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 margin based on depth
70 const int razor_margin = 600;
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_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 during 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 new PV when needed
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;
291 mainThread->failedLow = false;
292 mainThread->bestMoveChanges = 0;
295 size_t multiPV = Options["MultiPV"];
296 Skill skill(Options["Skill Level"]);
298 // When playing with strength handicap enable MultiPV search that we will
299 // use behind the scenes to retrieve a set of possible moves.
301 multiPV = std::max(multiPV, (size_t)4);
303 multiPV = std::min(multiPV, rootMoves.size());
305 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
306 Eval::Contempt = (us == WHITE ? make_score(contempt, contempt / 2)
307 : -make_score(contempt, contempt / 2));
309 // Iterative deepening loop until requested to stop or the target depth is reached
310 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
312 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
314 // Distribute search depths across the threads
317 int i = (idx - 1) % 20;
318 if (((rootDepth / ONE_PLY + rootPos.game_ply() + skipPhase[i]) / skipSize[i]) % 2)
322 // Age out PV variability metric
324 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
326 // Save the last iteration's scores before first PV line is searched and
327 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
328 for (RootMove& rm : rootMoves)
329 rm.previousScore = rm.score;
331 // MultiPV loop. We perform a full root search for each PV line
332 for (PVIdx = 0; PVIdx < multiPV && !Threads.stop; ++PVIdx)
334 // Reset UCI info selDepth for each depth and each PV line
337 // Reset aspiration window starting size
338 if (rootDepth >= 5 * ONE_PLY)
341 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
342 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
344 // Adjust contempt based on current situation
345 contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
346 contempt += bestValue > 500 ? 50: // Dynamic contempt
347 bestValue < -500 ? -50:
350 Eval::Contempt = (us == WHITE ? make_score(contempt, contempt / 2)
351 : -make_score(contempt, contempt / 2));
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 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
448 // if the bestMove is stable over several iterations, reduce time for this move,
449 // the longer the move has been stable, the more.
450 // Use part of the gained time from a previous stable move for the current move.
452 for (int i : {3, 4, 5})
453 if (lastBestMoveDepth * i < completedDepth )
454 timeReduction *= 1.3;
455 unstablePvFactor *= std::pow(mainThread->previousTimeReduction, 0.51) / timeReduction;
457 if ( rootMoves.size() == 1
458 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 605)
460 // If we are allowed to ponder do not stop the search now but
461 // keep pondering until the GUI sends "ponderhit" or "stop".
463 Threads.stopOnPonderhit = true;
474 mainThread->previousTimeReduction = timeReduction;
476 // If skill level is enabled, swap best PV line with the sub-optimal one
478 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
479 skill.best ? skill.best : skill.pick_best(multiPV)));
485 // search<>() is the main search function for both PV and non-PV nodes
487 template <NodeType NT>
488 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
490 const bool PvNode = NT == PV;
491 const bool rootNode = PvNode && ss->ply == 0;
493 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
494 assert(PvNode || (alpha == beta - 1));
495 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
496 assert(!(PvNode && cutNode));
497 assert(depth / ONE_PLY * ONE_PLY == depth);
499 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
503 Move ttMove, move, excludedMove, bestMove;
504 Depth extension, newDepth;
505 Value bestValue, value, ttValue, eval, maxValue;
506 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
507 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
509 int moveCount, captureCount, quietCount;
511 // Step 1. Initialize node
512 Thread* thisThread = pos.this_thread();
513 inCheck = pos.checkers();
514 moveCount = captureCount = quietCount = ss->moveCount = 0;
516 bestValue = -VALUE_INFINITE;
517 maxValue = VALUE_INFINITE;
519 // Check for the available remaining time
520 if (thisThread == Threads.main())
521 static_cast<MainThread*>(thisThread)->check_time();
523 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
524 if (PvNode && thisThread->selDepth < ss->ply + 1)
525 thisThread->selDepth = ss->ply + 1;
529 // Step 2. Check for aborted search and immediate draw
530 if (Threads.stop.load(std::memory_order_relaxed) || pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
531 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos) : VALUE_DRAW;
533 // Step 3. Mate distance pruning. Even if we mate at the next move our score
534 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
535 // a shorter mate was found upward in the tree then there is no need to search
536 // because we will never beat the current alpha. Same logic but with reversed
537 // signs applies also in the opposite condition of being mated instead of giving
538 // mate. In this case return a fail-high score.
539 alpha = std::max(mated_in(ss->ply), alpha);
540 beta = std::min(mate_in(ss->ply+1), beta);
545 assert(0 <= ss->ply && ss->ply < MAX_PLY);
547 (ss+1)->ply = ss->ply + 1;
548 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
549 ss->contHistory = &thisThread->contHistory[NO_PIECE][0];
550 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
551 Square prevSq = to_sq((ss-1)->currentMove);
553 // Step 4. Transposition table lookup. We don't want the score of a partial
554 // search to overwrite a previous full search TT value, so we use a different
555 // position key in case of an excluded move.
556 excludedMove = ss->excludedMove;
557 posKey = pos.key() ^ Key(excludedMove << 16); // isn't a very good hash
558 tte = TT.probe(posKey, ttHit);
559 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
560 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
561 : ttHit ? tte->move() : MOVE_NONE;
563 // At non-PV nodes we check for an early TT cutoff
566 && tte->depth() >= depth
567 && ttValue != VALUE_NONE // Possible in case of TT access race
568 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
569 : (tte->bound() & BOUND_UPPER)))
571 // If ttMove is quiet, update move sorting heuristics on TT hit
576 if (!pos.capture_or_promotion(ttMove))
577 update_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
579 // Extra penalty for a quiet TT move in previous ply when it gets refuted
580 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
581 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
583 // Penalty for a quiet ttMove that fails low
584 else if (!pos.capture_or_promotion(ttMove))
586 int penalty = -stat_bonus(depth);
587 thisThread->mainHistory.update(pos.side_to_move(), ttMove, penalty);
588 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
594 // Step 4a. Tablebase probe
595 if (!rootNode && TB::Cardinality)
597 int piecesCount = pos.count<ALL_PIECES>();
599 if ( piecesCount <= TB::Cardinality
600 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
601 && pos.rule50_count() == 0
602 && !pos.can_castle(ANY_CASTLING))
605 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
607 if (err != TB::ProbeState::FAIL)
609 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
611 int drawScore = TB::UseRule50 ? 1 : 0;
613 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
614 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
615 : VALUE_DRAW + 2 * wdl * drawScore;
617 Bound b = wdl < -drawScore ? BOUND_UPPER
618 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
620 if ( b == BOUND_EXACT
621 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
623 tte->save(posKey, value_to_tt(value, ss->ply), b,
624 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
625 MOVE_NONE, VALUE_NONE, TT.generation());
632 if (b == BOUND_LOWER)
633 bestValue = value, alpha = std::max(alpha, bestValue);
641 // Step 5. Evaluate the position statically
644 ss->staticEval = eval = VALUE_NONE;
650 // Never assume anything on values stored in TT
651 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
652 eval = ss->staticEval = evaluate(pos);
654 // Can ttValue be used as a better position evaluation?
655 if ( ttValue != VALUE_NONE
656 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
661 eval = ss->staticEval =
662 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
663 : -(ss-1)->staticEval + 2 * Eval::Tempo;
665 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
666 ss->staticEval, TT.generation());
669 if (skipEarlyPruning || !pos.non_pawn_material(pos.side_to_move()))
672 // Step 6. Razoring (skipped when in check)
674 && depth < 4 * ONE_PLY
675 && eval + razor_margin <= alpha)
677 if (depth <= ONE_PLY)
678 return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
680 Value ralpha = alpha - razor_margin;
681 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1);
686 // Step 7. Futility pruning: child node (skipped when in check)
688 && depth < 7 * ONE_PLY
689 && eval - futility_margin(depth) >= beta
690 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
693 // Step 8. Null move search with verification search (is omitted in PV nodes)
696 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
697 && (ss->ply >= thisThread->nmp_ply || ss->ply % 2 != thisThread->nmp_odd))
700 assert(eval - beta >= 0);
702 // Null move dynamic reduction based on depth and value
703 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
705 ss->currentMove = MOVE_NULL;
706 ss->contHistory = &thisThread->contHistory[NO_PIECE][0];
708 pos.do_null_move(st);
709 Value nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1)
710 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
711 pos.undo_null_move();
713 if (nullValue >= beta)
715 // Do not return unproven mate scores
716 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
719 if (abs(beta) < VALUE_KNOWN_WIN && (depth < 12 * ONE_PLY || thisThread->nmp_ply))
722 // Do verification search at high depths
723 // disable null move pruning for side to move for the first part of the remaining search tree
724 thisThread->nmp_ply = ss->ply + 3 * (depth-R) / 4;
725 thisThread->nmp_odd = ss->ply % 2;
727 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
728 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
730 thisThread->nmp_odd = thisThread->nmp_ply = 0;
737 // Step 9. ProbCut (skipped when in check)
738 // If we have a good enough capture and a reduced search returns a value
739 // much above beta, we can (almost) safely prune the previous move.
741 && depth >= 5 * ONE_PLY
742 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
744 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
746 assert(is_ok((ss-1)->currentMove));
748 MovePicker mp(pos, ttMove, rbeta - ss->staticEval, &thisThread->captureHistory);
750 while ((move = mp.next_move()) != MOVE_NONE)
753 ss->currentMove = move;
754 ss->contHistory = &thisThread->contHistory[pos.moved_piece(move)][to_sq(move)];
756 assert(depth >= 5 * ONE_PLY);
757 pos.do_move(move, st);
758 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode, false);
765 // Step 10. Internal iterative deepening (skipped when in check)
766 if ( depth >= 6 * ONE_PLY
768 && (PvNode || ss->staticEval + 256 >= beta))
770 Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
771 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
773 tte = TT.probe(posKey, ttHit);
774 ttMove = ttHit ? tte->move() : MOVE_NONE;
777 moves_loop: // When in check search starts from here
779 const PieceToHistory* contHist[] = { (ss-1)->contHistory, (ss-2)->contHistory, nullptr, (ss-4)->contHistory };
780 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
782 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory, contHist, countermove, ss->killers);
783 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
784 improving = ss->staticEval >= (ss-2)->staticEval
785 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
786 ||(ss-2)->staticEval == VALUE_NONE;
788 singularExtensionNode = !rootNode
789 && depth >= 8 * ONE_PLY
790 && ttMove != MOVE_NONE
791 && ttValue != VALUE_NONE
792 && !excludedMove // Recursive singular search is not allowed
793 && (tte->bound() & BOUND_LOWER)
794 && tte->depth() >= depth - 3 * ONE_PLY;
797 pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
799 // Step 11. Loop through moves
800 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
801 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
805 if (move == excludedMove)
808 // At root obey the "searchmoves" option and skip moves not listed in Root
809 // Move List. As a consequence any illegal move is also skipped. In MultiPV
810 // mode we also skip PV moves which have been already searched.
811 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
812 thisThread->rootMoves.end(), move))
815 ss->moveCount = ++moveCount;
817 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
818 sync_cout << "info depth " << depth / ONE_PLY
819 << " currmove " << UCI::move(move, pos.is_chess960())
820 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
823 (ss+1)->pv = nullptr;
825 extension = DEPTH_ZERO;
826 captureOrPromotion = pos.capture_or_promotion(move);
827 movedPiece = pos.moved_piece(move);
829 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
830 ? pos.check_squares(type_of(movedPiece)) & to_sq(move)
831 : pos.gives_check(move);
833 moveCountPruning = depth < 16 * ONE_PLY
834 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
836 // Step 12. Singular and Gives Check Extensions
838 // Singular extension search. If all moves but one fail low on a search of
839 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
840 // is singular and should be extended. To verify this we do a reduced search
841 // on all the other moves but the ttMove and if the result is lower than
842 // ttValue minus a margin then we will extend the ttMove.
843 if ( singularExtensionNode
847 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
848 Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
849 ss->excludedMove = move;
850 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode, true);
851 ss->excludedMove = MOVE_NONE;
861 // Calculate new depth for this move
862 newDepth = depth - ONE_PLY + extension;
864 // Step 13. Pruning at shallow depth
866 && pos.non_pawn_material(pos.side_to_move())
867 && bestValue > VALUE_MATED_IN_MAX_PLY)
869 if ( !captureOrPromotion
871 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
873 // Move count based pruning
874 if (moveCountPruning)
880 // Reduced depth of the next LMR search
881 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
883 // Countermoves based pruning
885 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
886 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
889 // Futility pruning: parent node
892 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
895 // Prune moves with negative SEE
897 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
900 else if ( depth < 7 * ONE_PLY
902 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
906 // Speculative prefetch as early as possible
907 prefetch(TT.first_entry(pos.key_after(move)));
909 // Check for legality just before making the move
910 if (!rootNode && !pos.legal(move))
912 ss->moveCount = --moveCount;
916 if (move == ttMove && captureOrPromotion)
919 // Update the current move (this must be done after singular extension search)
920 ss->currentMove = move;
921 ss->contHistory = &thisThread->contHistory[movedPiece][to_sq(move)];
923 // Step 14. Make the move
924 pos.do_move(move, st, givesCheck);
926 // Step 15. Reduced depth search (LMR). If the move fails high it will be
927 // re-searched at full depth.
928 if ( depth >= 3 * ONE_PLY
930 && (!captureOrPromotion || moveCountPruning))
932 Depth r = reduction<PvNode>(improving, depth, moveCount);
934 if (captureOrPromotion)
935 r -= r ? ONE_PLY : DEPTH_ZERO;
938 // Decrease reduction if opponent's move count is high
939 if ((ss-1)->moveCount > 15)
942 // Decrease reduction for exact PV nodes
946 // Increase reduction if ttMove is a capture
950 // Increase reduction for cut nodes
954 // Decrease reduction for moves that escape a capture. Filter out
955 // castling moves, because they are coded as "king captures rook" and
956 // hence break make_move().
957 else if ( type_of(move) == NORMAL
958 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
961 ss->statScore = thisThread->mainHistory[~pos.side_to_move()][from_to(move)]
962 + (*contHist[0])[movedPiece][to_sq(move)]
963 + (*contHist[1])[movedPiece][to_sq(move)]
964 + (*contHist[3])[movedPiece][to_sq(move)]
967 // Decrease/increase reduction by comparing opponent's stat score
968 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
971 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
974 // Decrease/increase reduction for moves with a good/bad history
975 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->statScore / 20000) * ONE_PLY);
978 Depth d = std::max(newDepth - r, ONE_PLY);
980 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
982 doFullDepthSearch = (value > alpha && d != newDepth);
985 doFullDepthSearch = !PvNode || moveCount > 1;
987 // Step 16. Full depth search when LMR is skipped or fails high
988 if (doFullDepthSearch)
989 value = newDepth < ONE_PLY ?
990 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha)
991 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha)
992 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
994 // For PV nodes only, do a full PV search on the first move or after a fail
995 // high (in the latter case search only if value < beta), otherwise let the
996 // parent node fail low with value <= alpha and try another move.
997 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1000 (ss+1)->pv[0] = MOVE_NONE;
1002 value = newDepth < ONE_PLY ?
1003 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha)
1004 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha)
1005 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1008 // Step 17. Undo move
1009 pos.undo_move(move);
1011 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1013 // Step 18. Check for a new best move
1014 // Finished searching the move. If a stop occurred, the return value of
1015 // the search cannot be trusted, and we return immediately without
1016 // updating best move, PV and TT.
1017 if (Threads.stop.load(std::memory_order_relaxed))
1022 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1023 thisThread->rootMoves.end(), move);
1025 // PV move or new best move ?
1026 if (moveCount == 1 || value > alpha)
1029 rm.selDepth = thisThread->selDepth;
1034 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1035 rm.pv.push_back(*m);
1037 // We record how often the best move has been changed in each
1038 // iteration. This information is used for time management: When
1039 // the best move changes frequently, we allocate some more time.
1040 if (moveCount > 1 && thisThread == Threads.main())
1041 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1044 // All other moves but the PV are set to the lowest value: this
1045 // is not a problem when sorting because the sort is stable and the
1046 // move position in the list is preserved - just the PV is pushed up.
1047 rm.score = -VALUE_INFINITE;
1050 if (value > bestValue)
1058 if (PvNode && !rootNode) // Update pv even in fail-high case
1059 update_pv(ss->pv, move, (ss+1)->pv);
1061 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1065 assert(value >= beta); // Fail high
1071 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1072 quietsSearched[quietCount++] = move;
1073 else if (captureOrPromotion && move != bestMove && captureCount < 32)
1074 capturesSearched[captureCount++] = move;
1077 // The following condition would detect a stop only after move loop has been
1078 // completed. But in this case bestValue is valid because we have fully
1079 // searched our subtree, and we can anyhow save the result in TT.
1085 // Step 20. Check for mate and stalemate
1086 // All legal moves have been searched and if there are no legal moves, it
1087 // must be a mate or a stalemate. If we are in a singular extension search then
1088 // return a fail low score.
1090 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1093 bestValue = excludedMove ? alpha
1094 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1097 // Quiet best move: update move sorting heuristics
1098 if (!pos.capture_or_promotion(bestMove))
1099 update_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1101 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth));
1103 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1104 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1105 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1107 // Bonus for prior countermove that caused the fail low
1108 else if ( depth >= 3 * ONE_PLY
1109 && !pos.captured_piece()
1110 && is_ok((ss-1)->currentMove))
1111 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1114 bestValue = std::min(bestValue, maxValue);
1117 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1118 bestValue >= beta ? BOUND_LOWER :
1119 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1120 depth, bestMove, ss->staticEval, TT.generation());
1122 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1128 // qsearch() is the quiescence search function, which is called by the main
1129 // search function with depth zero, or recursively with depth less than ONE_PLY.
1131 template <NodeType NT, bool InCheck>
1132 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1134 const bool PvNode = NT == PV;
1136 assert(InCheck == bool(pos.checkers()));
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);
1146 Move ttMove, move, bestMove;
1147 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1148 bool ttHit, givesCheck, evasionPrunable;
1154 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1156 ss->pv[0] = MOVE_NONE;
1159 ss->currentMove = bestMove = MOVE_NONE;
1160 (ss+1)->ply = ss->ply + 1;
1163 // Check for an instant draw or if the maximum ply has been reached
1164 if (pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
1165 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos) : VALUE_DRAW;
1167 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1169 // Decide whether or not to include checks: this fixes also the type of
1170 // TT entry depth that we are going to use. Note that in qsearch we use
1171 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1172 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1173 : DEPTH_QS_NO_CHECKS;
1174 // Transposition table lookup
1176 tte = TT.probe(posKey, ttHit);
1177 ttMove = ttHit ? tte->move() : MOVE_NONE;
1178 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1182 && tte->depth() >= ttDepth
1183 && ttValue != VALUE_NONE // Only in case of TT access race
1184 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1185 : (tte->bound() & BOUND_UPPER)))
1188 // Evaluate the position statically
1191 ss->staticEval = VALUE_NONE;
1192 bestValue = futilityBase = -VALUE_INFINITE;
1198 // Never assume anything on values stored in TT
1199 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1200 ss->staticEval = bestValue = evaluate(pos);
1202 // Can ttValue be used as a better position evaluation?
1203 if ( ttValue != VALUE_NONE
1204 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1205 bestValue = ttValue;
1208 ss->staticEval = bestValue =
1209 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1210 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1212 // Stand pat. Return immediately if static value is at least beta
1213 if (bestValue >= beta)
1216 tte->save(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1217 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1222 if (PvNode && bestValue > alpha)
1225 futilityBase = bestValue + 128;
1228 // Initialize a MovePicker object for the current position, and prepare
1229 // to search the moves. Because the depth is <= 0 here, only captures,
1230 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1232 MovePicker mp(pos, ttMove, depth, &pos.this_thread()->mainHistory, &pos.this_thread()->captureHistory, to_sq((ss-1)->currentMove));
1234 // Loop through the moves until no moves remain or a beta cutoff occurs
1235 while ((move = mp.next_move()) != MOVE_NONE)
1237 assert(is_ok(move));
1239 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
1240 ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
1241 : pos.gives_check(move);
1248 && futilityBase > -VALUE_KNOWN_WIN
1249 && !pos.advanced_pawn_push(move))
1251 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1253 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1255 if (futilityValue <= alpha)
1257 bestValue = std::max(bestValue, futilityValue);
1261 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1263 bestValue = std::max(bestValue, futilityBase);
1268 // Detect non-capture evasions that are candidates to be pruned
1269 evasionPrunable = InCheck
1270 && (depth != DEPTH_ZERO || moveCount > 2)
1271 && bestValue > VALUE_MATED_IN_MAX_PLY
1272 && !pos.capture(move);
1274 // Don't search moves with negative SEE values
1275 if ( (!InCheck || evasionPrunable)
1276 && !pos.see_ge(move))
1279 // Speculative prefetch as early as possible
1280 prefetch(TT.first_entry(pos.key_after(move)));
1282 // Check for legality just before making the move
1283 if (!pos.legal(move))
1289 ss->currentMove = move;
1291 // Make and search the move
1292 pos.do_move(move, st, givesCheck);
1293 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1294 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1295 pos.undo_move(move);
1297 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1299 // Check for a new best move
1300 if (value > bestValue)
1306 if (PvNode) // Update pv even in fail-high case
1307 update_pv(ss->pv, move, (ss+1)->pv);
1309 if (PvNode && value < beta) // Update alpha here!
1316 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1317 ttDepth, move, ss->staticEval, TT.generation());
1325 // All legal moves have been searched. A special case: If we're in check
1326 // and no legal moves were found, it is checkmate.
1327 if (InCheck && bestValue == -VALUE_INFINITE)
1328 return mated_in(ss->ply); // Plies to mate from the root
1330 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1331 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1332 ttDepth, bestMove, ss->staticEval, TT.generation());
1334 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1340 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1341 // "plies to mate from the current position". Non-mate scores are unchanged.
1342 // The function is called before storing a value in the transposition table.
1344 Value value_to_tt(Value v, int ply) {
1346 assert(v != VALUE_NONE);
1348 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1349 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1353 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1354 // from the transposition table (which refers to the plies to mate/be mated
1355 // from current position) to "plies to mate/be mated from the root".
1357 Value value_from_tt(Value v, int ply) {
1359 return v == VALUE_NONE ? VALUE_NONE
1360 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1361 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1365 // update_pv() adds current move and appends child pv[]
1367 void update_pv(Move* pv, Move move, Move* childPv) {
1369 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1375 // update_continuation_histories() updates histories of the move pairs formed
1376 // by moves at ply -1, -2, and -4 with current move.
1378 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1380 for (int i : {1, 2, 4})
1381 if (is_ok((ss-i)->currentMove))
1382 (ss-i)->contHistory->update(pc, to, bonus);
1386 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1388 void update_capture_stats(const Position& pos, Move move,
1389 Move* captures, int captureCnt, int bonus) {
1391 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1392 Piece moved_piece = pos.moved_piece(move);
1393 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1394 captureHistory.update(moved_piece, to_sq(move), captured, bonus);
1396 // Decrease all the other played capture moves
1397 for (int i = 0; i < captureCnt; ++i)
1399 moved_piece = pos.moved_piece(captures[i]);
1400 captured = type_of(pos.piece_on(to_sq(captures[i])));
1401 captureHistory.update(moved_piece, to_sq(captures[i]), captured, -bonus);
1406 // update_stats() updates move sorting heuristics when a new quiet best move is found
1408 void update_stats(const Position& pos, Stack* ss, Move move,
1409 Move* quiets, int quietsCnt, int bonus) {
1411 if (ss->killers[0] != move)
1413 ss->killers[1] = ss->killers[0];
1414 ss->killers[0] = move;
1417 Color c = pos.side_to_move();
1418 Thread* thisThread = pos.this_thread();
1419 thisThread->mainHistory.update(c, move, bonus);
1420 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1422 if (is_ok((ss-1)->currentMove))
1424 Square prevSq = to_sq((ss-1)->currentMove);
1425 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1428 // Decrease all the other played quiet moves
1429 for (int i = 0; i < quietsCnt; ++i)
1431 thisThread->mainHistory.update(c, quiets[i], -bonus);
1432 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1436 // When playing with strength handicap, choose best move among a set of RootMoves
1437 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1439 Move Skill::pick_best(size_t multiPV) {
1441 const RootMoves& rootMoves = Threads.main()->rootMoves;
1442 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1444 // RootMoves are already sorted by score in descending order
1445 Value topScore = rootMoves[0].score;
1446 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1447 int weakness = 120 - 2 * level;
1448 int maxScore = -VALUE_INFINITE;
1450 // Choose best move. For each move score we add two terms, both dependent on
1451 // weakness. One is deterministic and bigger for weaker levels, and one is
1452 // random. Then we choose the move with the resulting highest score.
1453 for (size_t i = 0; i < multiPV; ++i)
1455 // This is our magic formula
1456 int push = ( weakness * int(topScore - rootMoves[i].score)
1457 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1459 if (rootMoves[i].score + push >= maxScore)
1461 maxScore = rootMoves[i].score + push;
1462 best = rootMoves[i].pv[0];
1471 // check_time() is used to print debug info and, more importantly, to detect
1472 // when we are out of available time and thus stop the search.
1474 void MainThread::check_time() {
1479 // At low node count increase the checking rate to about 0.1% of nodes
1480 // otherwise use a default value.
1481 callsCnt = Limits.nodes ? std::min(4096, int(Limits.nodes / 1024)) : 4096;
1483 static TimePoint lastInfoTime = now();
1485 int elapsed = Time.elapsed();
1486 TimePoint tick = Limits.startTime + elapsed;
1488 if (tick - lastInfoTime >= 1000)
1490 lastInfoTime = tick;
1494 // An engine may not stop pondering until told so by the GUI
1498 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1499 || (Limits.movetime && elapsed >= Limits.movetime)
1500 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1501 Threads.stop = true;
1505 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1506 /// that all (if any) unsearched PV lines are sent using a previous search score.
1508 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1510 std::stringstream ss;
1511 int elapsed = Time.elapsed() + 1;
1512 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1513 size_t PVIdx = pos.this_thread()->PVIdx;
1514 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1515 uint64_t nodesSearched = Threads.nodes_searched();
1516 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1518 for (size_t i = 0; i < multiPV; ++i)
1520 bool updated = (i <= PVIdx && rootMoves[i].score != -VALUE_INFINITE);
1522 if (depth == ONE_PLY && !updated)
1525 Depth d = updated ? depth : depth - ONE_PLY;
1526 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1528 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1529 v = tb ? TB::Score : v;
1531 if (ss.rdbuf()->in_avail()) // Not at first line
1535 << " depth " << d / ONE_PLY
1536 << " seldepth " << rootMoves[i].selDepth
1537 << " multipv " << i + 1
1538 << " score " << UCI::value(v);
1540 if (!tb && i == PVIdx)
1541 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1543 ss << " nodes " << nodesSearched
1544 << " nps " << nodesSearched * 1000 / elapsed;
1546 if (elapsed > 1000) // Earlier makes little sense
1547 ss << " hashfull " << TT.hashfull();
1549 ss << " tbhits " << tbHits
1550 << " time " << elapsed
1553 for (Move m : rootMoves[i].pv)
1554 ss << " " << UCI::move(m, pos.is_chess960());
1561 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1562 /// before exiting the search, for instance, in case we stop the search during a
1563 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1564 /// otherwise in case of 'ponder on' we have nothing to think on.
1566 bool RootMove::extract_ponder_from_tt(Position& pos) {
1571 assert(pv.size() == 1);
1576 pos.do_move(pv[0], st);
1577 TTEntry* tte = TT.probe(pos.key(), ttHit);
1581 Move m = tte->move(); // Local copy to be SMP safe
1582 if (MoveList<LEGAL>(pos).contains(m))
1586 pos.undo_move(pv[0]);
1587 return pv.size() > 1;
1590 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1593 UseRule50 = Options["Syzygy50MoveRule"];
1594 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1595 Cardinality = Options["SyzygyProbeLimit"];
1597 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1598 if (Cardinality > MaxCardinality)
1600 Cardinality = MaxCardinality;
1601 ProbeDepth = DEPTH_ZERO;
1604 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1607 // Don't filter any moves if the user requested analysis on multiple
1608 if (Options["MultiPV"] != 1)
1611 // If the current root position is in the tablebases, then RootMoves
1612 // contains only moves that preserve the draw or the win.
1613 RootInTB = root_probe(pos, rootMoves, TB::Score);
1616 Cardinality = 0; // Do not probe tablebases during the search
1618 else // If DTZ tables are missing, use WDL tables as a fallback
1620 // Filter out moves that do not preserve the draw or the win.
1621 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1623 // Only probe during search if winning
1624 if (RootInTB && TB::Score <= VALUE_DRAW)
1628 if (RootInTB && !UseRule50)
1629 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1630 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
1633 // Since root_probe() and root_probe_wdl() dirty the root move scores,
1634 // we reset them to -VALUE_INFINITE
1635 for (RootMove& rm : rootMoves)
1636 rm.score = -VALUE_INFINITE;