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 {
53 namespace TB = Tablebases;
57 using namespace Search;
61 // Different node types, used as a template parameter
62 enum NodeType { NonPV, PV };
64 // Sizes and phases of the skip-blocks, used for distributing search depths across the threads
65 constexpr int SkipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
66 constexpr int SkipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
68 // Razor and futility margins
69 constexpr int RazorMargin[] = {0, 590, 604};
70 Value futility_margin(Depth d, bool improving) {
71 return Value((175 - 50 * improving) * d / ONE_PLY);
74 // Margin for pruning capturing moves: almost linear in depth
75 constexpr int CapturePruneMargin[] = { 0,
76 1 * PawnValueEg * 1055 / 1000,
77 2 * PawnValueEg * 1042 / 1000,
78 3 * PawnValueEg * 963 / 1000,
79 4 * PawnValueEg * 1038 / 1000,
80 5 * PawnValueEg * 950 / 1000,
81 6 * PawnValueEg * 930 / 1000
84 // Futility and reductions lookup tables, initialized at startup
85 int FutilityMoveCounts[2][16]; // [improving][depth]
86 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
88 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
89 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
92 // History and stats update bonus, based on depth
93 int stat_bonus(Depth depth) {
94 int d = depth / ONE_PLY;
95 return d > 17 ? 0 : 32 * d * d + 64 * d - 64;
98 // Skill structure is used to implement strength limit
100 explicit Skill(int l) : level(l) {}
101 bool enabled() const { return level < 20; }
102 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
103 Move pick_best(size_t multiPV);
106 Move best = MOVE_NONE;
109 template <NodeType NT>
110 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
112 template <NodeType NT>
113 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
115 Value value_to_tt(Value v, int ply);
116 Value value_from_tt(Value v, int ply);
117 void update_pv(Move* pv, Move move, Move* childPv);
118 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
119 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
120 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCnt, int bonus);
122 inline bool gives_check(const Position& pos, Move move) {
123 Color us = pos.side_to_move();
124 return type_of(move) == NORMAL && !(pos.blockers_for_king(~us) & pos.pieces(us))
125 ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
126 : pos.gives_check(move);
129 // perft() is our utility to verify move generation. All the leaf nodes up
130 // to the given depth are generated and counted, and the sum is returned.
132 uint64_t perft(Position& pos, Depth depth) {
135 uint64_t cnt, nodes = 0;
136 const bool leaf = (depth == 2 * ONE_PLY);
138 for (const auto& m : MoveList<LEGAL>(pos))
140 if (Root && depth <= ONE_PLY)
145 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
150 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
158 /// Search::init() is called at startup to initialize various lookup tables
160 void Search::init() {
162 for (int imp = 0; imp <= 1; ++imp)
163 for (int d = 1; d < 64; ++d)
164 for (int mc = 1; mc < 64; ++mc)
166 double r = log(d) * log(mc) / 1.95;
168 Reductions[NonPV][imp][d][mc] = int(std::round(r));
169 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
171 // Increase reduction for non-PV nodes when eval is not improving
173 Reductions[NonPV][imp][d][mc]++;
176 for (int d = 0; d < 16; ++d)
178 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
179 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
184 /// Search::clear() resets search state to its initial value
186 void Search::clear() {
188 Threads.main()->wait_for_search_finished();
190 Time.availableNodes = 0;
196 /// MainThread::search() is called by the main thread when the program receives
197 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
199 void MainThread::search() {
203 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
204 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
208 Color us = rootPos.side_to_move();
209 Time.init(Limits, us, rootPos.game_ply());
212 if (rootMoves.empty())
214 rootMoves.emplace_back(MOVE_NONE);
215 sync_cout << "info depth 0 score "
216 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
221 for (Thread* th : Threads)
223 th->start_searching();
225 Thread::search(); // Let's start searching!
228 // When we reach the maximum depth, we can arrive here without a raise of
229 // Threads.stop. However, if we are pondering or in an infinite search,
230 // the UCI protocol states that we shouldn't print the best move before the
231 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
232 // until the GUI sends one of those commands (which also raises Threads.stop).
233 Threads.stopOnPonderhit = true;
235 while (!Threads.stop && (Threads.ponder || Limits.infinite))
236 {} // Busy wait for a stop or a ponder reset
238 // Stop the threads if not already stopped (also raise the stop if
239 // "ponderhit" just reset Threads.ponder).
242 // Wait until all threads have finished
243 for (Thread* th : Threads)
245 th->wait_for_search_finished();
247 // When playing in 'nodes as time' mode, subtract the searched nodes from
248 // the available ones before exiting.
250 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
252 // Check if there are threads with a better score than main thread
253 Thread* bestThread = this;
254 if ( Options["MultiPV"] == 1
256 && !Skill(Options["Skill Level"]).enabled()
257 && rootMoves[0].pv[0] != MOVE_NONE)
259 for (Thread* th : Threads)
261 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
262 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
264 // Select the thread with the best score, always if it is a mate
266 && (depthDiff >= 0 || th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY))
271 previousScore = bestThread->rootMoves[0].score;
273 // Send again PV info if we have a new best thread
274 if (bestThread != this)
275 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
277 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
279 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
280 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
282 std::cout << sync_endl;
286 /// Thread::search() is the main iterative deepening loop. It calls search()
287 /// repeatedly with increasing depth until the allocated thinking time has been
288 /// consumed, the user stops the search, or the maximum search depth is reached.
290 void Thread::search() {
292 Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
293 Value bestValue, alpha, beta, delta;
294 Move lastBestMove = MOVE_NONE;
295 Depth lastBestMoveDepth = DEPTH_ZERO;
296 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
297 double timeReduction = 1.0;
298 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, 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 = int(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, 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;
360 // MultiPV loop. We perform a full root search for each PV line
361 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
366 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
367 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
371 // Reset UCI info selDepth for each depth and each PV line
374 // Reset aspiration window starting size
375 if (rootDepth >= 5 * ONE_PLY)
377 Value previousScore = rootMoves[pvIdx].previousScore;
379 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
380 beta = std::min(previousScore + delta, VALUE_INFINITE);
382 // Adjust contempt based on root move's previousScore (dynamic contempt)
383 int dct = ct + 88 * previousScore / (abs(previousScore) + 200);
385 contempt = (us == WHITE ? make_score(dct, dct / 2)
386 : -make_score(dct, dct / 2));
389 // Start with a small aspiration window and, in the case of a fail
390 // high/low, re-search with a bigger window until we don't fail
394 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false);
396 // Bring the best move to the front. It is critical that sorting
397 // is done with a stable algorithm because all the values but the
398 // first and eventually the new best one are set to -VALUE_INFINITE
399 // and we want to keep the same order for all the moves except the
400 // new PV that goes to the front. Note that in case of MultiPV
401 // search the already searched PV lines are preserved.
402 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
404 // If search has been stopped, we break immediately. Sorting is
405 // safe because RootMoves is still valid, although it refers to
406 // the previous iteration.
410 // When failing high/low give some update (without cluttering
411 // the UI) before a re-search.
414 && (bestValue <= alpha || bestValue >= beta)
415 && Time.elapsed() > 3000)
416 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
418 // In case of failing low/high increase aspiration window and
419 // re-search, otherwise exit the loop.
420 if (bestValue <= alpha)
422 beta = (alpha + beta) / 2;
423 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
428 Threads.stopOnPonderhit = false;
431 else if (bestValue >= beta)
432 beta = std::min(bestValue + delta, VALUE_INFINITE);
436 delta += delta / 4 + 5;
438 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
441 // Sort the PV lines searched so far and update the GUI
442 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
445 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
446 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
450 completedDepth = rootDepth;
452 if (rootMoves[0].pv[0] != lastBestMove) {
453 lastBestMove = rootMoves[0].pv[0];
454 lastBestMoveDepth = rootDepth;
457 // Have we found a "mate in x"?
459 && bestValue >= VALUE_MATE_IN_MAX_PLY
460 && VALUE_MATE - bestValue <= 2 * Limits.mate)
466 // If skill level is enabled and time is up, pick a sub-optimal best move
467 if (skill.enabled() && skill.time_to_pick(rootDepth))
468 skill.pick_best(multiPV);
470 // Do we have time for the next iteration? Can we stop searching now?
471 if ( Limits.use_time_management()
473 && !Threads.stopOnPonderhit)
475 const int F[] = { failedLow,
476 bestValue - mainThread->previousScore };
478 int improvingFactor = std::max(246, std::min(832, 306 + 119 * F[0] - 6 * F[1]));
480 // If the bestMove is stable over several iterations, reduce time accordingly
482 for (int i : {3, 4, 5})
483 if (lastBestMoveDepth * i < completedDepth)
484 timeReduction *= 1.25;
486 // Use part of the gained time from a previous stable move for the current move
487 double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
488 bestMoveInstability *= std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
490 // Stop the search if we have only one legal move, or if available time elapsed
491 if ( rootMoves.size() == 1
492 || Time.elapsed() > Time.optimum() * bestMoveInstability * improvingFactor / 581)
494 // If we are allowed to ponder do not stop the search now but
495 // keep pondering until the GUI sends "ponderhit" or "stop".
497 Threads.stopOnPonderhit = true;
507 mainThread->previousTimeReduction = timeReduction;
509 // If skill level is enabled, swap best PV line with the sub-optimal one
511 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
512 skill.best ? skill.best : skill.pick_best(multiPV)));
518 // search<>() is the main search function for both PV and non-PV nodes
520 template <NodeType NT>
521 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
523 constexpr bool PvNode = NT == PV;
524 const bool rootNode = PvNode && ss->ply == 0;
526 // Check if we have an upcoming move which draws by repetition, or
527 // if the opponent had an alternative move earlier to this position.
528 if ( pos.rule50_count() >= 3
529 && alpha < VALUE_DRAW
531 && pos.has_game_cycle(ss->ply))
538 // Dive into quiescence search when the depth reaches zero
540 return qsearch<NT>(pos, ss, alpha, beta);
542 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
543 assert(PvNode || (alpha == beta - 1));
544 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
545 assert(!(PvNode && cutNode));
546 assert(depth / ONE_PLY * ONE_PLY == depth);
548 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
552 Move ttMove, move, excludedMove, bestMove;
553 Depth extension, newDepth;
554 Value bestValue, value, ttValue, eval, maxValue;
555 bool ttHit, inCheck, givesCheck, improving;
556 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
558 int moveCount, captureCount, quietCount;
560 // Step 1. Initialize node
561 Thread* thisThread = pos.this_thread();
562 inCheck = pos.checkers();
563 Color us = pos.side_to_move();
564 moveCount = captureCount = quietCount = ss->moveCount = 0;
565 bestValue = -VALUE_INFINITE;
566 maxValue = VALUE_INFINITE;
568 // Check for the available remaining time
569 if (thisThread == Threads.main())
570 static_cast<MainThread*>(thisThread)->check_time();
572 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
573 if (PvNode && thisThread->selDepth < ss->ply + 1)
574 thisThread->selDepth = ss->ply + 1;
578 // Step 2. Check for aborted search and immediate draw
579 if ( Threads.stop.load(std::memory_order_relaxed)
580 || pos.is_draw(ss->ply)
581 || ss->ply >= MAX_PLY)
582 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
584 // Step 3. Mate distance pruning. Even if we mate at the next move our score
585 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
586 // a shorter mate was found upward in the tree then there is no need to search
587 // because we will never beat the current alpha. Same logic but with reversed
588 // signs applies also in the opposite condition of being mated instead of giving
589 // mate. In this case return a fail-high score.
590 alpha = std::max(mated_in(ss->ply), alpha);
591 beta = std::min(mate_in(ss->ply+1), beta);
596 assert(0 <= ss->ply && ss->ply < MAX_PLY);
598 (ss+1)->ply = ss->ply + 1;
599 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
600 ss->contHistory = thisThread->contHistory[NO_PIECE][0].get();
601 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
602 Square prevSq = to_sq((ss-1)->currentMove);
604 // Initialize statScore to zero for the grandchildren of the current position.
605 // So statScore is shared between all grandchildren and only the first grandchild
606 // starts with statScore = 0. Later grandchildren start with the last calculated
607 // statScore of the previous grandchild. This influences the reduction rules in
608 // LMR which are based on the statScore of parent position.
609 (ss+2)->statScore = 0;
611 // Step 4. Transposition table lookup. We don't want the score of a partial
612 // search to overwrite a previous full search TT value, so we use a different
613 // position key in case of an excluded move.
614 excludedMove = ss->excludedMove;
615 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
616 tte = TT.probe(posKey, ttHit);
617 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
618 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
619 : ttHit ? tte->move() : MOVE_NONE;
621 // At non-PV nodes we check for an early TT cutoff
624 && tte->depth() >= depth
625 && ttValue != VALUE_NONE // Possible in case of TT access race
626 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
627 : (tte->bound() & BOUND_UPPER)))
629 // If ttMove is quiet, update move sorting heuristics on TT hit
634 if (!pos.capture_or_promotion(ttMove))
635 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
637 // Extra penalty for a quiet TT move in previous ply when it gets refuted
638 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
639 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
641 // Penalty for a quiet ttMove that fails low
642 else if (!pos.capture_or_promotion(ttMove))
644 int penalty = -stat_bonus(depth);
645 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
646 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
652 // Step 5. Tablebases probe
653 if (!rootNode && TB::Cardinality)
655 int piecesCount = pos.count<ALL_PIECES>();
657 if ( piecesCount <= TB::Cardinality
658 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
659 && pos.rule50_count() == 0
660 && !pos.can_castle(ANY_CASTLING))
663 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
665 if (err != TB::ProbeState::FAIL)
667 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
669 int drawScore = TB::UseRule50 ? 1 : 0;
671 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
672 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
673 : VALUE_DRAW + 2 * wdl * drawScore;
675 Bound b = wdl < -drawScore ? BOUND_UPPER
676 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
678 if ( b == BOUND_EXACT
679 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
681 tte->save(posKey, value_to_tt(value, ss->ply), b,
682 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
683 MOVE_NONE, VALUE_NONE, TT.generation());
690 if (b == BOUND_LOWER)
691 bestValue = value, alpha = std::max(alpha, bestValue);
699 // Step 6. Static evaluation of the position
702 ss->staticEval = eval = VALUE_NONE;
704 goto moves_loop; // Skip early pruning when in check
708 // Never assume anything on values stored in TT
709 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
710 eval = ss->staticEval = evaluate(pos);
712 // Can ttValue be used as a better position evaluation?
713 if ( ttValue != VALUE_NONE
714 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
719 ss->staticEval = eval =
720 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
721 : -(ss-1)->staticEval + 2 * Eval::Tempo;
723 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
724 ss->staticEval, TT.generation());
727 // Step 7. Razoring (~2 Elo)
729 && depth < 3 * ONE_PLY
730 && eval <= alpha - RazorMargin[depth / ONE_PLY])
732 Value ralpha = alpha - (depth >= 2 * ONE_PLY) * RazorMargin[depth / ONE_PLY];
733 Value v = qsearch<NonPV>(pos, ss, ralpha, ralpha+1);
734 if (depth < 2 * ONE_PLY || v <= ralpha)
738 improving = ss->staticEval >= (ss-2)->staticEval
739 || (ss-2)->staticEval == VALUE_NONE;
741 // Step 8. Futility pruning: child node (~30 Elo)
743 && depth < 7 * ONE_PLY
744 && eval - futility_margin(depth, improving) >= beta
745 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
748 // Step 9. Null move search with verification search (~40 Elo)
750 && (ss-1)->currentMove != MOVE_NULL
751 && (ss-1)->statScore < 22500
753 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
755 && pos.non_pawn_material(us)
756 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
758 assert(eval - beta >= 0);
760 // Null move dynamic reduction based on depth and value
761 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
763 ss->currentMove = MOVE_NULL;
764 ss->contHistory = thisThread->contHistory[NO_PIECE][0].get();
766 pos.do_null_move(st);
768 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
770 pos.undo_null_move();
772 if (nullValue >= beta)
774 // Do not return unproven mate scores
775 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
778 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 12 * ONE_PLY))
781 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
783 // Do verification search at high depths, with null move pruning disabled
784 // for us, until ply exceeds nmpMinPly.
785 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
786 thisThread->nmpColor = us;
788 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
790 thisThread->nmpMinPly = 0;
797 // Step 10. ProbCut (~10 Elo)
798 // If we have a good enough capture and a reduced search returns a value
799 // much above beta, we can (almost) safely prune the previous move.
801 && depth >= 5 * ONE_PLY
802 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
804 Value rbeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
805 MovePicker mp(pos, ttMove, rbeta - ss->staticEval, &thisThread->captureHistory);
806 int probCutCount = 0;
808 while ( (move = mp.next_move()) != MOVE_NONE
814 ss->currentMove = move;
815 ss->contHistory = thisThread->contHistory[pos.moved_piece(move)][to_sq(move)].get();
817 assert(depth >= 5 * ONE_PLY);
819 pos.do_move(move, st);
821 // Perform a preliminary qsearch to verify that the move holds
822 value = -qsearch<NonPV>(pos, ss+1, -rbeta, -rbeta+1);
824 // If the qsearch held perform the regular search
826 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode);
835 // Step 11. Internal iterative deepening (~2 Elo)
836 if ( depth >= 8 * ONE_PLY
839 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
841 tte = TT.probe(posKey, ttHit);
842 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
843 ttMove = ttHit ? tte->move() : MOVE_NONE;
846 moves_loop: // When in check, search starts from here
848 const PieceToHistory* contHist[] = { (ss-1)->contHistory, (ss-2)->contHistory, nullptr, (ss-4)->contHistory };
849 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
851 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
852 &thisThread->captureHistory,
856 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
860 pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
862 // Step 12. Loop through all pseudo-legal moves until no moves remain
863 // or a beta cutoff occurs.
864 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
868 if (move == excludedMove)
871 // At root obey the "searchmoves" option and skip moves not listed in Root
872 // Move List. As a consequence any illegal move is also skipped. In MultiPV
873 // mode we also skip PV moves which have been already searched and those
874 // of lower "TB rank" if we are in a TB root position.
875 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
876 thisThread->rootMoves.begin() + thisThread->pvLast, move))
879 ss->moveCount = ++moveCount;
881 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
882 sync_cout << "info depth " << depth / ONE_PLY
883 << " currmove " << UCI::move(move, pos.is_chess960())
884 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
886 (ss+1)->pv = nullptr;
888 extension = DEPTH_ZERO;
889 captureOrPromotion = pos.capture_or_promotion(move);
890 movedPiece = pos.moved_piece(move);
891 givesCheck = gives_check(pos, move);
893 moveCountPruning = depth < 16 * ONE_PLY
894 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
896 // Step 13. Extensions (~70 Elo)
898 // Singular extension search (~60 Elo). If all moves but one fail low on a
899 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
900 // then that move is singular and should be extended. To verify this we do
901 // a reduced search on on all the other moves but the ttMove and if the
902 // result is lower than ttValue minus a margin then we will extend the ttMove.
903 if ( depth >= 8 * ONE_PLY
906 && !excludedMove // Recursive singular search is not allowed
907 && ttValue != VALUE_NONE
908 && (tte->bound() & BOUND_LOWER)
909 && tte->depth() >= depth - 3 * ONE_PLY
912 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
913 ss->excludedMove = move;
914 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
915 ss->excludedMove = MOVE_NONE;
920 else if ( givesCheck // Check extension (~2 Elo)
925 // Calculate new depth for this move
926 newDepth = depth - ONE_PLY + extension;
928 // Step 14. Pruning at shallow depth (~170 Elo)
930 && pos.non_pawn_material(us)
931 && bestValue > VALUE_MATED_IN_MAX_PLY)
933 if ( !captureOrPromotion
935 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
937 // Move count based pruning (~30 Elo)
938 if (moveCountPruning)
944 // Reduced depth of the next LMR search
945 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
947 // Countermoves based pruning (~20 Elo)
949 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
950 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
953 // Futility pruning: parent node (~2 Elo)
956 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
959 // Prune moves with negative SEE (~10 Elo)
961 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
964 else if ( depth < 7 * ONE_PLY // (~20 Elo)
966 && !pos.see_ge(move, -Value(CapturePruneMargin[depth / ONE_PLY])))
970 // Speculative prefetch as early as possible
971 prefetch(TT.first_entry(pos.key_after(move)));
973 // Check for legality just before making the move
974 if (!rootNode && !pos.legal(move))
976 ss->moveCount = --moveCount;
980 if (move == ttMove && captureOrPromotion)
983 // Update the current move (this must be done after singular extension search)
984 ss->currentMove = move;
985 ss->contHistory = thisThread->contHistory[movedPiece][to_sq(move)].get();
987 // Step 15. Make the move
988 pos.do_move(move, st, givesCheck);
990 // Step 16. Reduced depth search (LMR). If the move fails high it will be
991 // re-searched at full depth.
992 if ( depth >= 3 * ONE_PLY
994 && (!captureOrPromotion || moveCountPruning))
996 Depth r = reduction<PvNode>(improving, depth, moveCount);
998 if (captureOrPromotion) // (~5 Elo)
1000 // Increase reduction by comparing opponent's stat score
1001 if ((ss-1)->statScore >= 0)
1004 r -= r ? ONE_PLY : DEPTH_ZERO;
1008 // Decrease reduction if opponent's move count is high (~5 Elo)
1009 if ((ss-1)->moveCount > 15)
1012 // Decrease reduction for exact PV nodes (~0 Elo)
1016 // Increase reduction if ttMove is a capture (~0 Elo)
1020 // Increase reduction for cut nodes (~5 Elo)
1024 // Decrease reduction for moves that escape a capture. Filter out
1025 // castling moves, because they are coded as "king captures rook" and
1026 // hence break make_move(). (~5 Elo)
1027 else if ( type_of(move) == NORMAL
1028 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1031 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1032 + (*contHist[0])[movedPiece][to_sq(move)]
1033 + (*contHist[1])[movedPiece][to_sq(move)]
1034 + (*contHist[3])[movedPiece][to_sq(move)]
1037 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1038 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1041 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1044 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1045 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->statScore / 20000) * ONE_PLY);
1048 Depth d = std::max(newDepth - r, ONE_PLY);
1050 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1052 doFullDepthSearch = (value > alpha && d != newDepth);
1055 doFullDepthSearch = !PvNode || moveCount > 1;
1057 // Step 17. Full depth search when LMR is skipped or fails high
1058 if (doFullDepthSearch)
1059 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1061 // For PV nodes only, do a full PV search on the first move or after a fail
1062 // high (in the latter case search only if value < beta), otherwise let the
1063 // parent node fail low with value <= alpha and try another move.
1064 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1067 (ss+1)->pv[0] = MOVE_NONE;
1069 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1072 // Step 18. Undo move
1073 pos.undo_move(move);
1075 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1077 // Step 19. Check for a new best move
1078 // Finished searching the move. If a stop occurred, the return value of
1079 // the search cannot be trusted, and we return immediately without
1080 // updating best move, PV and TT.
1081 if (Threads.stop.load(std::memory_order_relaxed))
1086 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1087 thisThread->rootMoves.end(), move);
1089 // PV move or new best move?
1090 if (moveCount == 1 || value > alpha)
1093 rm.selDepth = thisThread->selDepth;
1098 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1099 rm.pv.push_back(*m);
1101 // We record how often the best move has been changed in each
1102 // iteration. This information is used for time management: When
1103 // the best move changes frequently, we allocate some more time.
1104 if (moveCount > 1 && thisThread == Threads.main())
1105 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1108 // All other moves but the PV are set to the lowest value: this
1109 // is not a problem when sorting because the sort is stable and the
1110 // move position in the list is preserved - just the PV is pushed up.
1111 rm.score = -VALUE_INFINITE;
1114 if (value > bestValue)
1122 if (PvNode && !rootNode) // Update pv even in fail-high case
1123 update_pv(ss->pv, move, (ss+1)->pv);
1125 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1129 assert(value >= beta); // Fail high
1136 if (move != bestMove)
1138 if (captureOrPromotion && captureCount < 32)
1139 capturesSearched[captureCount++] = move;
1141 else if (!captureOrPromotion && quietCount < 64)
1142 quietsSearched[quietCount++] = move;
1146 // The following condition would detect a stop only after move loop has been
1147 // completed. But in this case bestValue is valid because we have fully
1148 // searched our subtree, and we can anyhow save the result in TT.
1154 // Step 20. Check for mate and stalemate
1155 // All legal moves have been searched and if there are no legal moves, it
1156 // must be a mate or a stalemate. If we are in a singular extension search then
1157 // return a fail low score.
1159 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1162 bestValue = excludedMove ? alpha
1163 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1166 // Quiet best move: update move sorting heuristics
1167 if (!pos.capture_or_promotion(bestMove))
1168 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1169 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1171 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1173 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1174 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1175 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1177 // Bonus for prior countermove that caused the fail low
1178 else if ( (depth >= 3 * ONE_PLY || PvNode)
1179 && !pos.captured_piece()
1180 && is_ok((ss-1)->currentMove))
1181 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1184 bestValue = std::min(bestValue, maxValue);
1187 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1188 bestValue >= beta ? BOUND_LOWER :
1189 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1190 depth, bestMove, ss->staticEval, TT.generation());
1192 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1198 // qsearch() is the quiescence search function, which is called by the main
1199 // search function with depth zero, or recursively with depth less than ONE_PLY.
1200 template <NodeType NT>
1201 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1203 constexpr bool PvNode = NT == PV;
1205 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1206 assert(PvNode || (alpha == beta - 1));
1207 assert(depth <= DEPTH_ZERO);
1208 assert(depth / ONE_PLY * ONE_PLY == depth);
1214 Move ttMove, move, bestMove;
1216 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1217 bool ttHit, inCheck, givesCheck, evasionPrunable;
1222 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1224 ss->pv[0] = MOVE_NONE;
1227 (ss+1)->ply = ss->ply + 1;
1228 ss->currentMove = bestMove = MOVE_NONE;
1229 inCheck = pos.checkers();
1232 // Check for an immediate draw or maximum ply reached
1233 if ( pos.is_draw(ss->ply)
1234 || ss->ply >= MAX_PLY)
1235 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1237 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1239 // Decide whether or not to include checks: this fixes also the type of
1240 // TT entry depth that we are going to use. Note that in qsearch we use
1241 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1242 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1243 : DEPTH_QS_NO_CHECKS;
1244 // Transposition table lookup
1246 tte = TT.probe(posKey, ttHit);
1247 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1248 ttMove = ttHit ? tte->move() : MOVE_NONE;
1252 && tte->depth() >= ttDepth
1253 && ttValue != VALUE_NONE // Only in case of TT access race
1254 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1255 : (tte->bound() & BOUND_UPPER)))
1258 // Evaluate the position statically
1261 ss->staticEval = VALUE_NONE;
1262 bestValue = futilityBase = -VALUE_INFINITE;
1268 // Never assume anything on values stored in TT
1269 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1270 ss->staticEval = bestValue = evaluate(pos);
1272 // Can ttValue be used as a better position evaluation?
1273 if ( ttValue != VALUE_NONE
1274 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1275 bestValue = ttValue;
1278 ss->staticEval = bestValue =
1279 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1280 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1282 // Stand pat. Return immediately if static value is at least beta
1283 if (bestValue >= beta)
1286 tte->save(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1287 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1292 if (PvNode && bestValue > alpha)
1295 futilityBase = bestValue + 128;
1298 // Initialize a MovePicker object for the current position, and prepare
1299 // to search the moves. Because the depth is <= 0 here, only captures,
1300 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1302 MovePicker mp(pos, ttMove, depth, &pos.this_thread()->mainHistory,
1303 &pos.this_thread()->captureHistory,
1304 to_sq((ss-1)->currentMove));
1306 // Loop through the moves until no moves remain or a beta cutoff occurs
1307 while ((move = mp.next_move()) != MOVE_NONE)
1309 assert(is_ok(move));
1311 givesCheck = gives_check(pos, move);
1318 && futilityBase > -VALUE_KNOWN_WIN
1319 && !pos.advanced_pawn_push(move))
1321 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1323 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1325 if (futilityValue <= alpha)
1327 bestValue = std::max(bestValue, futilityValue);
1331 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1333 bestValue = std::max(bestValue, futilityBase);
1338 // Detect non-capture evasions that are candidates to be pruned
1339 evasionPrunable = inCheck
1340 && (depth != DEPTH_ZERO || moveCount > 2)
1341 && bestValue > VALUE_MATED_IN_MAX_PLY
1342 && !pos.capture(move);
1344 // Don't search moves with negative SEE values
1345 if ( (!inCheck || evasionPrunable)
1346 && !pos.see_ge(move))
1349 // Speculative prefetch as early as possible
1350 prefetch(TT.first_entry(pos.key_after(move)));
1352 // Check for legality just before making the move
1353 if (!pos.legal(move))
1359 ss->currentMove = move;
1361 // Make and search the move
1362 pos.do_move(move, st, givesCheck);
1363 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1364 pos.undo_move(move);
1366 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1368 // Check for a new best move
1369 if (value > bestValue)
1375 if (PvNode) // Update pv even in fail-high case
1376 update_pv(ss->pv, move, (ss+1)->pv);
1378 if (PvNode && value < beta) // Update alpha here!
1385 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1386 ttDepth, move, ss->staticEval, TT.generation());
1394 // All legal moves have been searched. A special case: If we're in check
1395 // and no legal moves were found, it is checkmate.
1396 if (inCheck && bestValue == -VALUE_INFINITE)
1397 return mated_in(ss->ply); // Plies to mate from the root
1399 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1400 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1401 ttDepth, bestMove, ss->staticEval, TT.generation());
1403 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1409 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1410 // "plies to mate from the current position". Non-mate scores are unchanged.
1411 // The function is called before storing a value in the transposition table.
1413 Value value_to_tt(Value v, int ply) {
1415 assert(v != VALUE_NONE);
1417 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1418 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1422 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1423 // from the transposition table (which refers to the plies to mate/be mated
1424 // from current position) to "plies to mate/be mated from the root".
1426 Value value_from_tt(Value v, int ply) {
1428 return v == VALUE_NONE ? VALUE_NONE
1429 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1430 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1434 // update_pv() adds current move and appends child pv[]
1436 void update_pv(Move* pv, Move move, Move* childPv) {
1438 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1444 // update_continuation_histories() updates histories of the move pairs formed
1445 // by moves at ply -1, -2, and -4 with current move.
1447 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1449 for (int i : {1, 2, 4})
1450 if (is_ok((ss-i)->currentMove))
1451 (*(ss-i)->contHistory)[pc][to] << bonus;
1455 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1457 void update_capture_stats(const Position& pos, Move move,
1458 Move* captures, int captureCnt, int bonus) {
1460 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1461 Piece moved_piece = pos.moved_piece(move);
1462 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1463 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1465 // Decrease all the other played capture moves
1466 for (int i = 0; i < captureCnt; ++i)
1468 moved_piece = pos.moved_piece(captures[i]);
1469 captured = type_of(pos.piece_on(to_sq(captures[i])));
1470 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1475 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1477 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1478 Move* quiets, int quietsCnt, int bonus) {
1480 if (ss->killers[0] != move)
1482 ss->killers[1] = ss->killers[0];
1483 ss->killers[0] = move;
1486 Color us = pos.side_to_move();
1487 Thread* thisThread = pos.this_thread();
1488 thisThread->mainHistory[us][from_to(move)] << bonus;
1489 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1491 if (is_ok((ss-1)->currentMove))
1493 Square prevSq = to_sq((ss-1)->currentMove);
1494 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1497 // Decrease all the other played quiet moves
1498 for (int i = 0; i < quietsCnt; ++i)
1500 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1501 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1505 // When playing with strength handicap, choose best move among a set of RootMoves
1506 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1508 Move Skill::pick_best(size_t multiPV) {
1510 const RootMoves& rootMoves = Threads.main()->rootMoves;
1511 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1513 // RootMoves are already sorted by score in descending order
1514 Value topScore = rootMoves[0].score;
1515 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1516 int weakness = 120 - 2 * level;
1517 int maxScore = -VALUE_INFINITE;
1519 // Choose best move. For each move score we add two terms, both dependent on
1520 // weakness. One is deterministic and bigger for weaker levels, and one is
1521 // random. Then we choose the move with the resulting highest score.
1522 for (size_t i = 0; i < multiPV; ++i)
1524 // This is our magic formula
1525 int push = ( weakness * int(topScore - rootMoves[i].score)
1526 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1528 if (rootMoves[i].score + push >= maxScore)
1530 maxScore = rootMoves[i].score + push;
1531 best = rootMoves[i].pv[0];
1540 /// MainThread::check_time() is used to print debug info and, more importantly,
1541 /// to detect when we are out of available time and thus stop the search.
1543 void MainThread::check_time() {
1548 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1549 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1551 static TimePoint lastInfoTime = now();
1553 TimePoint elapsed = Time.elapsed();
1554 TimePoint tick = Limits.startTime + elapsed;
1556 if (tick - lastInfoTime >= 1000)
1558 lastInfoTime = tick;
1562 // We should not stop pondering until told so by the GUI
1566 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1567 || (Limits.movetime && elapsed >= Limits.movetime)
1568 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1569 Threads.stop = true;
1573 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1574 /// that all (if any) unsearched PV lines are sent using a previous search score.
1576 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1578 std::stringstream ss;
1579 TimePoint elapsed = Time.elapsed() + 1;
1580 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1581 size_t pvIdx = pos.this_thread()->pvIdx;
1582 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1583 uint64_t nodesSearched = Threads.nodes_searched();
1584 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1586 for (size_t i = 0; i < multiPV; ++i)
1588 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1590 if (depth == ONE_PLY && !updated)
1593 Depth d = updated ? depth : depth - ONE_PLY;
1594 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1596 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1597 v = tb ? rootMoves[i].tbScore : v;
1599 if (ss.rdbuf()->in_avail()) // Not at first line
1603 << " depth " << d / ONE_PLY
1604 << " seldepth " << rootMoves[i].selDepth
1605 << " multipv " << i + 1
1606 << " score " << UCI::value(v);
1608 if (!tb && i == pvIdx)
1609 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1611 ss << " nodes " << nodesSearched
1612 << " nps " << nodesSearched * 1000 / elapsed;
1614 if (elapsed > 1000) // Earlier makes little sense
1615 ss << " hashfull " << TT.hashfull();
1617 ss << " tbhits " << tbHits
1618 << " time " << elapsed
1621 for (Move m : rootMoves[i].pv)
1622 ss << " " << UCI::move(m, pos.is_chess960());
1629 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1630 /// before exiting the search, for instance, in case we stop the search during a
1631 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1632 /// otherwise in case of 'ponder on' we have nothing to think on.
1634 bool RootMove::extract_ponder_from_tt(Position& pos) {
1639 assert(pv.size() == 1);
1644 pos.do_move(pv[0], st);
1645 TTEntry* tte = TT.probe(pos.key(), ttHit);
1649 Move m = tte->move(); // Local copy to be SMP safe
1650 if (MoveList<LEGAL>(pos).contains(m))
1654 pos.undo_move(pv[0]);
1655 return pv.size() > 1;
1658 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1661 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1662 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1663 Cardinality = int(Options["SyzygyProbeLimit"]);
1664 bool dtz_available = true;
1666 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1667 // ProbeDepth == DEPTH_ZERO
1668 if (Cardinality > MaxCardinality)
1670 Cardinality = MaxCardinality;
1671 ProbeDepth = DEPTH_ZERO;
1674 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1676 // Rank moves using DTZ tables
1677 RootInTB = root_probe(pos, rootMoves);
1681 // DTZ tables are missing; try to rank moves using WDL tables
1682 dtz_available = false;
1683 RootInTB = root_probe_wdl(pos, rootMoves);
1689 // Sort moves according to TB rank
1690 std::sort(rootMoves.begin(), rootMoves.end(),
1691 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1693 // Probe during search only if DTZ is not available and we are winning
1694 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1699 // Assign the same rank to all moves
1700 for (auto& m : rootMoves)