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 constexpr int SkipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
67 constexpr int SkipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
69 // Razor and futility margins
70 constexpr int RazorMargin[] = {0, 590, 604};
71 Value futility_margin(Depth d, bool improving) {
72 return Value((175 - 50 * improving) * d / ONE_PLY);
75 // Margin for pruning capturing moves: almost linear in depth
76 constexpr int CapturePruneMargin[] = { 0,
77 1 * PawnValueEg * 1055 / 1000,
78 2 * PawnValueEg * 1042 / 1000,
79 3 * PawnValueEg * 963 / 1000,
80 4 * PawnValueEg * 1038 / 1000,
81 5 * PawnValueEg * 950 / 1000,
82 6 * PawnValueEg * 930 / 1000
85 // Futility and reductions lookup tables, initialized at startup
86 int FutilityMoveCounts[2][16]; // [improving][depth]
87 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
89 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
90 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
93 // History and stats update bonus, based on depth
94 int stat_bonus(Depth depth) {
95 int d = depth / ONE_PLY;
96 return d > 17 ? 0 : d * d + 2 * d - 2;
99 // Skill structure is used to implement strength limit
101 explicit Skill(int l) : level(l) {}
102 bool enabled() const { return level < 20; }
103 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
104 Move pick_best(size_t multiPV);
107 Move best = MOVE_NONE;
110 template <NodeType NT>
111 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning);
113 template <NodeType NT>
114 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
116 Value value_to_tt(Value v, int ply);
117 Value value_from_tt(Value v, int ply);
118 void update_pv(Move* pv, Move move, Move* childPv);
119 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
120 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
121 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCnt, int bonus);
123 inline bool gives_check(const Position& pos, Move move) {
124 Color us = pos.side_to_move();
125 return type_of(move) == NORMAL && !(pos.blockers_for_king(~us) & pos.pieces(us))
126 ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
127 : pos.gives_check(move);
130 // perft() is our utility to verify move generation. All the leaf nodes up
131 // to the given depth are generated and counted, and the sum is returned.
133 uint64_t perft(Position& pos, Depth depth) {
136 uint64_t cnt, nodes = 0;
137 const bool leaf = (depth == 2 * ONE_PLY);
139 for (const auto& m : MoveList<LEGAL>(pos))
141 if (Root && depth <= ONE_PLY)
146 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
151 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
159 /// Search::init() is called at startup to initialize various lookup tables
161 void Search::init() {
163 for (int imp = 0; imp <= 1; ++imp)
164 for (int d = 1; d < 64; ++d)
165 for (int mc = 1; mc < 64; ++mc)
167 double r = log(d) * log(mc) / 1.95;
169 Reductions[NonPV][imp][d][mc] = int(std::round(r));
170 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
172 // Increase reduction for non-PV nodes when eval is not improving
174 Reductions[NonPV][imp][d][mc]++;
177 for (int d = 0; d < 16; ++d)
179 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
180 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
185 /// Search::clear() resets search state to its initial value
187 void Search::clear() {
189 Threads.main()->wait_for_search_finished();
191 Time.availableNodes = 0;
197 /// MainThread::search() is called by the main thread when the program receives
198 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
200 void MainThread::search() {
204 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
205 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
209 Color us = rootPos.side_to_move();
210 Time.init(Limits, us, rootPos.game_ply());
213 if (rootMoves.empty())
215 rootMoves.emplace_back(MOVE_NONE);
216 sync_cout << "info depth 0 score "
217 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
222 for (Thread* th : Threads)
224 th->start_searching();
226 Thread::search(); // Let's start searching!
229 // When we reach the maximum depth, we can arrive here without a raise of
230 // Threads.stop. However, if we are pondering or in an infinite search,
231 // the UCI protocol states that we shouldn't print the best move before the
232 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
233 // until the GUI sends one of those commands (which also raises Threads.stop).
234 Threads.stopOnPonderhit = true;
236 while (!Threads.stop && (Threads.ponder || Limits.infinite))
237 {} // Busy wait for a stop or a ponder reset
239 // Stop the threads if not already stopped (also raise the stop if
240 // "ponderhit" just reset Threads.ponder).
243 // Wait until all threads have finished
244 for (Thread* th : Threads)
246 th->wait_for_search_finished();
248 // When playing in 'nodes as time' mode, subtract the searched nodes from
249 // the available ones before exiting.
251 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
253 // Check if there are threads with a better score than main thread
254 Thread* bestThread = this;
255 if ( Options["MultiPV"] == 1
257 && !Skill(Options["Skill Level"]).enabled()
258 && rootMoves[0].pv[0] != MOVE_NONE)
260 for (Thread* th : Threads)
262 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
263 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
265 // Select the thread with the best score, always if it is a mate
267 && (depthDiff >= 0 || th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY))
272 previousScore = bestThread->rootMoves[0].score;
274 // Send again PV info if we have a new best thread
275 if (bestThread != this)
276 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
278 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
280 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
281 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
283 std::cout << sync_endl;
287 /// Thread::search() is the main iterative deepening loop. It calls search()
288 /// repeatedly with increasing depth until the allocated thinking time has been
289 /// consumed, the user stops the search, or the maximum search depth is reached.
291 void Thread::search() {
293 Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
294 Value bestValue, alpha, beta, delta;
295 Move lastBestMove = MOVE_NONE;
296 Depth lastBestMoveDepth = DEPTH_ZERO;
297 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
298 double timeReduction = 1.0;
299 Color us = rootPos.side_to_move();
301 std::memset(ss-4, 0, 7 * sizeof(Stack));
302 for (int i = 4; i > 0; i--)
303 (ss-i)->contHistory = this->contHistory[NO_PIECE][0].get(); // Use as sentinel
305 bestValue = delta = alpha = -VALUE_INFINITE;
306 beta = VALUE_INFINITE;
309 mainThread->bestMoveChanges = 0, mainThread->failedLow = false;
311 size_t multiPV = Options["MultiPV"];
312 Skill skill(Options["Skill Level"]);
314 // When playing with strength handicap enable MultiPV search that we will
315 // use behind the scenes to retrieve a set of possible moves.
317 multiPV = std::max(multiPV, (size_t)4);
319 multiPV = std::min(multiPV, rootMoves.size());
321 int ct = Options["Contempt"] * PawnValueEg / 100; // From centipawns
323 // In analysis mode, adjust contempt in accordance with user preference
324 if (Limits.infinite || Options["UCI_AnalyseMode"])
325 ct = Options["Analysis Contempt"] == "Off" ? 0
326 : Options["Analysis Contempt"] == "Both" ? ct
327 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
328 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
331 // In evaluate.cpp the evaluation is from the white point of view
332 contempt = (us == WHITE ? make_score(ct, ct / 2)
333 : -make_score(ct, ct / 2));
335 // Iterative deepening loop until requested to stop or the target depth is reached
336 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
338 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
340 // Distribute search depths across the helper threads
343 int i = (idx - 1) % 20;
344 if (((rootDepth / ONE_PLY + rootPos.game_ply() + SkipPhase[i]) / SkipSize[i]) % 2)
345 continue; // Retry with an incremented rootDepth
348 // Age out PV variability metric
350 mainThread->bestMoveChanges *= 0.517, mainThread->failedLow = false;
352 // Save the last iteration's scores before first PV line is searched and
353 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
354 for (RootMove& rm : rootMoves)
355 rm.previousScore = rm.score;
357 // MultiPV loop. We perform a full root search for each PV line
358 for (PVIdx = 0; PVIdx < multiPV && !Threads.stop; ++PVIdx)
360 // Reset UCI info selDepth for each depth and each PV line
363 // Reset aspiration window starting size
364 if (rootDepth >= 5 * ONE_PLY)
366 Value previousScore = rootMoves[PVIdx].previousScore;
368 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
369 beta = std::min(previousScore + delta, VALUE_INFINITE);
371 // Adjust contempt based on root move's previousScore (dynamic contempt)
372 int dct = ct + int(std::round(48 * atan(float(previousScore) / 128)));
374 contempt = (us == WHITE ? make_score(dct, dct / 2)
375 : -make_score(dct, dct / 2));
378 // Start with a small aspiration window and, in the case of a fail
379 // high/low, re-search with a bigger window until we don't fail
383 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
385 // Bring the best move to the front. It is critical that sorting
386 // is done with a stable algorithm because all the values but the
387 // first and eventually the new best one are set to -VALUE_INFINITE
388 // and we want to keep the same order for all the moves except the
389 // new PV that goes to the front. Note that in case of MultiPV
390 // search the already searched PV lines are preserved.
391 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
393 // If search has been stopped, we break immediately. Sorting is
394 // safe because RootMoves is still valid, although it refers to
395 // the previous iteration.
399 // When failing high/low give some update (without cluttering
400 // the UI) before a re-search.
403 && (bestValue <= alpha || bestValue >= beta)
404 && Time.elapsed() > 3000)
405 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
407 // In case of failing low/high increase aspiration window and
408 // re-search, otherwise exit the loop.
409 if (bestValue <= alpha)
411 beta = (alpha + beta) / 2;
412 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
416 mainThread->failedLow = true;
417 Threads.stopOnPonderhit = false;
420 else if (bestValue >= beta)
421 beta = std::min(bestValue + delta, VALUE_INFINITE);
425 delta += delta / 4 + 5;
427 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
430 // Sort the PV lines searched so far and update the GUI
431 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
434 && (Threads.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000))
435 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
439 completedDepth = rootDepth;
441 if (rootMoves[0].pv[0] != lastBestMove) {
442 lastBestMove = rootMoves[0].pv[0];
443 lastBestMoveDepth = rootDepth;
446 // Have we found a "mate in x"?
448 && bestValue >= VALUE_MATE_IN_MAX_PLY
449 && VALUE_MATE - bestValue <= 2 * Limits.mate)
455 // If skill level is enabled and time is up, pick a sub-optimal best move
456 if (skill.enabled() && skill.time_to_pick(rootDepth))
457 skill.pick_best(multiPV);
459 // Do we have time for the next iteration? Can we stop searching now?
460 if ( Limits.use_time_management()
462 && !Threads.stopOnPonderhit)
464 const int F[] = { mainThread->failedLow,
465 bestValue - mainThread->previousScore };
467 int improvingFactor = std::max(246, std::min(832, 306 + 119 * F[0] - 6 * F[1]));
469 // If the bestMove is stable over several iterations, reduce time accordingly
471 for (int i : {3, 4, 5})
472 if (lastBestMoveDepth * i < completedDepth)
473 timeReduction *= 1.25;
475 // Use part of the gained time from a previous stable move for the current move
476 double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
477 bestMoveInstability *= std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
479 // Stop the search if we have only one legal move, or if available time elapsed
480 if ( rootMoves.size() == 1
481 || Time.elapsed() > Time.optimum() * bestMoveInstability * improvingFactor / 581)
483 // If we are allowed to ponder do not stop the search now but
484 // keep pondering until the GUI sends "ponderhit" or "stop".
486 Threads.stopOnPonderhit = true;
496 mainThread->previousTimeReduction = timeReduction;
498 // If skill level is enabled, swap best PV line with the sub-optimal one
500 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
501 skill.best ? skill.best : skill.pick_best(multiPV)));
507 // search<>() is the main search function for both PV and non-PV nodes
509 template <NodeType NT>
510 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
512 // Use quiescence search when needed
514 return qsearch<NT>(pos, ss, alpha, beta);
516 constexpr bool PvNode = NT == PV;
517 const bool rootNode = PvNode && ss->ply == 0;
519 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
520 assert(PvNode || (alpha == beta - 1));
521 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
522 assert(!(PvNode && cutNode));
523 assert(depth / ONE_PLY * ONE_PLY == depth);
525 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
529 Move ttMove, move, excludedMove, bestMove;
530 Depth extension, newDepth;
531 Value bestValue, value, ttValue, eval, maxValue;
532 bool ttHit, inCheck, givesCheck, improving;
533 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
535 int moveCount, captureCount, quietCount;
537 // Step 1. Initialize node
538 Thread* thisThread = pos.this_thread();
539 inCheck = pos.checkers();
540 moveCount = captureCount = quietCount = ss->moveCount = 0;
541 bestValue = -VALUE_INFINITE;
542 maxValue = VALUE_INFINITE;
544 // Check for the available remaining time
545 if (thisThread == Threads.main())
546 static_cast<MainThread*>(thisThread)->check_time();
548 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
549 if (PvNode && thisThread->selDepth < ss->ply + 1)
550 thisThread->selDepth = ss->ply + 1;
554 // Step 2. Check for aborted search and immediate draw
555 if ( Threads.stop.load(std::memory_order_relaxed)
556 || pos.is_draw(ss->ply)
557 || ss->ply >= MAX_PLY)
558 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
560 // Step 3. Mate distance pruning. Even if we mate at the next move our score
561 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
562 // a shorter mate was found upward in the tree then there is no need to search
563 // because we will never beat the current alpha. Same logic but with reversed
564 // signs applies also in the opposite condition of being mated instead of giving
565 // mate. In this case return a fail-high score.
566 alpha = std::max(mated_in(ss->ply), alpha);
567 beta = std::min(mate_in(ss->ply+1), beta);
572 assert(0 <= ss->ply && ss->ply < MAX_PLY);
574 (ss+1)->ply = ss->ply + 1;
575 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
576 ss->contHistory = thisThread->contHistory[NO_PIECE][0].get();
577 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
578 Square prevSq = to_sq((ss-1)->currentMove);
580 // Initialize statScore to zero for the grandchildren of the current position.
581 // So statScore is shared between all grandchildren and only the first grandchild
582 // starts with statScore = 0. Later grandchildren start with the last calculated
583 // statScore of the previous grandchild. This influences the reduction rules in
584 // LMR which are based on the statScore of parent position.
585 (ss+2)->statScore = 0;
587 // Step 4. Transposition table lookup. We don't want the score of a partial
588 // search to overwrite a previous full search TT value, so we use a different
589 // position key in case of an excluded move.
590 excludedMove = ss->excludedMove;
591 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
592 tte = TT.probe(posKey, ttHit);
593 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
594 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
595 : ttHit ? tte->move() : MOVE_NONE;
597 // At non-PV nodes we check for an early TT cutoff
600 && tte->depth() >= depth
601 && ttValue != VALUE_NONE // Possible in case of TT access race
602 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
603 : (tte->bound() & BOUND_UPPER)))
605 // If ttMove is quiet, update move sorting heuristics on TT hit
610 if (!pos.capture_or_promotion(ttMove))
611 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
613 // Extra penalty for a quiet TT move in previous ply when it gets refuted
614 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
615 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
617 // Penalty for a quiet ttMove that fails low
618 else if (!pos.capture_or_promotion(ttMove))
620 int penalty = -stat_bonus(depth);
621 thisThread->mainHistory[pos.side_to_move()][from_to(ttMove)] << penalty;
622 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
628 // Step 5. Tablebases probe
629 if (!rootNode && TB::Cardinality)
631 int piecesCount = pos.count<ALL_PIECES>();
633 if ( piecesCount <= TB::Cardinality
634 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
635 && pos.rule50_count() == 0
636 && !pos.can_castle(ANY_CASTLING))
639 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
641 if (err != TB::ProbeState::FAIL)
643 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
645 int drawScore = TB::UseRule50 ? 1 : 0;
647 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
648 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
649 : VALUE_DRAW + 2 * wdl * drawScore;
651 Bound b = wdl < -drawScore ? BOUND_UPPER
652 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
654 if ( b == BOUND_EXACT
655 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
657 tte->save(posKey, value_to_tt(value, ss->ply), b,
658 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
659 MOVE_NONE, VALUE_NONE, TT.generation());
666 if (b == BOUND_LOWER)
667 bestValue = value, alpha = std::max(alpha, bestValue);
675 // Step 6. Evaluate the position statically
678 ss->staticEval = eval = VALUE_NONE;
684 // Never assume anything on values stored in TT
685 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
686 eval = ss->staticEval = evaluate(pos);
688 // Can ttValue be used as a better position evaluation?
689 if ( ttValue != VALUE_NONE
690 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
695 ss->staticEval = eval =
696 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
697 : -(ss-1)->staticEval + 2 * Eval::Tempo;
699 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
700 ss->staticEval, TT.generation());
703 improving = ss->staticEval >= (ss-2)->staticEval
704 ||(ss-2)->staticEval == VALUE_NONE;
706 if (skipEarlyPruning || !pos.non_pawn_material(pos.side_to_move()))
709 // Step 7. Razoring (skipped when in check, ~2 Elo)
711 && depth < 3 * ONE_PLY
712 && eval <= alpha - RazorMargin[depth / ONE_PLY])
714 Value ralpha = alpha - (depth >= 2 * ONE_PLY) * RazorMargin[depth / ONE_PLY];
715 Value v = qsearch<NonPV>(pos, ss, ralpha, ralpha+1);
716 if (depth < 2 * ONE_PLY || v <= ralpha)
720 // Step 8. Futility pruning: child node (skipped when in check, ~30 Elo)
722 && depth < 7 * ONE_PLY
723 && eval - futility_margin(depth, improving) >= beta
724 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
727 // Step 9. Null move search with verification search (~40 Elo)
730 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
731 && (ss->ply >= thisThread->nmp_ply || ss->ply % 2 != thisThread->nmp_odd))
733 assert(eval - beta >= 0);
735 // Null move dynamic reduction based on depth and value
736 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
738 ss->currentMove = MOVE_NULL;
739 ss->contHistory = thisThread->contHistory[NO_PIECE][0].get();
741 pos.do_null_move(st);
743 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
745 pos.undo_null_move();
747 if (nullValue >= beta)
749 // Do not return unproven mate scores
750 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
753 if (abs(beta) < VALUE_KNOWN_WIN && (depth < 12 * ONE_PLY || thisThread->nmp_ply))
756 // Do verification search at high depths. Disable null move pruning
757 // for side to move for the first part of the remaining search tree.
758 thisThread->nmp_ply = ss->ply + 3 * (depth-R) / 4;
759 thisThread->nmp_odd = ss->ply % 2;
761 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
763 thisThread->nmp_odd = thisThread->nmp_ply = 0;
770 // Step 10. ProbCut (skipped when in check, ~10 Elo)
771 // If we have a good enough capture and a reduced search returns a value
772 // much above beta, we can (almost) safely prune the previous move.
774 && depth >= 5 * ONE_PLY
775 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
777 assert(is_ok((ss-1)->currentMove));
779 Value rbeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
780 MovePicker mp(pos, ttMove, rbeta - ss->staticEval, &thisThread->captureHistory);
781 int probCutCount = 0;
783 while ( (move = mp.next_move()) != MOVE_NONE
789 ss->currentMove = move;
790 ss->contHistory = thisThread->contHistory[pos.moved_piece(move)][to_sq(move)].get();
792 assert(depth >= 5 * ONE_PLY);
794 pos.do_move(move, st);
796 // Perform a preliminary qsearch to verify that the move holds
797 value = -qsearch<NonPV>(pos, ss+1, -rbeta, -rbeta+1);
799 // If the qsearch held perform the regular search
801 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode, false);
810 // Step 11. Internal iterative deepening (skipped when in check, ~2 Elo)
811 if ( depth >= 6 * ONE_PLY
813 && (PvNode || ss->staticEval + 128 >= beta))
815 Depth d = 3 * depth / 4 - 2 * ONE_PLY;
816 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
818 tte = TT.probe(posKey, ttHit);
819 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
820 ttMove = ttHit ? tte->move() : MOVE_NONE;
823 moves_loop: // When in check, search starts from here
825 const PieceToHistory* contHist[] = { (ss-1)->contHistory, (ss-2)->contHistory, nullptr, (ss-4)->contHistory };
826 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
828 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory, contHist, countermove, ss->killers);
829 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
833 pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
835 // Step 12. Loop through all pseudo-legal moves until no moves remain
836 // or a beta cutoff occurs.
837 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
841 if (move == excludedMove)
844 // At root obey the "searchmoves" option and skip moves not listed in Root
845 // Move List. As a consequence any illegal move is also skipped. In MultiPV
846 // mode we also skip PV moves which have been already searched.
847 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
848 thisThread->rootMoves.end(), move))
851 ss->moveCount = ++moveCount;
853 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
854 sync_cout << "info depth " << depth / ONE_PLY
855 << " currmove " << UCI::move(move, pos.is_chess960())
856 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
858 (ss+1)->pv = nullptr;
860 extension = DEPTH_ZERO;
861 captureOrPromotion = pos.capture_or_promotion(move);
862 movedPiece = pos.moved_piece(move);
863 givesCheck = gives_check(pos, move);
865 moveCountPruning = depth < 16 * ONE_PLY
866 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
868 // Step 13. Extensions (~70 Elo)
870 // Singular extension search (~60 Elo). If all moves but one fail low on a
871 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
872 // then that move is singular and should be extended. To verify this we do
873 // a reduced search on on all the other moves but the ttMove and if the
874 // result is lower than ttValue minus a margin then we will extend the ttMove.
875 if ( depth >= 8 * ONE_PLY
878 && !excludedMove // Recursive singular search is not allowed
879 && ttValue != VALUE_NONE
880 && (tte->bound() & BOUND_LOWER)
881 && tte->depth() >= depth - 3 * ONE_PLY
884 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
885 ss->excludedMove = move;
886 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode, true);
887 ss->excludedMove = MOVE_NONE;
892 else if ( givesCheck // Check extension (~2 Elo)
897 // Calculate new depth for this move
898 newDepth = depth - ONE_PLY + extension;
900 // Step 14. Pruning at shallow depth (~170 Elo)
902 && pos.non_pawn_material(pos.side_to_move())
903 && bestValue > VALUE_MATED_IN_MAX_PLY)
905 if ( !captureOrPromotion
907 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
909 // Move count based pruning (~30 Elo)
910 if (moveCountPruning)
916 // Reduced depth of the next LMR search
917 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
919 // Countermoves based pruning (~20 Elo)
921 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
922 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
925 // Futility pruning: parent node (~2 Elo)
928 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
931 // Prune moves with negative SEE (~10 Elo)
933 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
936 else if ( depth < 7 * ONE_PLY // (~20 Elo)
938 && !pos.see_ge(move, -Value(CapturePruneMargin[depth / ONE_PLY])))
942 // Speculative prefetch as early as possible
943 prefetch(TT.first_entry(pos.key_after(move)));
945 // Check for legality just before making the move
946 if (!rootNode && !pos.legal(move))
948 ss->moveCount = --moveCount;
952 if (move == ttMove && captureOrPromotion)
955 // Update the current move (this must be done after singular extension search)
956 ss->currentMove = move;
957 ss->contHistory = thisThread->contHistory[movedPiece][to_sq(move)].get();
959 // Step 15. Make the move
960 pos.do_move(move, st, givesCheck);
962 // Step 16. Reduced depth search (LMR). If the move fails high it will be
963 // re-searched at full depth.
964 if ( depth >= 3 * ONE_PLY
966 && (!captureOrPromotion || moveCountPruning))
968 Depth r = reduction<PvNode>(improving, depth, moveCount);
970 if (captureOrPromotion)
971 r -= r ? ONE_PLY : DEPTH_ZERO;
974 // Decrease reduction if opponent's move count is high
975 if ((ss-1)->moveCount > 15)
978 // Decrease reduction for exact PV nodes
982 // Increase reduction if ttMove is a capture
986 // Increase reduction for cut nodes
990 // Decrease reduction for moves that escape a capture. Filter out
991 // castling moves, because they are coded as "king captures rook" and
992 // hence break make_move().
993 else if ( type_of(move) == NORMAL
994 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
997 ss->statScore = thisThread->mainHistory[~pos.side_to_move()][from_to(move)]
998 + (*contHist[0])[movedPiece][to_sq(move)]
999 + (*contHist[1])[movedPiece][to_sq(move)]
1000 + (*contHist[3])[movedPiece][to_sq(move)]
1003 // Decrease/increase reduction by comparing opponent's stat score
1004 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1007 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1010 // Decrease/increase reduction for moves with a good/bad history
1011 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->statScore / 20000) * ONE_PLY);
1014 Depth d = std::max(newDepth - r, ONE_PLY);
1016 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
1018 doFullDepthSearch = (value > alpha && d != newDepth);
1021 doFullDepthSearch = !PvNode || moveCount > 1;
1023 // Step 17. Full depth search when LMR is skipped or fails high
1024 if (doFullDepthSearch)
1025 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
1027 // For PV nodes only, do a full PV search on the first move or after a fail
1028 // high (in the latter case search only if value < beta), otherwise let the
1029 // parent node fail low with value <= alpha and try another move.
1030 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1033 (ss+1)->pv[0] = MOVE_NONE;
1035 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1038 // Step 18. Undo move
1039 pos.undo_move(move);
1041 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1043 // Step 19. Check for a new best move
1044 // Finished searching the move. If a stop occurred, the return value of
1045 // the search cannot be trusted, and we return immediately without
1046 // updating best move, PV and TT.
1047 if (Threads.stop.load(std::memory_order_relaxed))
1052 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1053 thisThread->rootMoves.end(), move);
1055 // PV move or new best move?
1056 if (moveCount == 1 || value > alpha)
1059 rm.selDepth = thisThread->selDepth;
1064 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1065 rm.pv.push_back(*m);
1067 // We record how often the best move has been changed in each
1068 // iteration. This information is used for time management: When
1069 // the best move changes frequently, we allocate some more time.
1070 if (moveCount > 1 && thisThread == Threads.main())
1071 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1074 // All other moves but the PV are set to the lowest value: this
1075 // is not a problem when sorting because the sort is stable and the
1076 // move position in the list is preserved - just the PV is pushed up.
1077 rm.score = -VALUE_INFINITE;
1080 if (value > bestValue)
1088 if (PvNode && !rootNode) // Update pv even in fail-high case
1089 update_pv(ss->pv, move, (ss+1)->pv);
1091 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1095 assert(value >= beta); // Fail high
1096 ss->statScore = std::max(ss->statScore, 0);
1102 if (move != bestMove)
1104 if (captureOrPromotion && captureCount < 32)
1105 capturesSearched[captureCount++] = move;
1107 else if (!captureOrPromotion && quietCount < 64)
1108 quietsSearched[quietCount++] = move;
1112 // The following condition would detect a stop only after move loop has been
1113 // completed. But in this case bestValue is valid because we have fully
1114 // searched our subtree, and we can anyhow save the result in TT.
1120 // Step 20. Check for mate and stalemate
1121 // All legal moves have been searched and if there are no legal moves, it
1122 // must be a mate or a stalemate. If we are in a singular extension search then
1123 // return a fail low score.
1125 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1128 bestValue = excludedMove ? alpha
1129 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1132 // Quiet best move: update move sorting heuristics
1133 if (!pos.capture_or_promotion(bestMove))
1134 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1136 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth));
1138 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1139 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1140 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1142 // Bonus for prior countermove that caused the fail low
1143 else if ( depth >= 3 * ONE_PLY
1144 && !pos.captured_piece()
1145 && is_ok((ss-1)->currentMove))
1146 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1149 bestValue = std::min(bestValue, maxValue);
1152 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1153 bestValue >= beta ? BOUND_LOWER :
1154 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1155 depth, bestMove, ss->staticEval, TT.generation());
1157 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1163 // qsearch() is the quiescence search function, which is called by the main
1164 // search function with depth zero, or recursively with depth less than ONE_PLY.
1165 template <NodeType NT>
1166 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1168 constexpr bool PvNode = NT == PV;
1170 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1171 assert(PvNode || (alpha == beta - 1));
1172 assert(depth <= DEPTH_ZERO);
1173 assert(depth / ONE_PLY * ONE_PLY == depth);
1179 Move ttMove, move, bestMove;
1181 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1182 bool ttHit, inCheck, givesCheck, evasionPrunable;
1187 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1189 ss->pv[0] = MOVE_NONE;
1192 (ss+1)->ply = ss->ply + 1;
1193 ss->currentMove = bestMove = MOVE_NONE;
1194 inCheck = pos.checkers();
1197 // Check for an immediate draw or maximum ply reached
1198 if ( pos.is_draw(ss->ply)
1199 || ss->ply >= MAX_PLY)
1200 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1202 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1204 // Decide whether or not to include checks: this fixes also the type of
1205 // TT entry depth that we are going to use. Note that in qsearch we use
1206 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1207 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1208 : DEPTH_QS_NO_CHECKS;
1209 // Transposition table lookup
1211 tte = TT.probe(posKey, ttHit);
1212 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1213 ttMove = ttHit ? tte->move() : MOVE_NONE;
1217 && tte->depth() >= ttDepth
1218 && ttValue != VALUE_NONE // Only in case of TT access race
1219 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1220 : (tte->bound() & BOUND_UPPER)))
1223 // Evaluate the position statically
1226 ss->staticEval = VALUE_NONE;
1227 bestValue = futilityBase = -VALUE_INFINITE;
1233 // Never assume anything on values stored in TT
1234 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1235 ss->staticEval = bestValue = evaluate(pos);
1237 // Can ttValue be used as a better position evaluation?
1238 if ( ttValue != VALUE_NONE
1239 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1240 bestValue = ttValue;
1243 ss->staticEval = bestValue =
1244 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1245 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1247 // Stand pat. Return immediately if static value is at least beta
1248 if (bestValue >= beta)
1251 tte->save(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1252 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1257 if (PvNode && bestValue > alpha)
1260 futilityBase = bestValue + 128;
1263 // Initialize a MovePicker object for the current position, and prepare
1264 // to search the moves. Because the depth is <= 0 here, only captures,
1265 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1267 MovePicker mp(pos, ttMove, depth, &pos.this_thread()->mainHistory, &pos.this_thread()->captureHistory, to_sq((ss-1)->currentMove));
1269 // Loop through the moves until no moves remain or a beta cutoff occurs
1270 while ((move = mp.next_move()) != MOVE_NONE)
1272 assert(is_ok(move));
1274 givesCheck = gives_check(pos, move);
1281 && futilityBase > -VALUE_KNOWN_WIN
1282 && !pos.advanced_pawn_push(move))
1284 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1286 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1288 if (futilityValue <= alpha)
1290 bestValue = std::max(bestValue, futilityValue);
1294 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1296 bestValue = std::max(bestValue, futilityBase);
1301 // Detect non-capture evasions that are candidates to be pruned
1302 evasionPrunable = inCheck
1303 && (depth != DEPTH_ZERO || moveCount > 2)
1304 && bestValue > VALUE_MATED_IN_MAX_PLY
1305 && !pos.capture(move);
1307 // Don't search moves with negative SEE values
1308 if ( (!inCheck || evasionPrunable)
1309 && !pos.see_ge(move))
1312 // Speculative prefetch as early as possible
1313 prefetch(TT.first_entry(pos.key_after(move)));
1315 // Check for legality just before making the move
1316 if (!pos.legal(move))
1322 ss->currentMove = move;
1324 // Make and search the move
1325 pos.do_move(move, st, givesCheck);
1326 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1327 pos.undo_move(move);
1329 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1331 // Check for a new best move
1332 if (value > bestValue)
1338 if (PvNode) // Update pv even in fail-high case
1339 update_pv(ss->pv, move, (ss+1)->pv);
1341 if (PvNode && value < beta) // Update alpha here!
1348 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1349 ttDepth, move, ss->staticEval, TT.generation());
1357 // All legal moves have been searched. A special case: If we're in check
1358 // and no legal moves were found, it is checkmate.
1359 if (inCheck && bestValue == -VALUE_INFINITE)
1360 return mated_in(ss->ply); // Plies to mate from the root
1362 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1363 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1364 ttDepth, bestMove, ss->staticEval, TT.generation());
1366 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1372 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1373 // "plies to mate from the current position". Non-mate scores are unchanged.
1374 // The function is called before storing a value in the transposition table.
1376 Value value_to_tt(Value v, int ply) {
1378 assert(v != VALUE_NONE);
1380 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1381 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1385 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1386 // from the transposition table (which refers to the plies to mate/be mated
1387 // from current position) to "plies to mate/be mated from the root".
1389 Value value_from_tt(Value v, int ply) {
1391 return v == VALUE_NONE ? VALUE_NONE
1392 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1393 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1397 // update_pv() adds current move and appends child pv[]
1399 void update_pv(Move* pv, Move move, Move* childPv) {
1401 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1407 // update_continuation_histories() updates histories of the move pairs formed
1408 // by moves at ply -1, -2, and -4 with current move.
1410 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1412 for (int i : {1, 2, 4})
1413 if (is_ok((ss-i)->currentMove))
1414 (*(ss-i)->contHistory)[pc][to] << bonus;
1418 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1420 void update_capture_stats(const Position& pos, Move move,
1421 Move* captures, int captureCnt, int bonus) {
1423 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1424 Piece moved_piece = pos.moved_piece(move);
1425 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1426 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1428 // Decrease all the other played capture moves
1429 for (int i = 0; i < captureCnt; ++i)
1431 moved_piece = pos.moved_piece(captures[i]);
1432 captured = type_of(pos.piece_on(to_sq(captures[i])));
1433 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1438 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1440 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1441 Move* quiets, int quietsCnt, int bonus) {
1443 if (ss->killers[0] != move)
1445 ss->killers[1] = ss->killers[0];
1446 ss->killers[0] = move;
1449 Color us = pos.side_to_move();
1450 Thread* thisThread = pos.this_thread();
1451 thisThread->mainHistory[us][from_to(move)] << bonus;
1452 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1454 if (is_ok((ss-1)->currentMove))
1456 Square prevSq = to_sq((ss-1)->currentMove);
1457 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1460 // Decrease all the other played quiet moves
1461 for (int i = 0; i < quietsCnt; ++i)
1463 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1464 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1468 // When playing with strength handicap, choose best move among a set of RootMoves
1469 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1471 Move Skill::pick_best(size_t multiPV) {
1473 const RootMoves& rootMoves = Threads.main()->rootMoves;
1474 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1476 // RootMoves are already sorted by score in descending order
1477 Value topScore = rootMoves[0].score;
1478 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1479 int weakness = 120 - 2 * level;
1480 int maxScore = -VALUE_INFINITE;
1482 // Choose best move. For each move score we add two terms, both dependent on
1483 // weakness. One is deterministic and bigger for weaker levels, and one is
1484 // random. Then we choose the move with the resulting highest score.
1485 for (size_t i = 0; i < multiPV; ++i)
1487 // This is our magic formula
1488 int push = ( weakness * int(topScore - rootMoves[i].score)
1489 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1491 if (rootMoves[i].score + push >= maxScore)
1493 maxScore = rootMoves[i].score + push;
1494 best = rootMoves[i].pv[0];
1503 /// MainThread::check_time() is used to print debug info and, more importantly,
1504 /// to detect when we are out of available time and thus stop the search.
1506 void MainThread::check_time() {
1511 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1512 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1514 static TimePoint lastInfoTime = now();
1516 TimePoint elapsed = Time.elapsed();
1517 TimePoint tick = Limits.startTime + elapsed;
1519 if (tick - lastInfoTime >= 1000)
1521 lastInfoTime = tick;
1525 // We should not stop pondering until told so by the GUI
1529 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1530 || (Limits.movetime && elapsed >= Limits.movetime)
1531 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1532 Threads.stop = true;
1536 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1537 /// that all (if any) unsearched PV lines are sent using a previous search score.
1539 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1541 std::stringstream ss;
1542 TimePoint elapsed = Time.elapsed() + 1;
1543 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1544 size_t PVIdx = pos.this_thread()->PVIdx;
1545 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1546 uint64_t nodesSearched = Threads.nodes_searched();
1547 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1549 for (size_t i = 0; i < multiPV; ++i)
1551 bool updated = (i <= PVIdx && rootMoves[i].score != -VALUE_INFINITE);
1553 if (depth == ONE_PLY && !updated)
1556 Depth d = updated ? depth : depth - ONE_PLY;
1557 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1559 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1560 v = tb ? TB::Score : v;
1562 if (ss.rdbuf()->in_avail()) // Not at first line
1566 << " depth " << d / ONE_PLY
1567 << " seldepth " << rootMoves[i].selDepth
1568 << " multipv " << i + 1
1569 << " score " << UCI::value(v);
1571 if (!tb && i == PVIdx)
1572 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1574 ss << " nodes " << nodesSearched
1575 << " nps " << nodesSearched * 1000 / elapsed;
1577 if (elapsed > 1000) // Earlier makes little sense
1578 ss << " hashfull " << TT.hashfull();
1580 ss << " tbhits " << tbHits
1581 << " time " << elapsed
1584 for (Move m : rootMoves[i].pv)
1585 ss << " " << UCI::move(m, pos.is_chess960());
1592 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1593 /// before exiting the search, for instance, in case we stop the search during a
1594 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1595 /// otherwise in case of 'ponder on' we have nothing to think on.
1597 bool RootMove::extract_ponder_from_tt(Position& pos) {
1602 assert(pv.size() == 1);
1607 pos.do_move(pv[0], st);
1608 TTEntry* tte = TT.probe(pos.key(), ttHit);
1612 Move m = tte->move(); // Local copy to be SMP safe
1613 if (MoveList<LEGAL>(pos).contains(m))
1617 pos.undo_move(pv[0]);
1618 return pv.size() > 1;
1622 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1625 UseRule50 = Options["Syzygy50MoveRule"];
1626 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1627 Cardinality = Options["SyzygyProbeLimit"];
1629 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1630 if (Cardinality > MaxCardinality)
1632 Cardinality = MaxCardinality;
1633 ProbeDepth = DEPTH_ZERO;
1636 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1639 // Don't filter any moves if the user requested analysis on multiple
1640 if (Options["MultiPV"] != 1)
1643 // If the current root position is in the tablebases, then RootMoves
1644 // contains only moves that preserve the draw or the win.
1645 RootInTB = root_probe(pos, rootMoves, TB::Score);
1648 Cardinality = 0; // Do not probe tablebases during the search
1650 else // If DTZ tables are missing, use WDL tables as a fallback
1652 // Filter out moves that do not preserve the draw or the win.
1653 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1655 // Only probe during search if winning
1656 if (RootInTB && TB::Score <= VALUE_DRAW)
1660 if (RootInTB && !UseRule50)
1661 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1662 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
1665 // Since root_probe() and root_probe_wdl() dirty the root move scores,
1666 // we reset them to -VALUE_INFINITE
1667 for (RootMove& rm : rootMoves)
1668 rm.score = -VALUE_INFINITE;