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 = 600;
70 Value futility_margin(Depth d, bool improving) {
71 return Value((175 - 50 * improving) * d / ONE_PLY);
74 // Futility and reductions lookup tables, initialized at startup
75 int FutilityMoveCounts[2][16]; // [improving][depth]
76 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
78 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
79 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
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 quietsCnt, int bonus);
117 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCnt, 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 imp = 0; imp <= 1; ++imp)
160 for (int d = 1; d < 64; ++d)
161 for (int mc = 1; mc < 64; ++mc)
163 double r = log(d) * log(mc) / 1.95;
165 Reductions[NonPV][imp][d][mc] = int(std::round(r));
166 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
168 // Increase reduction for non-PV nodes when eval is not improving
170 Reductions[NonPV][imp][d][mc]++;
173 for (int d = 0; d < 16; ++d)
175 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
176 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
181 /// Search::clear() resets search state to its initial value
183 void Search::clear() {
185 Threads.main()->wait_for_search_finished();
187 Time.availableNodes = 0;
193 /// MainThread::search() is called by the main thread when the program receives
194 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
196 void MainThread::search() {
200 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
201 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
205 Color us = rootPos.side_to_move();
206 Time.init(Limits, us, rootPos.game_ply());
209 if (rootMoves.empty())
211 rootMoves.emplace_back(MOVE_NONE);
212 sync_cout << "info depth 0 score "
213 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
218 for (Thread* th : Threads)
220 th->start_searching();
222 Thread::search(); // Let's start searching!
225 // When we reach the maximum depth, we can arrive here without a raise of
226 // Threads.stop. However, if we are pondering or in an infinite search,
227 // the UCI protocol states that we shouldn't print the best move before the
228 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
229 // until the GUI sends one of those commands (which also raises Threads.stop).
230 Threads.stopOnPonderhit = true;
232 while (!Threads.stop && (Threads.ponder || Limits.infinite))
233 {} // Busy wait for a stop or a ponder reset
235 // Stop the threads if not already stopped (also raise the stop if
236 // "ponderhit" just reset Threads.ponder).
239 // Wait until all threads have finished
240 for (Thread* th : Threads)
242 th->wait_for_search_finished();
244 // When playing in 'nodes as time' mode, subtract the searched nodes from
245 // the available ones before exiting.
247 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
249 // Check if there are threads with a better score than main thread
250 Thread* bestThread = this;
251 if ( Options["MultiPV"] == 1
253 && !Skill(Options["Skill Level"]).enabled()
254 && rootMoves[0].pv[0] != MOVE_NONE)
256 std::map<Move, int> votes;
257 Value minScore = this->rootMoves[0].score;
259 // Find out minimum score and reset votes for moves which can be voted
260 for (Thread* th: Threads)
262 minScore = std::min(minScore, th->rootMoves[0].score);
263 votes[th->rootMoves[0].pv[0]] = 0;
266 // Vote according to score and depth
267 for (Thread* th : Threads)
268 votes[th->rootMoves[0].pv[0]] += int(th->rootMoves[0].score - minScore)
269 + int(th->completedDepth);
271 // Select best thread
272 int bestVote = votes[this->rootMoves[0].pv[0]];
273 for (Thread* th : Threads)
275 if (votes[th->rootMoves[0].pv[0]] > bestVote)
277 bestVote = votes[th->rootMoves[0].pv[0]];
283 previousScore = bestThread->rootMoves[0].score;
285 // Send again PV info if we have a new best thread
286 if (bestThread != this)
287 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
289 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
291 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
292 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
294 std::cout << sync_endl;
298 /// Thread::search() is the main iterative deepening loop. It calls search()
299 /// repeatedly with increasing depth until the allocated thinking time has been
300 /// consumed, the user stops the search, or the maximum search depth is reached.
302 void Thread::search() {
304 Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
305 Value bestValue, alpha, beta, delta;
306 Move lastBestMove = MOVE_NONE;
307 Depth lastBestMoveDepth = DEPTH_ZERO;
308 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
309 double timeReduction = 1.0;
310 Color us = rootPos.side_to_move();
313 std::memset(ss-4, 0, 7 * sizeof(Stack));
314 for (int i = 4; i > 0; i--)
315 (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
317 bestValue = delta = alpha = -VALUE_INFINITE;
318 beta = VALUE_INFINITE;
321 mainThread->bestMoveChanges = 0, failedLow = false;
323 size_t multiPV = Options["MultiPV"];
324 Skill skill(Options["Skill Level"]);
326 // When playing with strength handicap enable MultiPV search that we will
327 // use behind the scenes to retrieve a set of possible moves.
329 multiPV = std::max(multiPV, (size_t)4);
331 multiPV = std::min(multiPV, rootMoves.size());
333 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
335 // In analysis mode, adjust contempt in accordance with user preference
336 if (Limits.infinite || Options["UCI_AnalyseMode"])
337 ct = Options["Analysis Contempt"] == "Off" ? 0
338 : Options["Analysis Contempt"] == "Both" ? ct
339 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
340 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
343 // In evaluate.cpp the evaluation is from the white point of view
344 contempt = (us == WHITE ? make_score(ct, ct / 2)
345 : -make_score(ct, ct / 2));
347 // Iterative deepening loop until requested to stop or the target depth is reached
348 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
350 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
352 // Distribute search depths across the helper threads
355 int i = (idx - 1) % 20;
356 if (((rootDepth / ONE_PLY + SkipPhase[i]) / SkipSize[i]) % 2)
357 continue; // Retry with an incremented rootDepth
360 // Age out PV variability metric
362 mainThread->bestMoveChanges *= 0.517, failedLow = false;
364 // Save the last iteration's scores before first PV line is searched and
365 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
366 for (RootMove& rm : rootMoves)
367 rm.previousScore = rm.score;
372 // MultiPV loop. We perform a full root search for each PV line
373 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
378 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
379 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
383 // Reset UCI info selDepth for each depth and each PV line
386 // Reset aspiration window starting size
387 if (rootDepth >= 5 * ONE_PLY)
389 Value previousScore = rootMoves[pvIdx].previousScore;
391 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
392 beta = std::min(previousScore + delta, VALUE_INFINITE);
394 // Adjust contempt based on root move's previousScore (dynamic contempt)
395 int dct = ct + 88 * previousScore / (abs(previousScore) + 200);
397 contempt = (us == WHITE ? make_score(dct, dct / 2)
398 : -make_score(dct, dct / 2));
401 // Start with a small aspiration window and, in the case of a fail
402 // high/low, re-search with a bigger window until we don't fail
404 int failedHighCnt = 0;
407 Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
408 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
410 // Bring the best move to the front. It is critical that sorting
411 // is done with a stable algorithm because all the values but the
412 // first and eventually the new best one are set to -VALUE_INFINITE
413 // and we want to keep the same order for all the moves except the
414 // new PV that goes to the front. Note that in case of MultiPV
415 // search the already searched PV lines are preserved.
416 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
418 // If search has been stopped, we break immediately. Sorting is
419 // safe because RootMoves is still valid, although it refers to
420 // the previous iteration.
424 // When failing high/low give some update (without cluttering
425 // the UI) before a re-search.
428 && (bestValue <= alpha || bestValue >= beta)
429 && Time.elapsed() > 3000)
430 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
432 // In case of failing low/high increase aspiration window and
433 // re-search, otherwise exit the loop.
434 if (bestValue <= alpha)
436 beta = (alpha + beta) / 2;
437 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
443 Threads.stopOnPonderhit = false;
446 else if (bestValue >= beta)
448 beta = std::min(bestValue + delta, VALUE_INFINITE);
455 delta += delta / 4 + 5;
457 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
460 // Sort the PV lines searched so far and update the GUI
461 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
464 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
465 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
469 completedDepth = rootDepth;
471 if (rootMoves[0].pv[0] != lastBestMove) {
472 lastBestMove = rootMoves[0].pv[0];
473 lastBestMoveDepth = rootDepth;
476 // Have we found a "mate in x"?
478 && bestValue >= VALUE_MATE_IN_MAX_PLY
479 && VALUE_MATE - bestValue <= 2 * Limits.mate)
485 // If skill level is enabled and time is up, pick a sub-optimal best move
486 if (skill.enabled() && skill.time_to_pick(rootDepth))
487 skill.pick_best(multiPV);
489 // Do we have time for the next iteration? Can we stop searching now?
490 if ( Limits.use_time_management()
492 && !Threads.stopOnPonderhit)
494 const int F[] = { failedLow,
495 bestValue - mainThread->previousScore };
497 int improvingFactor = std::max(246, std::min(832, 306 + 119 * F[0] - 6 * F[1]));
499 // If the bestMove is stable over several iterations, reduce time accordingly
501 for (int i : {3, 4, 5})
502 if (lastBestMoveDepth * i < completedDepth)
503 timeReduction *= 1.25;
505 // Use part of the gained time from a previous stable move for the current move
506 double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
507 bestMoveInstability *= std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
509 // Stop the search if we have only one legal move, or if available time elapsed
510 if ( rootMoves.size() == 1
511 || Time.elapsed() > Time.optimum() * bestMoveInstability * improvingFactor / 581)
513 // If we are allowed to ponder do not stop the search now but
514 // keep pondering until the GUI sends "ponderhit" or "stop".
516 Threads.stopOnPonderhit = true;
526 mainThread->previousTimeReduction = timeReduction;
528 // If skill level is enabled, swap best PV line with the sub-optimal one
530 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
531 skill.best ? skill.best : skill.pick_best(multiPV)));
537 // search<>() is the main search function for both PV and non-PV nodes
539 template <NodeType NT>
540 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
542 constexpr bool PvNode = NT == PV;
543 const bool rootNode = PvNode && ss->ply == 0;
545 // Check if we have an upcoming move which draws by repetition, or
546 // if the opponent had an alternative move earlier to this position.
547 if ( pos.rule50_count() >= 3
548 && alpha < VALUE_DRAW
550 && pos.has_game_cycle(ss->ply))
552 alpha = value_draw(depth, pos.this_thread());
557 // Dive into quiescence search when the depth reaches zero
559 return qsearch<NT>(pos, ss, alpha, beta);
561 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
562 assert(PvNode || (alpha == beta - 1));
563 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
564 assert(!(PvNode && cutNode));
565 assert(depth / ONE_PLY * ONE_PLY == depth);
567 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
571 Move ttMove, move, excludedMove, bestMove;
572 Depth extension, newDepth;
573 Value bestValue, value, ttValue, eval, maxValue, pureStaticEval;
574 bool ttHit, inCheck, givesCheck, improving;
575 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
577 int moveCount, captureCount, quietCount;
579 // Step 1. Initialize node
580 Thread* thisThread = pos.this_thread();
581 inCheck = pos.checkers();
582 Color us = pos.side_to_move();
583 moveCount = captureCount = quietCount = ss->moveCount = 0;
584 bestValue = -VALUE_INFINITE;
585 maxValue = VALUE_INFINITE;
587 // Check for the available remaining time
588 if (thisThread == Threads.main())
589 static_cast<MainThread*>(thisThread)->check_time();
591 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
592 if (PvNode && thisThread->selDepth < ss->ply + 1)
593 thisThread->selDepth = ss->ply + 1;
597 // Step 2. Check for aborted search and immediate draw
598 if ( Threads.stop.load(std::memory_order_relaxed)
599 || pos.is_draw(ss->ply)
600 || ss->ply >= MAX_PLY)
601 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
602 : value_draw(depth, pos.this_thread());
604 // Step 3. Mate distance pruning. Even if we mate at the next move our score
605 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
606 // a shorter mate was found upward in the tree then there is no need to search
607 // because we will never beat the current alpha. Same logic but with reversed
608 // signs applies also in the opposite condition of being mated instead of giving
609 // mate. In this case return a fail-high score.
610 alpha = std::max(mated_in(ss->ply), alpha);
611 beta = std::min(mate_in(ss->ply+1), beta);
616 assert(0 <= ss->ply && ss->ply < MAX_PLY);
618 (ss+1)->ply = ss->ply + 1;
619 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
620 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
621 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
622 Square prevSq = to_sq((ss-1)->currentMove);
624 // Initialize statScore to zero for the grandchildren of the current position.
625 // So statScore is shared between all grandchildren and only the first grandchild
626 // starts with statScore = 0. Later grandchildren start with the last calculated
627 // statScore of the previous grandchild. This influences the reduction rules in
628 // LMR which are based on the statScore of parent position.
629 (ss+2)->statScore = 0;
631 // Step 4. Transposition table lookup. We don't want the score of a partial
632 // search to overwrite a previous full search TT value, so we use a different
633 // position key in case of an excluded move.
634 excludedMove = ss->excludedMove;
635 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
636 tte = TT.probe(posKey, ttHit);
637 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
638 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
639 : ttHit ? tte->move() : MOVE_NONE;
641 // At non-PV nodes we check for an early TT cutoff
644 && tte->depth() >= depth
645 && ttValue != VALUE_NONE // Possible in case of TT access race
646 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
647 : (tte->bound() & BOUND_UPPER)))
649 // If ttMove is quiet, update move sorting heuristics on TT hit
654 if (!pos.capture_or_promotion(ttMove))
655 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
657 // Extra penalty for a quiet TT move in previous ply when it gets refuted
658 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
659 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
661 // Penalty for a quiet ttMove that fails low
662 else if (!pos.capture_or_promotion(ttMove))
664 int penalty = -stat_bonus(depth);
665 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
666 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
672 // Step 5. Tablebases probe
673 if (!rootNode && TB::Cardinality)
675 int piecesCount = pos.count<ALL_PIECES>();
677 if ( piecesCount <= TB::Cardinality
678 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
679 && pos.rule50_count() == 0
680 && !pos.can_castle(ANY_CASTLING))
683 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
685 if (err != TB::ProbeState::FAIL)
687 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
689 int drawScore = TB::UseRule50 ? 1 : 0;
691 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
692 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
693 : VALUE_DRAW + 2 * wdl * drawScore;
695 Bound b = wdl < -drawScore ? BOUND_UPPER
696 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
698 if ( b == BOUND_EXACT
699 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
701 tte->save(posKey, value_to_tt(value, ss->ply), b,
702 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
703 MOVE_NONE, VALUE_NONE);
710 if (b == BOUND_LOWER)
711 bestValue = value, alpha = std::max(alpha, bestValue);
719 // Step 6. Static evaluation of the position
722 ss->staticEval = eval = pureStaticEval = VALUE_NONE;
724 goto moves_loop; // Skip early pruning when in check
728 // Never assume anything on values stored in TT
729 ss->staticEval = eval = pureStaticEval = tte->eval();
730 if (eval == VALUE_NONE)
731 ss->staticEval = eval = pureStaticEval = evaluate(pos);
733 // Can ttValue be used as a better position evaluation?
734 if ( ttValue != VALUE_NONE
735 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
740 if ((ss-1)->currentMove != MOVE_NULL)
742 int p = (ss-1)->statScore;
743 int bonus = p > 0 ? (-p - 2500) / 512 :
744 p < 0 ? (-p + 2500) / 512 : 0;
746 pureStaticEval = evaluate(pos);
747 ss->staticEval = eval = pureStaticEval + bonus;
750 ss->staticEval = eval = pureStaticEval = -(ss-1)->staticEval + 2 * Eval::Tempo;
752 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, pureStaticEval);
755 // Step 7. Razoring (~2 Elo)
756 if ( depth < 2 * ONE_PLY
757 && eval <= alpha - RazorMargin)
758 return qsearch<NT>(pos, ss, alpha, beta);
760 improving = ss->staticEval >= (ss-2)->staticEval
761 || (ss-2)->staticEval == VALUE_NONE;
763 // Step 8. Futility pruning: child node (~30 Elo)
765 && depth < 7 * ONE_PLY
766 && eval - futility_margin(depth, improving) >= beta
767 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
770 // Step 9. Null move search with verification search (~40 Elo)
772 && (ss-1)->currentMove != MOVE_NULL
773 && (ss-1)->statScore < 23200
775 && pureStaticEval >= beta - 36 * depth / ONE_PLY + 225
777 && pos.non_pawn_material(us)
778 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
780 assert(eval - beta >= 0);
782 // Null move dynamic reduction based on depth and value
783 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 200, 3)) * ONE_PLY;
785 ss->currentMove = MOVE_NULL;
786 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
788 pos.do_null_move(st);
790 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
792 pos.undo_null_move();
794 if (nullValue >= beta)
796 // Do not return unproven mate scores
797 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
800 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 12 * ONE_PLY))
803 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
805 // Do verification search at high depths, with null move pruning disabled
806 // for us, until ply exceeds nmpMinPly.
807 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
808 thisThread->nmpColor = us;
810 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
812 thisThread->nmpMinPly = 0;
819 // Step 10. ProbCut (~10 Elo)
820 // If we have a good enough capture and a reduced search returns a value
821 // much above beta, we can (almost) safely prune the previous move.
823 && depth >= 5 * ONE_PLY
824 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
826 Value rbeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
827 MovePicker mp(pos, ttMove, rbeta - ss->staticEval, &thisThread->captureHistory);
828 int probCutCount = 0;
830 while ( (move = mp.next_move()) != MOVE_NONE
832 if (move != excludedMove && pos.legal(move))
836 ss->currentMove = move;
837 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
839 assert(depth >= 5 * ONE_PLY);
841 pos.do_move(move, st);
843 // Perform a preliminary qsearch to verify that the move holds
844 value = -qsearch<NonPV>(pos, ss+1, -rbeta, -rbeta+1);
846 // If the qsearch held perform the regular search
848 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode);
857 // Step 11. Internal iterative deepening (~2 Elo)
858 if ( depth >= 8 * ONE_PLY
861 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
863 tte = TT.probe(posKey, ttHit);
864 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
865 ttMove = ttHit ? tte->move() : MOVE_NONE;
868 moves_loop: // When in check, search starts from here
870 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, nullptr, (ss-4)->continuationHistory };
871 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
873 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
874 &thisThread->captureHistory,
878 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
881 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
882 pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
884 // Step 12. Loop through all pseudo-legal moves until no moves remain
885 // or a beta cutoff occurs.
886 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
890 if (move == excludedMove)
893 // At root obey the "searchmoves" option and skip moves not listed in Root
894 // Move List. As a consequence any illegal move is also skipped. In MultiPV
895 // mode we also skip PV moves which have been already searched and those
896 // of lower "TB rank" if we are in a TB root position.
897 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
898 thisThread->rootMoves.begin() + thisThread->pvLast, move))
901 ss->moveCount = ++moveCount;
903 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
904 sync_cout << "info depth " << depth / ONE_PLY
905 << " currmove " << UCI::move(move, pos.is_chess960())
906 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
908 (ss+1)->pv = nullptr;
910 extension = DEPTH_ZERO;
911 captureOrPromotion = pos.capture_or_promotion(move);
912 movedPiece = pos.moved_piece(move);
913 givesCheck = gives_check(pos, move);
915 moveCountPruning = depth < 16 * ONE_PLY
916 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
918 // Step 13. Extensions (~70 Elo)
920 // Singular extension search (~60 Elo). If all moves but one fail low on a
921 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
922 // then that move is singular and should be extended. To verify this we do
923 // a reduced search on all the other moves but the ttMove and if the
924 // result is lower than ttValue minus a margin then we will extend the ttMove.
925 if ( depth >= 8 * ONE_PLY
928 && !excludedMove // Recursive singular search is not allowed
929 && ttValue != VALUE_NONE
930 && (tte->bound() & BOUND_LOWER)
931 && tte->depth() >= depth - 3 * ONE_PLY
934 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
935 ss->excludedMove = move;
936 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
937 ss->excludedMove = MOVE_NONE;
942 else if ( givesCheck // Check extension (~2 Elo)
946 // Extension for king moves that change castling rights
947 if ( type_of(movedPiece) == KING
948 && pos.can_castle(us)
949 && depth < 12 * ONE_PLY)
952 // Calculate new depth for this move
953 newDepth = depth - ONE_PLY + extension;
955 // Step 14. Pruning at shallow depth (~170 Elo)
957 && pos.non_pawn_material(us)
958 && bestValue > VALUE_MATED_IN_MAX_PLY)
960 if ( !captureOrPromotion
962 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
964 // Move count based pruning (~30 Elo)
965 if (moveCountPruning)
971 // Reduced depth of the next LMR search
972 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
974 // Countermoves based pruning (~20 Elo)
975 if ( lmrDepth < 3 + ((ss-1)->statScore > 0)
976 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
977 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
980 // Futility pruning: parent node (~2 Elo)
983 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
986 // Prune moves with negative SEE (~10 Elo)
987 if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
990 else if ( !extension // (~20 Elo)
991 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
995 // Speculative prefetch as early as possible
996 prefetch(TT.first_entry(pos.key_after(move)));
998 // Check for legality just before making the move
999 if (!rootNode && !pos.legal(move))
1001 ss->moveCount = --moveCount;
1005 // Update the current move (this must be done after singular extension search)
1006 ss->currentMove = move;
1007 ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
1009 // Step 15. Make the move
1010 pos.do_move(move, st, givesCheck);
1012 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1013 // re-searched at full depth.
1014 if ( depth >= 3 * ONE_PLY
1016 && (!captureOrPromotion || moveCountPruning))
1018 Depth r = reduction<PvNode>(improving, depth, moveCount);
1020 // Decrease reduction if opponent's move count is high (~10 Elo)
1021 if ((ss-1)->moveCount > 15)
1024 if (!captureOrPromotion)
1026 // Decrease reduction for exact PV nodes (~0 Elo)
1030 // Increase reduction if ttMove is a capture (~0 Elo)
1034 // Increase reduction for cut nodes (~5 Elo)
1038 // Decrease reduction for moves that escape a capture. Filter out
1039 // castling moves, because they are coded as "king captures rook" and
1040 // hence break make_move(). (~5 Elo)
1041 else if ( type_of(move) == NORMAL
1042 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1045 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1046 + (*contHist[0])[movedPiece][to_sq(move)]
1047 + (*contHist[1])[movedPiece][to_sq(move)]
1048 + (*contHist[3])[movedPiece][to_sq(move)]
1051 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1052 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1055 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1058 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1059 r -= ss->statScore / 20000 * ONE_PLY;
1062 Depth d = std::max(newDepth - std::max(r, DEPTH_ZERO), ONE_PLY);
1064 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1066 doFullDepthSearch = (value > alpha && d != newDepth);
1069 doFullDepthSearch = !PvNode || moveCount > 1;
1071 // Step 17. Full depth search when LMR is skipped or fails high
1072 if (doFullDepthSearch)
1073 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1075 // For PV nodes only, do a full PV search on the first move or after a fail
1076 // high (in the latter case search only if value < beta), otherwise let the
1077 // parent node fail low with value <= alpha and try another move.
1078 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1081 (ss+1)->pv[0] = MOVE_NONE;
1083 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1086 // Step 18. Undo move
1087 pos.undo_move(move);
1089 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1091 // Step 19. Check for a new best move
1092 // Finished searching the move. If a stop occurred, the return value of
1093 // the search cannot be trusted, and we return immediately without
1094 // updating best move, PV and TT.
1095 if (Threads.stop.load(std::memory_order_relaxed))
1100 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1101 thisThread->rootMoves.end(), move);
1103 // PV move or new best move?
1104 if (moveCount == 1 || value > alpha)
1107 rm.selDepth = thisThread->selDepth;
1112 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1113 rm.pv.push_back(*m);
1115 // We record how often the best move has been changed in each
1116 // iteration. This information is used for time management: When
1117 // the best move changes frequently, we allocate some more time.
1118 if (moveCount > 1 && thisThread == Threads.main())
1119 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1122 // All other moves but the PV are set to the lowest value: this
1123 // is not a problem when sorting because the sort is stable and the
1124 // move position in the list is preserved - just the PV is pushed up.
1125 rm.score = -VALUE_INFINITE;
1128 if (value > bestValue)
1136 if (PvNode && !rootNode) // Update pv even in fail-high case
1137 update_pv(ss->pv, move, (ss+1)->pv);
1139 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1143 assert(value >= beta); // Fail high
1150 if (move != bestMove)
1152 if (captureOrPromotion && captureCount < 32)
1153 capturesSearched[captureCount++] = move;
1155 else if (!captureOrPromotion && quietCount < 64)
1156 quietsSearched[quietCount++] = move;
1160 // The following condition would detect a stop only after move loop has been
1161 // completed. But in this case bestValue is valid because we have fully
1162 // searched our subtree, and we can anyhow save the result in TT.
1168 // Step 20. Check for mate and stalemate
1169 // All legal moves have been searched and if there are no legal moves, it
1170 // must be a mate or a stalemate. If we are in a singular extension search then
1171 // return a fail low score.
1173 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1176 bestValue = excludedMove ? alpha
1177 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1180 // Quiet best move: update move sorting heuristics
1181 if (!pos.capture_or_promotion(bestMove))
1182 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1183 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1185 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1187 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1188 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1189 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1191 // Bonus for prior countermove that caused the fail low
1192 else if ( (depth >= 3 * ONE_PLY || PvNode)
1193 && !pos.captured_piece()
1194 && is_ok((ss-1)->currentMove))
1195 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1198 bestValue = std::min(bestValue, maxValue);
1201 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1202 bestValue >= beta ? BOUND_LOWER :
1203 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1204 depth, bestMove, pureStaticEval);
1206 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1212 // qsearch() is the quiescence search function, which is called by the main
1213 // search function with depth zero, or recursively with depth less than ONE_PLY.
1214 template <NodeType NT>
1215 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1217 constexpr bool PvNode = NT == PV;
1219 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1220 assert(PvNode || (alpha == beta - 1));
1221 assert(depth <= DEPTH_ZERO);
1222 assert(depth / ONE_PLY * ONE_PLY == depth);
1228 Move ttMove, move, bestMove;
1230 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1231 bool ttHit, inCheck, givesCheck, evasionPrunable;
1236 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1238 ss->pv[0] = MOVE_NONE;
1241 Thread* thisThread = pos.this_thread();
1242 (ss+1)->ply = ss->ply + 1;
1243 ss->currentMove = bestMove = MOVE_NONE;
1244 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
1245 inCheck = pos.checkers();
1248 // Check for an immediate draw or maximum ply reached
1249 if ( pos.is_draw(ss->ply)
1250 || ss->ply >= MAX_PLY)
1251 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1253 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1255 // Decide whether or not to include checks: this fixes also the type of
1256 // TT entry depth that we are going to use. Note that in qsearch we use
1257 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1258 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1259 : DEPTH_QS_NO_CHECKS;
1260 // Transposition table lookup
1262 tte = TT.probe(posKey, ttHit);
1263 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1264 ttMove = ttHit ? tte->move() : MOVE_NONE;
1268 && tte->depth() >= ttDepth
1269 && ttValue != VALUE_NONE // Only in case of TT access race
1270 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1271 : (tte->bound() & BOUND_UPPER)))
1274 // Evaluate the position statically
1277 ss->staticEval = VALUE_NONE;
1278 bestValue = futilityBase = -VALUE_INFINITE;
1284 // Never assume anything on values stored in TT
1285 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1286 ss->staticEval = bestValue = evaluate(pos);
1288 // Can ttValue be used as a better position evaluation?
1289 if ( ttValue != VALUE_NONE
1290 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1291 bestValue = ttValue;
1294 ss->staticEval = bestValue =
1295 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1296 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1298 // Stand pat. Return immediately if static value is at least beta
1299 if (bestValue >= beta)
1302 tte->save(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1303 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1308 if (PvNode && bestValue > alpha)
1311 futilityBase = bestValue + 128;
1314 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, nullptr, (ss-4)->continuationHistory };
1316 // Initialize a MovePicker object for the current position, and prepare
1317 // to search the moves. Because the depth is <= 0 here, only captures,
1318 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1320 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1321 &thisThread->captureHistory,
1323 to_sq((ss-1)->currentMove));
1325 // Loop through the moves until no moves remain or a beta cutoff occurs
1326 while ((move = mp.next_move()) != MOVE_NONE)
1328 assert(is_ok(move));
1330 givesCheck = gives_check(pos, move);
1337 && futilityBase > -VALUE_KNOWN_WIN
1338 && !pos.advanced_pawn_push(move))
1340 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1342 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1344 if (futilityValue <= alpha)
1346 bestValue = std::max(bestValue, futilityValue);
1350 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1352 bestValue = std::max(bestValue, futilityBase);
1357 // Detect non-capture evasions that are candidates to be pruned
1358 evasionPrunable = inCheck
1359 && (depth != DEPTH_ZERO || moveCount > 2)
1360 && bestValue > VALUE_MATED_IN_MAX_PLY
1361 && !pos.capture(move);
1363 // Don't search moves with negative SEE values
1364 if ( (!inCheck || evasionPrunable)
1365 && !pos.see_ge(move))
1368 // Speculative prefetch as early as possible
1369 prefetch(TT.first_entry(pos.key_after(move)));
1371 // Check for legality just before making the move
1372 if (!pos.legal(move))
1378 ss->currentMove = move;
1379 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
1381 // Make and search the move
1382 pos.do_move(move, st, givesCheck);
1383 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1384 pos.undo_move(move);
1386 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1388 // Check for a new best move
1389 if (value > bestValue)
1395 if (PvNode) // Update pv even in fail-high case
1396 update_pv(ss->pv, move, (ss+1)->pv);
1398 if (PvNode && value < beta) // Update alpha here!
1405 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1406 ttDepth, move, ss->staticEval);
1414 // All legal moves have been searched. A special case: If we're in check
1415 // and no legal moves were found, it is checkmate.
1416 if (inCheck && bestValue == -VALUE_INFINITE)
1417 return mated_in(ss->ply); // Plies to mate from the root
1419 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1420 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1421 ttDepth, bestMove, ss->staticEval);
1423 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1429 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1430 // "plies to mate from the current position". Non-mate scores are unchanged.
1431 // The function is called before storing a value in the transposition table.
1433 Value value_to_tt(Value v, int ply) {
1435 assert(v != VALUE_NONE);
1437 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1438 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1442 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1443 // from the transposition table (which refers to the plies to mate/be mated
1444 // from current position) to "plies to mate/be mated from the root".
1446 Value value_from_tt(Value v, int ply) {
1448 return v == VALUE_NONE ? VALUE_NONE
1449 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1450 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1454 // update_pv() adds current move and appends child pv[]
1456 void update_pv(Move* pv, Move move, Move* childPv) {
1458 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1464 // update_continuation_histories() updates histories of the move pairs formed
1465 // by moves at ply -1, -2, and -4 with current move.
1467 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1469 for (int i : {1, 2, 4})
1470 if (is_ok((ss-i)->currentMove))
1471 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1475 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1477 void update_capture_stats(const Position& pos, Move move,
1478 Move* captures, int captureCnt, int bonus) {
1480 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1481 Piece moved_piece = pos.moved_piece(move);
1482 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1484 if (pos.capture_or_promotion(move))
1485 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1487 // Decrease all the other played capture moves
1488 for (int i = 0; i < captureCnt; ++i)
1490 moved_piece = pos.moved_piece(captures[i]);
1491 captured = type_of(pos.piece_on(to_sq(captures[i])));
1492 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1497 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1499 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1500 Move* quiets, int quietsCnt, int bonus) {
1502 if (ss->killers[0] != move)
1504 ss->killers[1] = ss->killers[0];
1505 ss->killers[0] = move;
1508 Color us = pos.side_to_move();
1509 Thread* thisThread = pos.this_thread();
1510 thisThread->mainHistory[us][from_to(move)] << bonus;
1511 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1513 if (is_ok((ss-1)->currentMove))
1515 Square prevSq = to_sq((ss-1)->currentMove);
1516 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1519 // Decrease all the other played quiet moves
1520 for (int i = 0; i < quietsCnt; ++i)
1522 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1523 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1527 // When playing with strength handicap, choose best move among a set of RootMoves
1528 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1530 Move Skill::pick_best(size_t multiPV) {
1532 const RootMoves& rootMoves = Threads.main()->rootMoves;
1533 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1535 // RootMoves are already sorted by score in descending order
1536 Value topScore = rootMoves[0].score;
1537 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1538 int weakness = 120 - 2 * level;
1539 int maxScore = -VALUE_INFINITE;
1541 // Choose best move. For each move score we add two terms, both dependent on
1542 // weakness. One is deterministic and bigger for weaker levels, and one is
1543 // random. Then we choose the move with the resulting highest score.
1544 for (size_t i = 0; i < multiPV; ++i)
1546 // This is our magic formula
1547 int push = ( weakness * int(topScore - rootMoves[i].score)
1548 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1550 if (rootMoves[i].score + push >= maxScore)
1552 maxScore = rootMoves[i].score + push;
1553 best = rootMoves[i].pv[0];
1562 /// MainThread::check_time() is used to print debug info and, more importantly,
1563 /// to detect when we are out of available time and thus stop the search.
1565 void MainThread::check_time() {
1570 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1571 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1573 static TimePoint lastInfoTime = now();
1575 TimePoint elapsed = Time.elapsed();
1576 TimePoint tick = Limits.startTime + elapsed;
1578 if (tick - lastInfoTime >= 1000)
1580 lastInfoTime = tick;
1584 // We should not stop pondering until told so by the GUI
1588 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1589 || (Limits.movetime && elapsed >= Limits.movetime)
1590 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1591 Threads.stop = true;
1595 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1596 /// that all (if any) unsearched PV lines are sent using a previous search score.
1598 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1600 std::stringstream ss;
1601 TimePoint elapsed = Time.elapsed() + 1;
1602 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1603 size_t pvIdx = pos.this_thread()->pvIdx;
1604 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1605 uint64_t nodesSearched = Threads.nodes_searched();
1606 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1608 for (size_t i = 0; i < multiPV; ++i)
1610 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1612 if (depth == ONE_PLY && !updated)
1615 Depth d = updated ? depth : depth - ONE_PLY;
1616 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1618 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1619 v = tb ? rootMoves[i].tbScore : v;
1621 if (ss.rdbuf()->in_avail()) // Not at first line
1625 << " depth " << d / ONE_PLY
1626 << " seldepth " << rootMoves[i].selDepth
1627 << " multipv " << i + 1
1628 << " score " << UCI::value(v);
1630 if (!tb && i == pvIdx)
1631 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1633 ss << " nodes " << nodesSearched
1634 << " nps " << nodesSearched * 1000 / elapsed;
1636 if (elapsed > 1000) // Earlier makes little sense
1637 ss << " hashfull " << TT.hashfull();
1639 ss << " tbhits " << tbHits
1640 << " time " << elapsed
1643 for (Move m : rootMoves[i].pv)
1644 ss << " " << UCI::move(m, pos.is_chess960());
1651 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1652 /// before exiting the search, for instance, in case we stop the search during a
1653 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1654 /// otherwise in case of 'ponder on' we have nothing to think on.
1656 bool RootMove::extract_ponder_from_tt(Position& pos) {
1661 assert(pv.size() == 1);
1666 pos.do_move(pv[0], st);
1667 TTEntry* tte = TT.probe(pos.key(), ttHit);
1671 Move m = tte->move(); // Local copy to be SMP safe
1672 if (MoveList<LEGAL>(pos).contains(m))
1676 pos.undo_move(pv[0]);
1677 return pv.size() > 1;
1680 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1683 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1684 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1685 Cardinality = int(Options["SyzygyProbeLimit"]);
1686 bool dtz_available = true;
1688 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1689 // ProbeDepth == DEPTH_ZERO
1690 if (Cardinality > MaxCardinality)
1692 Cardinality = MaxCardinality;
1693 ProbeDepth = DEPTH_ZERO;
1696 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1698 // Rank moves using DTZ tables
1699 RootInTB = root_probe(pos, rootMoves);
1703 // DTZ tables are missing; try to rank moves using WDL tables
1704 dtz_available = false;
1705 RootInTB = root_probe_wdl(pos, rootMoves);
1711 // Sort moves according to TB rank
1712 std::sort(rootMoves.begin(), rootMoves.end(),
1713 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1715 // Probe during search only if DTZ is not available and we are winning
1716 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1721 // Assign the same rank to all moves
1722 for (auto& m : rootMoves)