move among a set of RootMoves
1439 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1441 Move Skill::pick_best(size_t multiPV) {
1443 const RootMoves& rootMoves = Threads.main()->rootMoves;
1444 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1446 // RootMoves are already sorted by score in descending order
1447 Value topScore = rootMoves[0].score;
1448 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1449 int weakness = 120 - 2 * level;
1450 int maxScore = -VALUE_INFINITE;
1452 // Choose best move. For each move score we add two terms, both dependent on
1453 // weakness. One is deterministic and bigger for weaker levels, and one is
1454 // random. Then we choose the move with the resulting highest score.
1455 for (size_t i = 0; i < multiPV; ++i)
1457 // This is our magic formula
1458 int push = ( weakness * int(topScore - rootMoves[i].score)
1459 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1461 if (rootMoves[i].score + push >= maxScore)
1463 maxScore = rootMoves[i].score + push;
1464 best = rootMoves[i].pv[0];
1473 /// MainThread::check_time() is used to print debug info and, more importantly,
1474 /// to detect when we are out of available time and thus stop the search.
1476 void MainThread::check_time() {
1481 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1482 callsCnt = Limits.nodes ? std::min(4096, int(Limits.nodes / 1024)) : 4096;
1484 static TimePoint lastInfoTime = now();
1486 int elapsed = Time.elapsed();
1487 TimePoint tick = Limits.startTime + elapsed;
1489 if (tick - lastInfoTime >= 1000)
1491 lastInfoTime = tick;
1495 // We should not stop pondering until told so by the GUI
1499 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1500 || (Limits.movetime && elapsed >= Limits.movetime)
1501 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1502 Threads.stop = true;
1506 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1507 /// that all (if any) unsearched PV lines are sent using a previous search score.
1509 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1511 std::stringstream ss;
1512 int elapsed = Time.elapsed() + 1;
1513 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1514 size_t PVIdx = pos.this_thread()->PVIdx;
1515 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1516 uint64_t nodesSearched = Threads.nodes_searched();
1517 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1519 for (size_t i = 0; i < multiPV; ++i)
1521 bool updated = (i <= PVIdx && rootMoves[i].score != -VALUE_INFINITE);
1523 if (depth == ONE_PLY && !updated)
1526 Depth d = updated ? depth : depth - ONE_PLY;
1527 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1529 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1530 v = tb ? TB::Score : v;
1532 if (ss.rdbuf()->in_avail()) // Not at first line
1536 << " depth " << d / ONE_PLY
1537 << " seldepth " << rootMoves[i].selDepth
1538 << " multipv " << i + 1
1539 << " score " << UCI::value(v);
1541 if (!tb && i == PVIdx)
1542 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1544 ss << " nodes " << nodesSearched
1545 << " nps " << nodesSearched * 1000 / elapsed;
1547 if (elapsed > 1000) // Earlier makes little sense
1548 ss << " hashfull " << TT.hashfull();
1550 ss << " tbhits " << tbHits
1551 << " time " << elapsed
1554 for (Move m : rootMoves[i].pv)
1555 ss << " " << UCI::move(m, pos.is_chess960());
1562 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1563 /// before exiting the search, for instance, in case we stop the search during a
1564 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1565 /// otherwise in case of 'ponder on' we have nothing to think on.
1567 bool RootMove::extract_ponder_from_tt(Position& pos) {
1572 assert(pv.size() == 1);
1577 pos.do_move(pv[0], st);
1578 TTEntry* tte = TT.probe(pos.key(), ttHit);
1582 Move m = tte->move(); // Local copy to be SMP safe
1583 if (MoveList<LEGAL>(pos).contains(m))
1587 pos.undo_move(pv[0]);
1588 return pv.size() > 1;
1592 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1595 UseRule50 = Options["Syzygy50MoveRule"];
1596 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1597 Cardinality = Options["SyzygyProbeLimit"];
1599 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1600 if (Cardinality > MaxCardinality)
1602 Cardinality = MaxCardinality;
1603 ProbeDepth = DEPTH_ZERO;
1606 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1609 // Don't filter any moves if the user requested analysis on multiple
1610 if (Options["MultiPV"] != 1)
1613 // If the current root position is in the tablebases, then RootMoves
1614 // contains only moves that preserve the draw or the win.
1615 RootInTB = root_probe(pos, rootMoves, TB::Score);
1618 Cardinality = 0; // Do not probe tablebases during the search
1620 else // If DTZ tables are missing, use WDL tables as a fallback
1622 // Filter out moves that do not preserve the draw or the win.
1623 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1625 // Only probe during search if winning
1626 if (RootInTB && TB::Score <= VALUE_DRAW)
1630 if (RootInTB && !UseRule50)
1631 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1632 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
1635 // Since root_probe() and root_probe_wdl() dirty the root move scores,
1636 // we reset them to -VALUE_INFINITE
1637 for (RootMove& rm : rootMoves)
1638 rm.score = -VALUE_INFINITE;