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) 2015-2019 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 // Razor and futility margins
65 constexpr int RazorMargin = 600;
66 Value futility_margin(Depth d, bool improving) {
67 return Value((175 - 50 * improving) * d / ONE_PLY);
70 // Reductions lookup table, initialized at startup
71 int Reductions[64]; // [depth or moveNumber]
73 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
74 int r = Reductions[std::min(d / ONE_PLY, 63)] * Reductions[std::min(mn, 63)] / 1024;
75 return ((r + 512) / 1024 + (!i && r > 1024) - PvNode) * ONE_PLY;
78 constexpr int futility_move_count(bool improving, int depth) {
79 return (5 + depth * depth) * (1 + improving) / 2;
82 // History and stats update bonus, based on depth
83 int stat_bonus(Depth depth) {
84 int d = depth / ONE_PLY;
85 return d > 17 ? 0 : 29 * d * d + 138 * d - 134;
88 // Add a small random component to draw evaluations to keep search dynamic
89 // and to avoid 3fold-blindness.
90 Value value_draw(Depth depth, Thread* thisThread) {
91 return depth < 4 ? VALUE_DRAW
92 : VALUE_DRAW + Value(2 * (thisThread->nodes.load(std::memory_order_relaxed) % 2) - 1);
95 // Skill structure is used to implement strength limit
97 explicit Skill(int l) : level(l) {}
98 bool enabled() const { return level < 20; }
99 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
100 Move pick_best(size_t multiPV);
103 Move best = MOVE_NONE;
106 template <NodeType NT>
107 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
109 template <NodeType NT>
110 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
112 Value value_to_tt(Value v, int ply);
113 Value value_from_tt(Value v, int ply);
114 void update_pv(Move* pv, Move move, Move* childPv);
115 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
116 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietCount, int bonus);
117 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCount, int bonus);
119 inline bool gives_check(const Position& pos, Move move) {
120 Color us = pos.side_to_move();
121 return type_of(move) == NORMAL && !(pos.blockers_for_king(~us) & pos.pieces(us))
122 ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
123 : pos.gives_check(move);
126 // perft() is our utility to verify move generation. All the leaf nodes up
127 // to the given depth are generated and counted, and the sum is returned.
129 uint64_t perft(Position& pos, Depth depth) {
132 uint64_t cnt, nodes = 0;
133 const bool leaf = (depth == 2 * ONE_PLY);
135 for (const auto& m : MoveList<LEGAL>(pos))
137 if (Root && depth <= ONE_PLY)
142 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
147 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
155 /// Search::init() is called at startup to initialize various lookup tables
157 void Search::init() {
159 for (int i = 1; i < 64; ++i)
160 Reductions[i] = int(1024 * std::log(i) / std::sqrt(1.95));
164 /// Search::clear() resets search state to its initial value
166 void Search::clear() {
168 Threads.main()->wait_for_search_finished();
170 Time.availableNodes = 0;
173 Tablebases::init(Options["SyzygyPath"]); // Free up mapped files
177 /// MainThread::search() is called by the main thread when the program receives
178 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
180 void MainThread::search() {
184 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
185 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
189 Color us = rootPos.side_to_move();
190 Time.init(Limits, us, rootPos.game_ply());
193 if (rootMoves.empty())
195 rootMoves.emplace_back(MOVE_NONE);
196 sync_cout << "info depth 0 score "
197 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
202 for (Thread* th : Threads)
204 th->start_searching();
206 Thread::search(); // Let's start searching!
209 // When we reach the maximum depth, we can arrive here without a raise of
210 // Threads.stop. However, if we are pondering or in an infinite search,
211 // the UCI protocol states that we shouldn't print the best move before the
212 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
213 // until the GUI sends one of those commands.
215 while (!Threads.stop && (ponder || Limits.infinite))
216 {} // Busy wait for a stop or a ponder reset
218 // Stop the threads if not already stopped (also raise the stop if
219 // "ponderhit" just reset Threads.ponder).
222 // Wait until all threads have finished
223 for (Thread* th : Threads)
225 th->wait_for_search_finished();
227 // When playing in 'nodes as time' mode, subtract the searched nodes from
228 // the available ones before exiting.
230 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
232 // Check if there are threads with a better score than main thread
233 Thread* bestThread = this;
234 if ( Options["MultiPV"] == 1
236 && !Skill(Options["Skill Level"]).enabled()
237 && rootMoves[0].pv[0] != MOVE_NONE)
239 std::map<Move, int64_t> votes;
240 Value minScore = this->rootMoves[0].score;
242 // Find out minimum score and reset votes for moves which can be voted
243 for (Thread* th: Threads)
244 minScore = std::min(minScore, th->rootMoves[0].score);
246 // Vote according to score and depth
247 for (Thread* th : Threads)
249 int64_t s = th->rootMoves[0].score - minScore + 1;
250 votes[th->rootMoves[0].pv[0]] += 200 + s * s * int(th->completedDepth);
253 // Select best thread
254 auto bestVote = votes[this->rootMoves[0].pv[0]];
255 for (Thread* th : Threads)
256 if (votes[th->rootMoves[0].pv[0]] > bestVote)
258 bestVote = votes[th->rootMoves[0].pv[0]];
263 previousScore = bestThread->rootMoves[0].score;
265 // Send again PV info if we have a new best thread
266 if (bestThread != this)
267 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
269 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
271 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
272 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
274 std::cout << sync_endl;
278 /// Thread::search() is the main iterative deepening loop. It calls search()
279 /// repeatedly with increasing depth until the allocated thinking time has been
280 /// consumed, the user stops the search, or the maximum search depth is reached.
282 void Thread::search() {
284 // To allow access to (ss-5) up to (ss+2), the stack must be oversized.
285 // The former is needed to allow update_continuation_histories(ss-1, ...),
286 // which accesses its argument at ss-4, also near the root.
287 // The latter is needed for statScores and killer initialization.
288 Stack stack[MAX_PLY+10], *ss = stack+7;
290 Value bestValue, alpha, beta, delta;
291 Move lastBestMove = MOVE_NONE;
292 Depth lastBestMoveDepth = DEPTH_ZERO;
293 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
294 double timeReduction = 1.0;
295 Color us = rootPos.side_to_move();
297 std::memset(ss-7, 0, 10 * sizeof(Stack));
298 for (int i = 7; i > 0; i--)
299 (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
302 bestValue = delta = alpha = -VALUE_INFINITE;
303 beta = VALUE_INFINITE;
306 mainThread->bestMoveChanges = 0;
308 size_t multiPV = Options["MultiPV"];
309 Skill skill(Options["Skill Level"]);
311 // When playing with strength handicap enable MultiPV search that we will
312 // use behind the scenes to retrieve a set of possible moves.
314 multiPV = std::max(multiPV, (size_t)4);
316 multiPV = std::min(multiPV, rootMoves.size());
318 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
320 // In analysis mode, adjust contempt in accordance with user preference
321 if (Limits.infinite || Options["UCI_AnalyseMode"])
322 ct = Options["Analysis Contempt"] == "Off" ? 0
323 : Options["Analysis Contempt"] == "Both" ? ct
324 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
325 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
328 // In evaluate.cpp the evaluation is from the white point of view
329 contempt = (us == WHITE ? make_score(ct, ct / 2)
330 : -make_score(ct, ct / 2));
332 // Iterative deepening loop until requested to stop or the target depth is reached
333 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
335 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
337 // Age out PV variability metric
339 mainThread->bestMoveChanges *= 0.517;
341 // Save the last iteration's scores before first PV line is searched and
342 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
343 for (RootMove& rm : rootMoves)
344 rm.previousScore = rm.score;
349 // MultiPV loop. We perform a full root search for each PV line
350 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
355 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
356 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
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 + 88 * previousScore / (abs(previousScore) + 200);
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
381 int failedHighCnt = 0;
384 Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
385 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
387 // Bring the best move to the front. It is critical that sorting
388 // is done with a stable algorithm because all the values but the
389 // first and eventually the new best one are set to -VALUE_INFINITE
390 // and we want to keep the same order for all the moves except the
391 // new PV that goes to the front. Note that in case of MultiPV
392 // search the already searched PV lines are preserved.
393 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
395 // If search has been stopped, we break immediately. Sorting is
396 // safe because RootMoves is still valid, although it refers to
397 // the previous iteration.
401 // When failing high/low give some update (without cluttering
402 // the UI) before a re-search.
405 && (bestValue <= alpha || bestValue >= beta)
406 && Time.elapsed() > 3000)
407 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
409 // In case of failing low/high increase aspiration window and
410 // re-search, otherwise exit the loop.
411 if (bestValue <= alpha)
413 beta = (alpha + beta) / 2;
414 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
419 mainThread->stopOnPonderhit = false;
422 else if (bestValue >= beta)
424 beta = std::min(bestValue + delta, VALUE_INFINITE);
431 delta += delta / 4 + 5;
433 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
436 // Sort the PV lines searched so far and update the GUI
437 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
440 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
441 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
445 completedDepth = rootDepth;
447 if (rootMoves[0].pv[0] != lastBestMove) {
448 lastBestMove = rootMoves[0].pv[0];
449 lastBestMoveDepth = rootDepth;
452 // Have we found a "mate in x"?
454 && bestValue >= VALUE_MATE_IN_MAX_PLY
455 && VALUE_MATE - bestValue <= 2 * Limits.mate)
461 // If skill level is enabled and time is up, pick a sub-optimal best move
462 if (skill.enabled() && skill.time_to_pick(rootDepth))
463 skill.pick_best(multiPV);
465 // Do we have time for the next iteration? Can we stop searching now?
466 if ( Limits.use_time_management()
468 && !mainThread->stopOnPonderhit)
470 double fallingEval = (306 + 9 * (mainThread->previousScore - bestValue)) / 581.0;
471 fallingEval = std::max(0.5, std::min(1.5, fallingEval));
473 // If the bestMove is stable over several iterations, reduce time accordingly
474 timeReduction = lastBestMoveDepth + 10 * ONE_PLY < completedDepth ? 1.95 : 1.0;
475 double reduction = std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
477 // Use part of the gained time from a previous stable move for the current move
478 double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
480 // Stop the search if we have only one legal move, or if available time elapsed
481 if ( rootMoves.size() == 1
482 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
484 // If we are allowed to ponder do not stop the search now but
485 // keep pondering until the GUI sends "ponderhit" or "stop".
486 if (mainThread->ponder)
487 mainThread->stopOnPonderhit = true;
497 mainThread->previousTimeReduction = timeReduction;
499 // If skill level is enabled, swap best PV line with the sub-optimal one
501 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
502 skill.best ? skill.best : skill.pick_best(multiPV)));
508 // search<>() is the main search function for both PV and non-PV nodes
510 template <NodeType NT>
511 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
513 constexpr bool PvNode = NT == PV;
514 const bool rootNode = PvNode && ss->ply == 0;
516 // Check if we have an upcoming move which draws by repetition, or
517 // if the opponent had an alternative move earlier to this position.
518 if ( pos.rule50_count() >= 3
519 && alpha < VALUE_DRAW
521 && pos.has_game_cycle(ss->ply))
523 alpha = value_draw(depth, pos.this_thread());
528 // Dive into quiescence search when the depth reaches zero
530 return qsearch<NT>(pos, ss, alpha, beta);
532 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
533 assert(PvNode || (alpha == beta - 1));
534 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
535 assert(!(PvNode && cutNode));
536 assert(depth / ONE_PLY * ONE_PLY == depth);
538 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
542 Move ttMove, move, excludedMove, bestMove;
543 Depth extension, newDepth;
544 Value bestValue, value, ttValue, eval, maxValue, pureStaticEval;
545 bool ttHit, ttPv, inCheck, givesCheck, improving;
546 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
548 int moveCount, captureCount, quietCount;
550 // Step 1. Initialize node
551 Thread* thisThread = pos.this_thread();
552 inCheck = pos.checkers();
553 Color us = pos.side_to_move();
554 moveCount = captureCount = quietCount = ss->moveCount = 0;
555 bestValue = -VALUE_INFINITE;
556 maxValue = VALUE_INFINITE;
558 // Check for the available remaining time
559 if (thisThread == Threads.main())
560 static_cast<MainThread*>(thisThread)->check_time();
562 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
563 if (PvNode && thisThread->selDepth < ss->ply + 1)
564 thisThread->selDepth = ss->ply + 1;
568 // Step 2. Check for aborted search and immediate draw
569 if ( Threads.stop.load(std::memory_order_relaxed)
570 || pos.is_draw(ss->ply)
571 || ss->ply >= MAX_PLY)
572 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
573 : value_draw(depth, pos.this_thread());
575 // Step 3. Mate distance pruning. Even if we mate at the next move our score
576 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
577 // a shorter mate was found upward in the tree then there is no need to search
578 // because we will never beat the current alpha. Same logic but with reversed
579 // signs applies also in the opposite condition of being mated instead of giving
580 // mate. In this case return a fail-high score.
581 alpha = std::max(mated_in(ss->ply), alpha);
582 beta = std::min(mate_in(ss->ply+1), beta);
587 assert(0 <= ss->ply && ss->ply < MAX_PLY);
589 (ss+1)->ply = ss->ply + 1;
590 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
591 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
592 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
593 Square prevSq = to_sq((ss-1)->currentMove);
595 // Initialize statScore to zero for the grandchildren of the current position.
596 // So statScore is shared between all grandchildren and only the first grandchild
597 // starts with statScore = 0. Later grandchildren start with the last calculated
598 // statScore of the previous grandchild. This influences the reduction rules in
599 // LMR which are based on the statScore of parent position.
600 (ss+2)->statScore = 0;
602 // Step 4. Transposition table lookup. We don't want the score of a partial
603 // search to overwrite a previous full search TT value, so we use a different
604 // position key in case of an excluded move.
605 excludedMove = ss->excludedMove;
606 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
607 tte = TT.probe(posKey, ttHit);
608 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
609 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
610 : ttHit ? tte->move() : MOVE_NONE;
611 ttPv = (ttHit && tte->is_pv()) || (PvNode && depth > 4 * ONE_PLY);
613 // At non-PV nodes we check for an early TT cutoff
616 && tte->depth() >= depth
617 && ttValue != VALUE_NONE // Possible in case of TT access race
618 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
619 : (tte->bound() & BOUND_UPPER)))
621 // If ttMove is quiet, update move sorting heuristics on TT hit
626 if (!pos.capture_or_promotion(ttMove))
627 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
629 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
630 if ( ((ss-1)->moveCount == 1 || (ss-1)->currentMove == (ss-1)->killers[0])
631 && !pos.captured_piece())
632 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
634 // Penalty for a quiet ttMove that fails low
635 else if (!pos.capture_or_promotion(ttMove))
637 int penalty = -stat_bonus(depth);
638 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
639 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
645 // Step 5. Tablebases probe
646 if (!rootNode && TB::Cardinality)
648 int piecesCount = pos.count<ALL_PIECES>();
650 if ( piecesCount <= TB::Cardinality
651 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
652 && pos.rule50_count() == 0
653 && !pos.can_castle(ANY_CASTLING))
656 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
658 // Force check of time on the next occasion
659 if (thisThread == Threads.main())
660 static_cast<MainThread*>(thisThread)->callsCnt = 0;
662 if (err != TB::ProbeState::FAIL)
664 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
666 int drawScore = TB::UseRule50 ? 1 : 0;
668 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
669 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
670 : VALUE_DRAW + 2 * wdl * drawScore;
672 Bound b = wdl < -drawScore ? BOUND_UPPER
673 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
675 if ( b == BOUND_EXACT
676 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
678 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
679 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
680 MOVE_NONE, VALUE_NONE);
687 if (b == BOUND_LOWER)
688 bestValue = value, alpha = std::max(alpha, bestValue);
696 // Step 6. Static evaluation of the position
699 ss->staticEval = eval = pureStaticEval = VALUE_NONE;
701 goto moves_loop; // Skip early pruning when in check
705 // Never assume anything on values stored in TT
706 ss->staticEval = eval = pureStaticEval = tte->eval();
707 if (eval == VALUE_NONE)
708 ss->staticEval = eval = pureStaticEval = evaluate(pos);
710 // Can ttValue be used as a better position evaluation?
711 if ( ttValue != VALUE_NONE
712 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
717 if ((ss-1)->currentMove != MOVE_NULL)
719 int bonus = -(ss-1)->statScore / 512;
721 pureStaticEval = evaluate(pos);
722 ss->staticEval = eval = pureStaticEval + bonus;
725 ss->staticEval = eval = pureStaticEval = -(ss-1)->staticEval + 2 * Eval::Tempo;
727 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, pureStaticEval);
730 // Step 7. Razoring (~2 Elo)
731 if ( !rootNode // The required rootNode PV handling is not available in qsearch
732 && depth < 2 * ONE_PLY
733 && eval <= alpha - RazorMargin)
734 return qsearch<NT>(pos, ss, alpha, beta);
736 improving = ss->staticEval >= (ss-2)->staticEval
737 || (ss-2)->staticEval == VALUE_NONE;
739 // Step 8. Futility pruning: child node (~30 Elo)
741 && depth < 7 * ONE_PLY
742 && eval - futility_margin(depth, improving) >= beta
743 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
746 // Step 9. Null move search with verification search (~40 Elo)
748 && (ss-1)->currentMove != MOVE_NULL
749 && (ss-1)->statScore < 23200
751 && pureStaticEval >= beta - 36 * depth / ONE_PLY + 225
753 && pos.non_pawn_material(us)
754 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
756 assert(eval - beta >= 0);
758 // Null move dynamic reduction based on depth and value
759 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 200, 3)) * ONE_PLY;
761 ss->currentMove = MOVE_NULL;
762 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
764 pos.do_null_move(st);
766 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
768 pos.undo_null_move();
770 if (nullValue >= beta)
772 // Do not return unproven mate scores
773 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
776 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 12 * ONE_PLY))
779 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
781 // Do verification search at high depths, with null move pruning disabled
782 // for us, until ply exceeds nmpMinPly.
783 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
784 thisThread->nmpColor = us;
786 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
788 thisThread->nmpMinPly = 0;
795 // Step 10. ProbCut (~10 Elo)
796 // If we have a good enough capture and a reduced search returns a value
797 // much above beta, we can (almost) safely prune the previous move.
799 && depth >= 5 * ONE_PLY
800 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
802 Value raisedBeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
803 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
804 int probCutCount = 0;
806 while ( (move = mp.next_move()) != MOVE_NONE
807 && probCutCount < 2 + 2 * cutNode)
808 if (move != excludedMove && pos.legal(move))
812 ss->currentMove = move;
813 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
815 assert(depth >= 5 * ONE_PLY);
817 pos.do_move(move, st);
819 // Perform a preliminary qsearch to verify that the move holds
820 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
822 // If the qsearch held, perform the regular search
823 if (value >= raisedBeta)
824 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
828 if (value >= raisedBeta)
833 // Step 11. Internal iterative deepening (~2 Elo)
834 if ( depth >= 8 * ONE_PLY
837 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
839 tte = TT.probe(posKey, ttHit);
840 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
841 ttMove = ttHit ? tte->move() : MOVE_NONE;
844 moves_loop: // When in check, search starts from here
846 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
847 nullptr, (ss-4)->continuationHistory,
848 nullptr, (ss-6)->continuationHistory };
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
858 moveCountPruning = false;
859 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
861 // Step 12. Loop through all pseudo-legal moves until no moves remain
862 // or a beta cutoff occurs.
863 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
867 if (move == excludedMove)
870 // At root obey the "searchmoves" option and skip moves not listed in Root
871 // Move List. As a consequence any illegal move is also skipped. In MultiPV
872 // mode we also skip PV moves which have been already searched and those
873 // of lower "TB rank" if we are in a TB root position.
874 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
875 thisThread->rootMoves.begin() + thisThread->pvLast, move))
878 ss->moveCount = ++moveCount;
880 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
881 sync_cout << "info depth " << depth / ONE_PLY
882 << " currmove " << UCI::move(move, pos.is_chess960())
883 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
885 (ss+1)->pv = nullptr;
887 extension = DEPTH_ZERO;
888 captureOrPromotion = pos.capture_or_promotion(move);
889 movedPiece = pos.moved_piece(move);
890 givesCheck = gives_check(pos, move);
892 // Step 13. Extensions (~70 Elo)
894 // Singular extension search (~60 Elo). If all moves but one fail low on a
895 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
896 // then that move is singular and should be extended. To verify this we do
897 // a reduced search on all the other moves but the ttMove and if the
898 // result is lower than ttValue minus a margin then we will extend the ttMove.
899 if ( depth >= 8 * ONE_PLY
902 && !excludedMove // Avoid recursive singular search
903 /* && ttValue != VALUE_NONE Already implicit in the next condition */
904 && abs(ttValue) < VALUE_KNOWN_WIN
905 && (tte->bound() & BOUND_LOWER)
906 && tte->depth() >= depth - 3 * ONE_PLY
909 Value singularBeta = ttValue - 2 * depth / ONE_PLY;
910 ss->excludedMove = move;
911 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, depth / 2, cutNode);
912 ss->excludedMove = MOVE_NONE;
914 if (value < singularBeta)
918 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
919 // search without the ttMove. So we assume this expected Cut-node is not singular,
920 // that is multiple moves fail high, and we can prune the whole subtree by returning
921 // the hard beta bound.
922 else if (cutNode && singularBeta > beta)
926 // Check extension (~2 Elo)
928 && (pos.blockers_for_king(~us) & from_sq(move) || pos.see_ge(move)))
931 // Castling extension
932 else if (type_of(move) == CASTLING)
935 // Calculate new depth for this move
936 newDepth = depth - ONE_PLY + extension;
938 // Step 14. Pruning at shallow depth (~170 Elo)
940 && pos.non_pawn_material(us)
941 && bestValue > VALUE_MATED_IN_MAX_PLY)
943 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
944 moveCountPruning = moveCount >= futility_move_count(improving,depth / ONE_PLY);
946 if ( !captureOrPromotion
948 && !pos.advanced_pawn_push(move))
950 // Move count based pruning (~30 Elo)
951 if (moveCountPruning)
954 // Reduced depth of the next LMR search
955 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
957 // Countermoves based pruning (~20 Elo)
958 if ( lmrDepth < 3 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
959 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
960 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
963 // Futility pruning: parent node (~2 Elo)
966 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
969 // Prune moves with negative SEE (~10 Elo)
970 if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
973 else if ( !extension // (~20 Elo)
974 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
978 // Speculative prefetch as early as possible
979 prefetch(TT.first_entry(pos.key_after(move)));
981 // Check for legality just before making the move
982 if (!rootNode && !pos.legal(move))
984 ss->moveCount = --moveCount;
988 // Update the current move (this must be done after singular extension search)
989 ss->currentMove = move;
990 ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
992 // Step 15. Make the move
993 pos.do_move(move, st, givesCheck);
995 // Step 16. Reduced depth search (LMR). If the move fails high it will be
996 // re-searched at full depth.
997 if ( depth >= 3 * ONE_PLY
999 && (!captureOrPromotion || moveCountPruning))
1001 Depth r = reduction<PvNode>(improving, depth, moveCount);
1003 // Decrease reduction if position is or has been on the PV
1007 // Decrease reduction if opponent's move count is high (~10 Elo)
1008 if ((ss-1)->moveCount > 15)
1011 if (!captureOrPromotion)
1013 // Increase reduction if ttMove is a capture (~0 Elo)
1017 // Increase reduction for cut nodes (~5 Elo)
1021 // Decrease reduction for moves that escape a capture. Filter out
1022 // castling moves, because they are coded as "king captures rook" and
1023 // hence break make_move(). (~5 Elo)
1024 else if ( type_of(move) == NORMAL
1025 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1028 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1029 + (*contHist[0])[movedPiece][to_sq(move)]
1030 + (*contHist[1])[movedPiece][to_sq(move)]
1031 + (*contHist[3])[movedPiece][to_sq(move)]
1034 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1035 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1038 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1041 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1042 r -= ss->statScore / 20000 * ONE_PLY;
1045 Depth d = std::max(newDepth - std::max(r, DEPTH_ZERO), ONE_PLY);
1047 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1049 doFullDepthSearch = (value > alpha && d != newDepth);
1052 doFullDepthSearch = !PvNode || moveCount > 1;
1054 // Step 17. Full depth search when LMR is skipped or fails high
1055 if (doFullDepthSearch)
1056 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1058 // For PV nodes only, do a full PV search on the first move or after a fail
1059 // high (in the latter case search only if value < beta), otherwise let the
1060 // parent node fail low with value <= alpha and try another move.
1061 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1064 (ss+1)->pv[0] = MOVE_NONE;
1066 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1069 // Step 18. Undo move
1070 pos.undo_move(move);
1072 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1074 // Step 19. Check for a new best move
1075 // Finished searching the move. If a stop occurred, the return value of
1076 // the search cannot be trusted, and we return immediately without
1077 // updating best move, PV and TT.
1078 if (Threads.stop.load(std::memory_order_relaxed))
1083 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1084 thisThread->rootMoves.end(), move);
1086 // PV move or new best move?
1087 if (moveCount == 1 || value > alpha)
1090 rm.selDepth = thisThread->selDepth;
1095 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1096 rm.pv.push_back(*m);
1098 // We record how often the best move has been changed in each
1099 // iteration. This information is used for time management: When
1100 // the best move changes frequently, we allocate some more time.
1101 if (moveCount > 1 && thisThread == Threads.main())
1102 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1105 // All other moves but the PV are set to the lowest value: this
1106 // is not a problem when sorting because the sort is stable and the
1107 // move position in the list is preserved - just the PV is pushed up.
1108 rm.score = -VALUE_INFINITE;
1111 if (value > bestValue)
1119 if (PvNode && !rootNode) // Update pv even in fail-high case
1120 update_pv(ss->pv, move, (ss+1)->pv);
1122 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1126 assert(value >= beta); // Fail high
1133 if (move != bestMove)
1135 if (captureOrPromotion && captureCount < 32)
1136 capturesSearched[captureCount++] = move;
1138 else if (!captureOrPromotion && quietCount < 64)
1139 quietsSearched[quietCount++] = move;
1143 // The following condition would detect a stop only after move loop has been
1144 // completed. But in this case bestValue is valid because we have fully
1145 // searched our subtree, and we can anyhow save the result in TT.
1151 // Step 20. Check for mate and stalemate
1152 // All legal moves have been searched and if there are no legal moves, it
1153 // must be a mate or a stalemate. If we are in a singular extension search then
1154 // return a fail low score.
1156 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1159 bestValue = excludedMove ? alpha
1160 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1163 // Quiet best move: update move sorting heuristics
1164 if (!pos.capture_or_promotion(bestMove))
1165 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1166 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1168 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1170 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1171 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1172 && !pos.captured_piece())
1173 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1176 // Bonus for prior countermove that caused the fail low
1177 else if ( (depth >= 3 * ONE_PLY || PvNode)
1178 && !pos.captured_piece())
1179 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1182 bestValue = std::min(bestValue, maxValue);
1185 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1186 bestValue >= beta ? BOUND_LOWER :
1187 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1188 depth, bestMove, pureStaticEval);
1190 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1196 // qsearch() is the quiescence search function, which is called by the main
1197 // search function with depth zero, or recursively with depth less than ONE_PLY.
1198 template <NodeType NT>
1199 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1201 constexpr bool PvNode = NT == PV;
1203 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1204 assert(PvNode || (alpha == beta - 1));
1205 assert(depth <= DEPTH_ZERO);
1206 assert(depth / ONE_PLY * ONE_PLY == depth);
1212 Move ttMove, move, bestMove;
1214 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1215 bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
1220 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1222 ss->pv[0] = MOVE_NONE;
1225 Thread* thisThread = pos.this_thread();
1226 (ss+1)->ply = ss->ply + 1;
1227 ss->currentMove = bestMove = MOVE_NONE;
1228 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
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;
1249 pvHit = ttHit && tte->is_pv();
1253 && tte->depth() >= ttDepth
1254 && ttValue != VALUE_NONE // Only in case of TT access race
1255 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1256 : (tte->bound() & BOUND_UPPER)))
1259 // Evaluate the position statically
1262 ss->staticEval = VALUE_NONE;
1263 bestValue = futilityBase = -VALUE_INFINITE;
1269 // Never assume anything on values stored in TT
1270 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1271 ss->staticEval = bestValue = evaluate(pos);
1273 // Can ttValue be used as a better position evaluation?
1274 if ( ttValue != VALUE_NONE
1275 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1276 bestValue = ttValue;
1279 ss->staticEval = bestValue =
1280 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1281 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1283 // Stand pat. Return immediately if static value is at least beta
1284 if (bestValue >= beta)
1287 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1288 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1293 if (PvNode && bestValue > alpha)
1296 futilityBase = bestValue + 128;
1299 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1300 nullptr, (ss-4)->continuationHistory,
1301 nullptr, (ss-6)->continuationHistory };
1303 // Initialize a MovePicker object for the current position, and prepare
1304 // to search the moves. Because the depth is <= 0 here, only captures,
1305 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1307 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1308 &thisThread->captureHistory,
1310 to_sq((ss-1)->currentMove));
1312 // Loop through the moves until no moves remain or a beta cutoff occurs
1313 while ((move = mp.next_move()) != MOVE_NONE)
1315 assert(is_ok(move));
1317 givesCheck = gives_check(pos, move);
1324 && futilityBase > -VALUE_KNOWN_WIN
1325 && !pos.advanced_pawn_push(move))
1327 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1329 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1331 if (futilityValue <= alpha)
1333 bestValue = std::max(bestValue, futilityValue);
1337 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1339 bestValue = std::max(bestValue, futilityBase);
1344 // Detect non-capture evasions that are candidates to be pruned
1345 evasionPrunable = inCheck
1346 && (depth != DEPTH_ZERO || moveCount > 2)
1347 && bestValue > VALUE_MATED_IN_MAX_PLY
1348 && !pos.capture(move);
1350 // Don't search moves with negative SEE values
1351 if ( (!inCheck || evasionPrunable)
1352 && !pos.see_ge(move))
1355 // Speculative prefetch as early as possible
1356 prefetch(TT.first_entry(pos.key_after(move)));
1358 // Check for legality just before making the move
1359 if (!pos.legal(move))
1365 ss->currentMove = move;
1366 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
1368 // Make and search the move
1369 pos.do_move(move, st, givesCheck);
1370 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1371 pos.undo_move(move);
1373 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1375 // Check for a new best move
1376 if (value > bestValue)
1384 if (PvNode) // Update pv even in fail-high case
1385 update_pv(ss->pv, move, (ss+1)->pv);
1387 if (PvNode && value < beta) // Update alpha here!
1395 // All legal moves have been searched. A special case: If we're in check
1396 // and no legal moves were found, it is checkmate.
1397 if (inCheck && bestValue == -VALUE_INFINITE)
1398 return mated_in(ss->ply); // Plies to mate from the root
1400 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1401 bestValue >= beta ? BOUND_LOWER :
1402 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1403 ttDepth, bestMove, ss->staticEval);
1405 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1411 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1412 // "plies to mate from the current position". Non-mate scores are unchanged.
1413 // The function is called before storing a value in the transposition table.
1415 Value value_to_tt(Value v, int ply) {
1417 assert(v != VALUE_NONE);
1419 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1420 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1424 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1425 // from the transposition table (which refers to the plies to mate/be mated
1426 // from current position) to "plies to mate/be mated from the root".
1428 Value value_from_tt(Value v, int ply) {
1430 return v == VALUE_NONE ? VALUE_NONE
1431 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1432 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1436 // update_pv() adds current move and appends child pv[]
1438 void update_pv(Move* pv, Move move, Move* childPv) {
1440 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1446 // update_continuation_histories() updates histories of the move pairs formed
1447 // by moves at ply -1, -2, and -4 with current move.
1449 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1451 for (int i : {1, 2, 4, 6})
1452 if (is_ok((ss-i)->currentMove))
1453 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1457 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1459 void update_capture_stats(const Position& pos, Move move,
1460 Move* captures, int captureCount, int bonus) {
1462 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1463 Piece moved_piece = pos.moved_piece(move);
1464 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1466 if (pos.capture_or_promotion(move))
1467 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1469 // Decrease all the other played capture moves
1470 for (int i = 0; i < captureCount; ++i)
1472 moved_piece = pos.moved_piece(captures[i]);
1473 captured = type_of(pos.piece_on(to_sq(captures[i])));
1474 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1479 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1481 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1482 Move* quiets, int quietCount, int bonus) {
1484 if (ss->killers[0] != move)
1486 ss->killers[1] = ss->killers[0];
1487 ss->killers[0] = move;
1490 Color us = pos.side_to_move();
1491 Thread* thisThread = pos.this_thread();
1492 thisThread->mainHistory[us][from_to(move)] << bonus;
1493 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1495 if (is_ok((ss-1)->currentMove))
1497 Square prevSq = to_sq((ss-1)->currentMove);
1498 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1501 // Decrease all the other played quiet moves
1502 for (int i = 0; i < quietCount; ++i)
1504 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1505 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1509 // When playing with strength handicap, choose best move among a set of RootMoves
1510 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1512 Move Skill::pick_best(size_t multiPV) {
1514 const RootMoves& rootMoves = Threads.main()->rootMoves;
1515 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1517 // RootMoves are already sorted by score in descending order
1518 Value topScore = rootMoves[0].score;
1519 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1520 int weakness = 120 - 2 * level;
1521 int maxScore = -VALUE_INFINITE;
1523 // Choose best move. For each move score we add two terms, both dependent on
1524 // weakness. One is deterministic and bigger for weaker levels, and one is
1525 // random. Then we choose the move with the resulting highest score.
1526 for (size_t i = 0; i < multiPV; ++i)
1528 // This is our magic formula
1529 int push = ( weakness * int(topScore - rootMoves[i].score)
1530 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1532 if (rootMoves[i].score + push >= maxScore)
1534 maxScore = rootMoves[i].score + push;
1535 best = rootMoves[i].pv[0];
1544 /// MainThread::check_time() is used to print debug info and, more importantly,
1545 /// to detect when we are out of available time and thus stop the search.
1547 void MainThread::check_time() {
1552 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1553 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1555 static TimePoint lastInfoTime = now();
1557 TimePoint elapsed = Time.elapsed();
1558 TimePoint tick = Limits.startTime + elapsed;
1560 if (tick - lastInfoTime >= 1000)
1562 lastInfoTime = tick;
1566 // We should not stop pondering until told so by the GUI
1570 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1571 || (Limits.movetime && elapsed >= Limits.movetime)
1572 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1573 Threads.stop = true;
1577 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1578 /// that all (if any) unsearched PV lines are sent using a previous search score.
1580 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1582 std::stringstream ss;
1583 TimePoint elapsed = Time.elapsed() + 1;
1584 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1585 size_t pvIdx = pos.this_thread()->pvIdx;
1586 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1587 uint64_t nodesSearched = Threads.nodes_searched();
1588 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1590 for (size_t i = 0; i < multiPV; ++i)
1592 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1594 if (depth == ONE_PLY && !updated)
1597 Depth d = updated ? depth : depth - ONE_PLY;
1598 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1600 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1601 v = tb ? rootMoves[i].tbScore : v;
1603 if (ss.rdbuf()->in_avail()) // Not at first line
1607 << " depth " << d / ONE_PLY
1608 << " seldepth " << rootMoves[i].selDepth
1609 << " multipv " << i + 1
1610 << " score " << UCI::value(v);
1612 if (!tb && i == pvIdx)
1613 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1615 ss << " nodes " << nodesSearched
1616 << " nps " << nodesSearched * 1000 / elapsed;
1618 if (elapsed > 1000) // Earlier makes little sense
1619 ss << " hashfull " << TT.hashfull();
1621 ss << " tbhits " << tbHits
1622 << " time " << elapsed
1625 for (Move m : rootMoves[i].pv)
1626 ss << " " << UCI::move(m, pos.is_chess960());
1633 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1634 /// before exiting the search, for instance, in case we stop the search during a
1635 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1636 /// otherwise in case of 'ponder on' we have nothing to think on.
1638 bool RootMove::extract_ponder_from_tt(Position& pos) {
1643 assert(pv.size() == 1);
1645 if (pv[0] == MOVE_NONE)
1648 pos.do_move(pv[0], st);
1649 TTEntry* tte = TT.probe(pos.key(), ttHit);
1653 Move m = tte->move(); // Local copy to be SMP safe
1654 if (MoveList<LEGAL>(pos).contains(m))
1658 pos.undo_move(pv[0]);
1659 return pv.size() > 1;
1662 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1665 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1666 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1667 Cardinality = int(Options["SyzygyProbeLimit"]);
1668 bool dtz_available = true;
1670 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1671 // ProbeDepth == DEPTH_ZERO
1672 if (Cardinality > MaxCardinality)
1674 Cardinality = MaxCardinality;
1675 ProbeDepth = DEPTH_ZERO;
1678 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1680 // Rank moves using DTZ tables
1681 RootInTB = root_probe(pos, rootMoves);
1685 // DTZ tables are missing; try to rank moves using WDL tables
1686 dtz_available = false;
1687 RootInTB = root_probe_wdl(pos, rootMoves);
1693 // Sort moves according to TB rank
1694 std::sort(rootMoves.begin(), rootMoves.end(),
1695 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1697 // Probe during search only if DTZ is not available and we are winning
1698 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1703 // Assign the same rank to all moves
1704 for (auto& m : rootMoves)