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[MAX_MOVES]; // [depth or moveNumber]
73 Depth reduction(bool i, Depth d, int mn) {
74 int r = Reductions[d / ONE_PLY] * Reductions[mn];
75 return ((r + 512) / 1024 + (!i && r > 1024)) * 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 avoid 3fold-blindness
89 Value value_draw(Depth depth, Thread* thisThread) {
90 return depth < 4 * ONE_PLY ? VALUE_DRAW
91 : VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
94 // Skill structure is used to implement strength limit
96 explicit Skill(int l) : level(l) {}
97 bool enabled() const { return level < 20; }
98 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
99 Move pick_best(size_t multiPV);
102 Move best = MOVE_NONE;
105 template <NodeType NT>
106 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
108 template <NodeType NT>
109 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
111 Value value_to_tt(Value v, int ply);
112 Value value_from_tt(Value v, int ply);
113 void update_pv(Move* pv, Move move, Move* childPv);
114 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
115 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietCount, int bonus);
116 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCount, int bonus);
118 // perft() is our utility to verify move generation. All the leaf nodes up
119 // to the given depth are generated and counted, and the sum is returned.
121 uint64_t perft(Position& pos, Depth depth) {
124 uint64_t cnt, nodes = 0;
125 const bool leaf = (depth == 2 * ONE_PLY);
127 for (const auto& m : MoveList<LEGAL>(pos))
129 if (Root && depth <= ONE_PLY)
134 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
139 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
147 /// Search::init() is called at startup to initialize various lookup tables
149 void Search::init() {
151 for (int i = 1; i < MAX_MOVES; ++i)
152 Reductions[i] = int(22.9 * std::log(i));
156 /// Search::clear() resets search state to its initial value
158 void Search::clear() {
160 Threads.main()->wait_for_search_finished();
162 Time.availableNodes = 0;
165 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
169 /// MainThread::search() is started when the program receives the UCI 'go'
170 /// command. It searches from the root position and outputs the "bestmove".
172 void MainThread::search() {
176 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
177 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
181 Color us = rootPos.side_to_move();
182 Time.init(Limits, us, rootPos.game_ply());
185 if (rootMoves.empty())
187 rootMoves.emplace_back(MOVE_NONE);
188 sync_cout << "info depth 0 score "
189 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
194 for (Thread* th : Threads)
196 th->bestMoveChanges = 0;
198 th->start_searching();
201 Thread::search(); // Let's start searching!
204 // When we reach the maximum depth, we can arrive here without a raise of
205 // Threads.stop. However, if we are pondering or in an infinite search,
206 // the UCI protocol states that we shouldn't print the best move before the
207 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
208 // until the GUI sends one of those commands.
210 while (!Threads.stop && (ponder || Limits.infinite))
211 {} // Busy wait for a stop or a ponder reset
213 // Stop the threads if not already stopped (also raise the stop if
214 // "ponderhit" just reset Threads.ponder).
217 // Wait until all threads have finished
218 for (Thread* th : Threads)
220 th->wait_for_search_finished();
222 // When playing in 'nodes as time' mode, subtract the searched nodes from
223 // the available ones before exiting.
225 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
227 Thread* bestThread = this;
229 // Check if there are threads with a better score than main thread
230 if ( Options["MultiPV"] == 1
232 && !Skill(Options["Skill Level"]).enabled()
233 && rootMoves[0].pv[0] != MOVE_NONE)
235 std::map<Move, int64_t> votes;
236 Value minScore = this->rootMoves[0].score;
238 // Find out minimum score and reset votes for moves which can be voted
239 for (Thread* th: Threads)
240 minScore = std::min(minScore, th->rootMoves[0].score);
242 // Vote according to score and depth, and select the best thread
243 for (Thread* th : Threads)
245 votes[th->rootMoves[0].pv[0]] +=
246 (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
248 if (votes[th->rootMoves[0].pv[0]] > votes[bestThread->rootMoves[0].pv[0]])
253 previousScore = bestThread->rootMoves[0].score;
255 // Send again PV info if we have a new best thread
256 if (bestThread != this)
257 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
259 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
261 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
262 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
264 std::cout << sync_endl;
268 /// Thread::search() is the main iterative deepening loop. It calls search()
269 /// repeatedly with increasing depth until the allocated thinking time has been
270 /// consumed, the user stops the search, or the maximum search depth is reached.
272 void Thread::search() {
274 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
275 // The former is needed to allow update_continuation_histories(ss-1, ...),
276 // which accesses its argument at ss-6, also near the root.
277 // The latter is needed for statScores and killer initialization.
278 Stack stack[MAX_PLY+10], *ss = stack+7;
280 Value bestValue, alpha, beta, delta;
281 Move lastBestMove = MOVE_NONE;
282 Depth lastBestMoveDepth = DEPTH_ZERO;
283 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
284 double timeReduction = 1, totBestMoveChanges = 0;
285 Color us = rootPos.side_to_move();
287 std::memset(ss-7, 0, 10 * sizeof(Stack));
288 for (int i = 7; i > 0; i--)
289 (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
292 bestValue = delta = alpha = -VALUE_INFINITE;
293 beta = VALUE_INFINITE;
295 multiPV = Options["MultiPV"];
296 Skill skill(Options["Skill Level"]);
298 // When playing with strength handicap enable MultiPV search that we will
299 // use behind the scenes to retrieve a set of possible moves.
301 multiPV = std::max(multiPV, (size_t)4);
303 multiPV = std::min(multiPV, rootMoves.size());
305 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
307 // In analysis mode, adjust contempt in accordance with user preference
308 if (Limits.infinite || Options["UCI_AnalyseMode"])
309 ct = Options["Analysis Contempt"] == "Off" ? 0
310 : Options["Analysis Contempt"] == "Both" ? ct
311 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
312 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
315 // Evaluation score is from the white point of view
316 contempt = (us == WHITE ? make_score(ct, ct / 2)
317 : -make_score(ct, ct / 2));
319 // Iterative deepening loop until requested to stop or the target depth is reached
320 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
322 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
324 // Age out PV variability metric
326 totBestMoveChanges /= 2;
328 // Save the last iteration's scores before first PV line is searched and
329 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
330 for (RootMove& rm : rootMoves)
331 rm.previousScore = rm.score;
336 // MultiPV loop. We perform a full root search for each PV line
337 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
342 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
343 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
347 // Reset UCI info selDepth for each depth and each PV line
350 // Reset aspiration window starting size
351 if (rootDepth >= 5 * ONE_PLY)
353 Value previousScore = rootMoves[pvIdx].previousScore;
355 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
356 beta = std::min(previousScore + delta, VALUE_INFINITE);
358 // Adjust contempt based on root move's previousScore (dynamic contempt)
359 int dct = ct + 88 * previousScore / (abs(previousScore) + 200);
361 contempt = (us == WHITE ? make_score(dct, dct / 2)
362 : -make_score(dct, dct / 2));
365 // Start with a small aspiration window and, in the case of a fail
366 // high/low, re-search with a bigger window until we don't fail
368 int failedHighCnt = 0;
371 Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
372 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
374 // Bring the best move to the front. It is critical that sorting
375 // is done with a stable algorithm because all the values but the
376 // first and eventually the new best one are set to -VALUE_INFINITE
377 // and we want to keep the same order for all the moves except the
378 // new PV that goes to the front. Note that in case of MultiPV
379 // search the already searched PV lines are preserved.
380 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
382 // If search has been stopped, we break immediately. Sorting is
383 // safe because RootMoves is still valid, although it refers to
384 // the previous iteration.
388 // When failing high/low give some update (without cluttering
389 // the UI) before a re-search.
392 && (bestValue <= alpha || bestValue >= beta)
393 && Time.elapsed() > 3000)
394 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
396 // In case of failing low/high increase aspiration window and
397 // re-search, otherwise exit the loop.
398 if (bestValue <= alpha)
400 beta = (alpha + beta) / 2;
401 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
405 mainThread->stopOnPonderhit = false;
407 else if (bestValue >= beta)
409 beta = std::min(bestValue + delta, VALUE_INFINITE);
415 delta += delta / 4 + 5;
417 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
420 // Sort the PV lines searched so far and update the GUI
421 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
424 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
425 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
429 completedDepth = rootDepth;
431 if (rootMoves[0].pv[0] != lastBestMove) {
432 lastBestMove = rootMoves[0].pv[0];
433 lastBestMoveDepth = rootDepth;
436 // Have we found a "mate in x"?
438 && bestValue >= VALUE_MATE_IN_MAX_PLY
439 && VALUE_MATE - bestValue <= 2 * Limits.mate)
445 // If skill level is enabled and time is up, pick a sub-optimal best move
446 if (skill.enabled() && skill.time_to_pick(rootDepth))
447 skill.pick_best(multiPV);
449 // Do we have time for the next iteration? Can we stop searching now?
450 if ( Limits.use_time_management()
452 && !mainThread->stopOnPonderhit)
454 double fallingEval = (314 + 9 * (mainThread->previousScore - bestValue)) / 581.0;
455 fallingEval = clamp(fallingEval, 0.5, 1.5);
457 // If the bestMove is stable over several iterations, reduce time accordingly
458 timeReduction = lastBestMoveDepth + 10 * ONE_PLY < completedDepth ? 1.95 : 1.0;
459 double reduction = std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
461 // Use part of the gained time from a previous stable move for the current move
462 for (Thread* th : Threads)
464 totBestMoveChanges += th->bestMoveChanges;
465 th->bestMoveChanges = 0;
467 double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
469 // Stop the search if we have only one legal move, or if available time elapsed
470 if ( rootMoves.size() == 1
471 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
473 // If we are allowed to ponder do not stop the search now but
474 // keep pondering until the GUI sends "ponderhit" or "stop".
475 if (mainThread->ponder)
476 mainThread->stopOnPonderhit = true;
486 mainThread->previousTimeReduction = timeReduction;
488 // If skill level is enabled, swap best PV line with the sub-optimal one
490 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
491 skill.best ? skill.best : skill.pick_best(multiPV)));
497 // search<>() is the main search function for both PV and non-PV nodes
499 template <NodeType NT>
500 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
502 constexpr bool PvNode = NT == PV;
503 const bool rootNode = PvNode && ss->ply == 0;
505 // Check if we have an upcoming move which draws by repetition, or
506 // if the opponent had an alternative move earlier to this position.
507 if ( pos.rule50_count() >= 3
508 && alpha < VALUE_DRAW
510 && pos.has_game_cycle(ss->ply))
512 alpha = value_draw(depth, pos.this_thread());
517 // Dive into quiescence search when the depth reaches zero
519 return qsearch<NT>(pos, ss, alpha, beta);
521 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
522 assert(PvNode || (alpha == beta - 1));
523 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
524 assert(!(PvNode && cutNode));
525 assert(depth / ONE_PLY * ONE_PLY == depth);
527 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
531 Move ttMove, move, excludedMove, bestMove;
532 Depth extension, newDepth;
533 Value bestValue, value, ttValue, eval, maxValue;
534 bool ttHit, ttPv, inCheck, givesCheck, improving;
535 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
537 int moveCount, captureCount, quietCount, singularLMR;
539 // Step 1. Initialize node
540 Thread* thisThread = pos.this_thread();
541 inCheck = pos.checkers();
542 Color us = pos.side_to_move();
543 moveCount = captureCount = quietCount = singularLMR = ss->moveCount = 0;
544 bestValue = -VALUE_INFINITE;
545 maxValue = VALUE_INFINITE;
547 // Check for the available remaining time
548 if (thisThread == Threads.main())
549 static_cast<MainThread*>(thisThread)->check_time();
551 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
552 if (PvNode && thisThread->selDepth < ss->ply + 1)
553 thisThread->selDepth = ss->ply + 1;
557 // Step 2. Check for aborted search and immediate draw
558 if ( Threads.stop.load(std::memory_order_relaxed)
559 || pos.is_draw(ss->ply)
560 || ss->ply >= MAX_PLY)
561 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
562 : value_draw(depth, pos.this_thread());
564 // Step 3. Mate distance pruning. Even if we mate at the next move our score
565 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
566 // a shorter mate was found upward in the tree then there is no need to search
567 // because we will never beat the current alpha. Same logic but with reversed
568 // signs applies also in the opposite condition of being mated instead of giving
569 // mate. In this case return a fail-high score.
570 alpha = std::max(mated_in(ss->ply), alpha);
571 beta = std::min(mate_in(ss->ply+1), beta);
576 assert(0 <= ss->ply && ss->ply < MAX_PLY);
578 (ss+1)->ply = ss->ply + 1;
579 (ss+1)->excludedMove = bestMove = MOVE_NONE;
580 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
581 Square prevSq = to_sq((ss-1)->currentMove);
583 // Initialize statScore to zero for the grandchildren of the current position.
584 // So statScore is shared between all grandchildren and only the first grandchild
585 // starts with statScore = 0. Later grandchildren start with the last calculated
586 // statScore of the previous grandchild. This influences the reduction rules in
587 // LMR which are based on the statScore of parent position.
589 (ss + 4)->statScore = 0;
591 (ss + 2)->statScore = 0;
593 // Step 4. Transposition table lookup. We don't want the score of a partial
594 // search to overwrite a previous full search TT value, so we use a different
595 // position key in case of an excluded move.
596 excludedMove = ss->excludedMove;
597 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
598 tte = TT.probe(posKey, ttHit);
599 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
600 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
601 : ttHit ? tte->move() : MOVE_NONE;
602 ttPv = PvNode || (ttHit && tte->is_pv());
604 // At non-PV nodes we check for an early TT cutoff
607 && tte->depth() >= depth
608 && ttValue != VALUE_NONE // Possible in case of TT access race
609 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
610 : (tte->bound() & BOUND_UPPER)))
612 // If ttMove is quiet, update move sorting heuristics on TT hit
617 if (!pos.capture_or_promotion(ttMove))
618 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
620 // Extra penalty for early quiet moves of the previous ply
621 if ((ss-1)->moveCount <= 2 && !pos.captured_piece())
622 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
624 // Penalty for a quiet ttMove that fails low
625 else if (!pos.capture_or_promotion(ttMove))
627 int penalty = -stat_bonus(depth);
628 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
629 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
635 // Step 5. Tablebases probe
636 if (!rootNode && TB::Cardinality)
638 int piecesCount = pos.count<ALL_PIECES>();
640 if ( piecesCount <= TB::Cardinality
641 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
642 && pos.rule50_count() == 0
643 && !pos.can_castle(ANY_CASTLING))
646 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
648 // Force check of time on the next occasion
649 if (thisThread == Threads.main())
650 static_cast<MainThread*>(thisThread)->callsCnt = 0;
652 if (err != TB::ProbeState::FAIL)
654 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
656 int drawScore = TB::UseRule50 ? 1 : 0;
658 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
659 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
660 : VALUE_DRAW + 2 * wdl * drawScore;
662 Bound b = wdl < -drawScore ? BOUND_UPPER
663 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
665 if ( b == BOUND_EXACT
666 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
668 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
669 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
670 MOVE_NONE, VALUE_NONE);
677 if (b == BOUND_LOWER)
678 bestValue = value, alpha = std::max(alpha, bestValue);
686 // Step 6. Static evaluation of the position
689 ss->staticEval = eval = VALUE_NONE;
691 goto moves_loop; // Skip early pruning when in check
695 // Never assume anything on values stored in TT
696 ss->staticEval = eval = tte->eval();
697 if (eval == VALUE_NONE)
698 ss->staticEval = eval = evaluate(pos);
700 // Can ttValue be used as a better position evaluation?
701 if ( ttValue != VALUE_NONE
702 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
707 if ((ss-1)->currentMove != MOVE_NULL)
709 int bonus = -(ss-1)->statScore / 512;
711 ss->staticEval = eval = evaluate(pos) + bonus;
714 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Eval::Tempo;
716 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
719 // Step 7. Razoring (~2 Elo)
720 if ( !rootNode // The required rootNode PV handling is not available in qsearch
721 && depth < 2 * ONE_PLY
722 && eval <= alpha - RazorMargin)
723 return qsearch<NT>(pos, ss, alpha, beta);
725 improving = ss->staticEval >= (ss-2)->staticEval
726 || (ss-2)->staticEval == VALUE_NONE;
728 // Step 8. Futility pruning: child node (~30 Elo)
730 && depth < 7 * ONE_PLY
731 && eval - futility_margin(depth, improving) >= beta
732 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
735 // Step 9. Null move search with verification search (~40 Elo)
737 && (ss-1)->currentMove != MOVE_NULL
738 && (ss-1)->statScore < 23200
740 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
742 && pos.non_pawn_material(us)
743 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
745 assert(eval - beta >= 0);
747 // Null move dynamic reduction based on depth and value
748 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 200, 3)) * ONE_PLY;
750 ss->currentMove = MOVE_NULL;
751 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
753 pos.do_null_move(st);
755 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
757 pos.undo_null_move();
759 if (nullValue >= beta)
761 // Do not return unproven mate scores
762 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
765 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 12 * ONE_PLY))
768 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
770 // Do verification search at high depths, with null move pruning disabled
771 // for us, until ply exceeds nmpMinPly.
772 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / (4 * ONE_PLY);
773 thisThread->nmpColor = us;
775 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
777 thisThread->nmpMinPly = 0;
784 // Step 10. ProbCut (~10 Elo)
785 // If we have a good enough capture and a reduced search returns a value
786 // much above beta, we can (almost) safely prune the previous move.
788 && depth >= 5 * ONE_PLY
789 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
791 Value raisedBeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
792 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
793 int probCutCount = 0;
795 while ( (move = mp.next_move()) != MOVE_NONE
796 && probCutCount < 2 + 2 * cutNode)
797 if (move != excludedMove && pos.legal(move))
801 ss->currentMove = move;
802 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
804 assert(depth >= 5 * ONE_PLY);
806 pos.do_move(move, st);
808 // Perform a preliminary qsearch to verify that the move holds
809 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
811 // If the qsearch held, perform the regular search
812 if (value >= raisedBeta)
813 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
817 if (value >= raisedBeta)
822 // Step 11. Internal iterative deepening (~2 Elo)
823 if (depth >= 8 * ONE_PLY && !ttMove)
825 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
827 tte = TT.probe(posKey, ttHit);
828 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
829 ttMove = ttHit ? tte->move() : MOVE_NONE;
832 moves_loop: // When in check, search starts from here
834 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
835 nullptr, (ss-4)->continuationHistory,
836 nullptr, (ss-6)->continuationHistory };
838 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
840 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
841 &thisThread->captureHistory,
846 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
847 moveCountPruning = false;
848 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
850 // Step 12. Loop through all pseudo-legal moves until no moves remain
851 // or a beta cutoff occurs.
852 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
856 if (move == excludedMove)
859 // At root obey the "searchmoves" option and skip moves not listed in Root
860 // Move List. As a consequence any illegal move is also skipped. In MultiPV
861 // mode we also skip PV moves which have been already searched and those
862 // of lower "TB rank" if we are in a TB root position.
863 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
864 thisThread->rootMoves.begin() + thisThread->pvLast, move))
867 ss->moveCount = ++moveCount;
869 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
870 sync_cout << "info depth " << depth / ONE_PLY
871 << " currmove " << UCI::move(move, pos.is_chess960())
872 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
874 // In MultiPV mode also skip moves which will be searched later as PV moves
875 if (rootNode && std::count(thisThread->rootMoves.begin() + thisThread->pvIdx + 1,
876 thisThread->rootMoves.begin() + thisThread->multiPV, move))
880 (ss+1)->pv = nullptr;
882 extension = DEPTH_ZERO;
883 captureOrPromotion = pos.capture_or_promotion(move);
884 movedPiece = pos.moved_piece(move);
885 givesCheck = pos.gives_check(move);
887 // Step 13. Extensions (~70 Elo)
889 // Singular extension search (~60 Elo). If all moves but one fail low on a
890 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
891 // then that move is singular and should be extended. To verify this we do
892 // a reduced search on all the other moves but the ttMove and if the
893 // result is lower than ttValue minus a margin then we will extend the ttMove.
894 if ( depth >= 8 * ONE_PLY
897 && !excludedMove // Avoid recursive singular search
898 /* && ttValue != VALUE_NONE Already implicit in the next condition */
899 && abs(ttValue) < VALUE_KNOWN_WIN
900 && (tte->bound() & BOUND_LOWER)
901 && tte->depth() >= depth - 3 * ONE_PLY
904 Value singularBeta = ttValue - 2 * depth / ONE_PLY;
905 Depth halfDepth = depth / (2 * ONE_PLY) * ONE_PLY; // ONE_PLY invariant
906 ss->excludedMove = move;
907 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, halfDepth, cutNode);
908 ss->excludedMove = MOVE_NONE;
910 if (value < singularBeta)
915 if (value < singularBeta - std::min(3 * depth / ONE_PLY, 39))
920 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
921 // search without the ttMove. So we assume this expected Cut-node is not singular,
922 // that multiple moves fail high, and we can prune the whole subtree by returning
924 else if ( eval >= beta
925 && singularBeta >= beta)
929 // Check extension (~2 Elo)
931 && (pos.blockers_for_king(~us) & from_sq(move) || pos.see_ge(move)))
934 // Castling extension
935 else if (type_of(move) == CASTLING)
940 && pos.rule50_count() > 18
941 && depth < 3 * ONE_PLY
942 && ++thisThread->shuffleExts < thisThread->nodes.load(std::memory_order_relaxed) / 4) // To avoid too many extensions
945 // Passed pawn extension
946 else if ( move == ss->killers[0]
947 && pos.advanced_pawn_push(move)
948 && pos.pawn_passed(us, to_sq(move)))
951 // Calculate new depth for this move
952 newDepth = depth - ONE_PLY + extension;
954 // Step 14. Pruning at shallow depth (~170 Elo)
956 && pos.non_pawn_material(us)
957 && bestValue > VALUE_MATED_IN_MAX_PLY)
959 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
960 moveCountPruning = moveCount >= futility_move_count(improving, depth / ONE_PLY);
962 if ( !captureOrPromotion
964 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material(~us) > BishopValueMg))
966 // Move count based pruning (~30 Elo)
967 if (moveCountPruning)
970 // Reduced depth of the next LMR search
971 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), DEPTH_ZERO);
974 // Countermoves based pruning (~20 Elo)
975 if ( lmrDepth < 3 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
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 ((!givesCheck || !extension)
991 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY))) // (~20 Elo)
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
1015 && moveCount > 1 + 3 * rootNode
1016 && ( !captureOrPromotion
1018 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha))
1020 Depth r = reduction(improving, depth, moveCount);
1022 // Decrease reduction if position is or has been on the PV
1026 // Decrease reduction if opponent's move count is high (~10 Elo)
1027 if ((ss-1)->moveCount > 15)
1030 // Decrease reduction if move has been singularly extended
1031 r -= singularLMR * ONE_PLY;
1033 if (!captureOrPromotion)
1035 // Increase reduction if ttMove is a capture (~0 Elo)
1039 // Increase reduction for cut nodes (~5 Elo)
1043 // Decrease reduction for moves that escape a capture. Filter out
1044 // castling moves, because they are coded as "king captures rook" and
1045 // hence break make_move(). (~5 Elo)
1046 else if ( type_of(move) == NORMAL
1047 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1050 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1051 + (*contHist[0])[movedPiece][to_sq(move)]
1052 + (*contHist[1])[movedPiece][to_sq(move)]
1053 + (*contHist[3])[movedPiece][to_sq(move)]
1056 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1057 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1060 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1063 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1064 r -= ss->statScore / 20000 * ONE_PLY;
1067 Depth d = clamp(newDepth - r, ONE_PLY, newDepth);
1069 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1071 doFullDepthSearch = (value > alpha && d != newDepth);
1074 doFullDepthSearch = !PvNode || moveCount > 1;
1076 // Step 17. Full depth search when LMR is skipped or fails high
1077 if (doFullDepthSearch)
1078 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1080 // For PV nodes only, do a full PV search on the first move or after a fail
1081 // high (in the latter case search only if value < beta), otherwise let the
1082 // parent node fail low with value <= alpha and try another move.
1083 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1086 (ss+1)->pv[0] = MOVE_NONE;
1088 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1091 // Step 18. Undo move
1092 pos.undo_move(move);
1094 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1096 // Step 19. Check for a new best move
1097 // Finished searching the move. If a stop occurred, the return value of
1098 // the search cannot be trusted, and we return immediately without
1099 // updating best move, PV and TT.
1100 if (Threads.stop.load(std::memory_order_relaxed))
1105 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1106 thisThread->rootMoves.end(), move);
1108 // PV move or new best move?
1109 if (moveCount == 1 || value > alpha)
1112 rm.selDepth = thisThread->selDepth;
1117 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1118 rm.pv.push_back(*m);
1120 // We record how often the best move has been changed in each
1121 // iteration. This information is used for time management: When
1122 // the best move changes frequently, we allocate some more time.
1124 ++thisThread->bestMoveChanges;
1127 // All other moves but the PV are set to the lowest value: this
1128 // is not a problem when sorting because the sort is stable and the
1129 // move position in the list is preserved - just the PV is pushed up.
1130 rm.score = -VALUE_INFINITE;
1133 if (value > bestValue)
1141 if (PvNode && !rootNode) // Update pv even in fail-high case
1142 update_pv(ss->pv, move, (ss+1)->pv);
1144 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1148 assert(value >= beta); // Fail high
1155 if (move != bestMove)
1157 if (captureOrPromotion && captureCount < 32)
1158 capturesSearched[captureCount++] = move;
1160 else if (!captureOrPromotion && quietCount < 64)
1161 quietsSearched[quietCount++] = move;
1165 // The following condition would detect a stop only after move loop has been
1166 // completed. But in this case bestValue is valid because we have fully
1167 // searched our subtree, and we can anyhow save the result in TT.
1173 // Step 20. Check for mate and stalemate
1174 // All legal moves have been searched and if there are no legal moves, it
1175 // must be a mate or a stalemate. If we are in a singular extension search then
1176 // return a fail low score.
1178 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1181 bestValue = excludedMove ? alpha
1182 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1185 // Quiet best move: update move sorting heuristics
1186 if (!pos.capture_or_promotion(bestMove))
1187 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1188 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1190 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1192 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1193 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1194 && !pos.captured_piece())
1195 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1198 // Bonus for prior countermove that caused the fail low
1199 else if ( (depth >= 3 * ONE_PLY || PvNode)
1200 && !pos.captured_piece())
1201 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1204 bestValue = std::min(bestValue, maxValue);
1207 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1208 bestValue >= beta ? BOUND_LOWER :
1209 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1210 depth, bestMove, ss->staticEval);
1212 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1218 // qsearch() is the quiescence search function, which is called by the main search
1219 // function with zero depth, or recursively with further decreasing depth per call.
1220 template <NodeType NT>
1221 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1223 constexpr bool PvNode = NT == PV;
1225 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1226 assert(PvNode || (alpha == beta - 1));
1227 assert(depth <= DEPTH_ZERO);
1228 assert(depth / ONE_PLY * ONE_PLY == depth);
1234 Move ttMove, move, bestMove;
1236 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1237 bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
1242 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1244 ss->pv[0] = MOVE_NONE;
1247 Thread* thisThread = pos.this_thread();
1248 (ss+1)->ply = ss->ply + 1;
1249 bestMove = MOVE_NONE;
1250 inCheck = pos.checkers();
1253 // Check for an immediate draw or maximum ply reached
1254 if ( pos.is_draw(ss->ply)
1255 || ss->ply >= MAX_PLY)
1256 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1258 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1260 // Decide whether or not to include checks: this fixes also the type of
1261 // TT entry depth that we are going to use. Note that in qsearch we use
1262 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1263 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1264 : DEPTH_QS_NO_CHECKS;
1265 // Transposition table lookup
1267 tte = TT.probe(posKey, ttHit);
1268 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1269 ttMove = ttHit ? tte->move() : MOVE_NONE;
1270 pvHit = ttHit && tte->is_pv();
1274 && tte->depth() >= ttDepth
1275 && ttValue != VALUE_NONE // Only in case of TT access race
1276 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1277 : (tte->bound() & BOUND_UPPER)))
1280 // Evaluate the position statically
1283 ss->staticEval = VALUE_NONE;
1284 bestValue = futilityBase = -VALUE_INFINITE;
1290 // Never assume anything on values stored in TT
1291 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1292 ss->staticEval = bestValue = evaluate(pos);
1294 // Can ttValue be used as a better position evaluation?
1295 if ( ttValue != VALUE_NONE
1296 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1297 bestValue = ttValue;
1300 ss->staticEval = bestValue =
1301 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1302 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1304 // Stand pat. Return immediately if static value is at least beta
1305 if (bestValue >= beta)
1308 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1309 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1314 if (PvNode && bestValue > alpha)
1317 futilityBase = bestValue + 128;
1320 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1321 nullptr, (ss-4)->continuationHistory,
1322 nullptr, (ss-6)->continuationHistory };
1324 // Initialize a MovePicker object for the current position, and prepare
1325 // to search the moves. Because the depth is <= 0 here, only captures,
1326 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1328 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1329 &thisThread->captureHistory,
1331 to_sq((ss-1)->currentMove));
1333 // Loop through the moves until no moves remain or a beta cutoff occurs
1334 while ((move = mp.next_move()) != MOVE_NONE)
1336 assert(is_ok(move));
1338 givesCheck = pos.gives_check(move);
1345 && futilityBase > -VALUE_KNOWN_WIN
1346 && !pos.advanced_pawn_push(move))
1348 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1350 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1352 if (futilityValue <= alpha)
1354 bestValue = std::max(bestValue, futilityValue);
1358 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1360 bestValue = std::max(bestValue, futilityBase);
1365 // Detect non-capture evasions that are candidates to be pruned
1366 evasionPrunable = inCheck
1367 && (depth != DEPTH_ZERO || moveCount > 2)
1368 && bestValue > VALUE_MATED_IN_MAX_PLY
1369 && !pos.capture(move);
1371 // Don't search moves with negative SEE values
1372 if ( (!inCheck || evasionPrunable)
1373 && (!givesCheck || !(pos.blockers_for_king(~pos.side_to_move()) & from_sq(move)))
1374 && !pos.see_ge(move))
1377 // Speculative prefetch as early as possible
1378 prefetch(TT.first_entry(pos.key_after(move)));
1380 // Check for legality just before making the move
1381 if (!pos.legal(move))
1387 ss->currentMove = move;
1388 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
1390 // Make and search the move
1391 pos.do_move(move, st, givesCheck);
1392 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1393 pos.undo_move(move);
1395 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1397 // Check for a new best move
1398 if (value > bestValue)
1406 if (PvNode) // Update pv even in fail-high case
1407 update_pv(ss->pv, move, (ss+1)->pv);
1409 if (PvNode && value < beta) // Update alpha here!
1417 // All legal moves have been searched. A special case: If we're in check
1418 // and no legal moves were found, it is checkmate.
1419 if (inCheck && bestValue == -VALUE_INFINITE)
1420 return mated_in(ss->ply); // Plies to mate from the root
1422 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1423 bestValue >= beta ? BOUND_LOWER :
1424 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1425 ttDepth, bestMove, ss->staticEval);
1427 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1433 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1434 // "plies to mate from the current position". Non-mate scores are unchanged.
1435 // The function is called before storing a value in the transposition table.
1437 Value value_to_tt(Value v, int ply) {
1439 assert(v != VALUE_NONE);
1441 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1442 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1446 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1447 // from the transposition table (which refers to the plies to mate/be mated
1448 // from current position) to "plies to mate/be mated from the root".
1450 Value value_from_tt(Value v, int ply) {
1452 return v == VALUE_NONE ? VALUE_NONE
1453 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1454 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1458 // update_pv() adds current move and appends child pv[]
1460 void update_pv(Move* pv, Move move, Move* childPv) {
1462 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1468 // update_continuation_histories() updates histories of the move pairs formed
1469 // by moves at ply -1, -2, and -4 with current move.
1471 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1473 for (int i : {1, 2, 4, 6})
1474 if (is_ok((ss-i)->currentMove))
1475 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1479 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1481 void update_capture_stats(const Position& pos, Move move,
1482 Move* captures, int captureCount, int bonus) {
1484 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1485 Piece moved_piece = pos.moved_piece(move);
1486 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1488 if (pos.capture_or_promotion(move))
1489 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1491 // Decrease all the other played capture moves
1492 for (int i = 0; i < captureCount; ++i)
1494 moved_piece = pos.moved_piece(captures[i]);
1495 captured = type_of(pos.piece_on(to_sq(captures[i])));
1496 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1501 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1503 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1504 Move* quiets, int quietCount, int bonus) {
1506 if (ss->killers[0] != move)
1508 ss->killers[1] = ss->killers[0];
1509 ss->killers[0] = move;
1512 Color us = pos.side_to_move();
1513 Thread* thisThread = pos.this_thread();
1514 thisThread->mainHistory[us][from_to(move)] << bonus;
1515 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1517 if (is_ok((ss-1)->currentMove))
1519 Square prevSq = to_sq((ss-1)->currentMove);
1520 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1523 // Decrease all the other played quiet moves
1524 for (int i = 0; i < quietCount; ++i)
1526 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1527 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1531 // When playing with strength handicap, choose best move among a set of RootMoves
1532 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1534 Move Skill::pick_best(size_t multiPV) {
1536 const RootMoves& rootMoves = Threads.main()->rootMoves;
1537 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1539 // RootMoves are already sorted by score in descending order
1540 Value topScore = rootMoves[0].score;
1541 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1542 int weakness = 120 - 2 * level;
1543 int maxScore = -VALUE_INFINITE;
1545 // Choose best move. For each move score we add two terms, both dependent on
1546 // weakness. One is deterministic and bigger for weaker levels, and one is
1547 // random. Then we choose the move with the resulting highest score.
1548 for (size_t i = 0; i < multiPV; ++i)
1550 // This is our magic formula
1551 int push = ( weakness * int(topScore - rootMoves[i].score)
1552 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1554 if (rootMoves[i].score + push >= maxScore)
1556 maxScore = rootMoves[i].score + push;
1557 best = rootMoves[i].pv[0];
1566 /// MainThread::check_time() is used to print debug info and, more importantly,
1567 /// to detect when we are out of available time and thus stop the search.
1569 void MainThread::check_time() {
1574 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1575 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1577 static TimePoint lastInfoTime = now();
1579 TimePoint elapsed = Time.elapsed();
1580 TimePoint tick = Limits.startTime + elapsed;
1582 if (tick - lastInfoTime >= 1000)
1584 lastInfoTime = tick;
1588 // We should not stop pondering until told so by the GUI
1592 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1593 || (Limits.movetime && elapsed >= Limits.movetime)
1594 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1595 Threads.stop = true;
1599 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1600 /// that all (if any) unsearched PV lines are sent using a previous search score.
1602 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1604 std::stringstream ss;
1605 TimePoint elapsed = Time.elapsed() + 1;
1606 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1607 size_t pvIdx = pos.this_thread()->pvIdx;
1608 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1609 uint64_t nodesSearched = Threads.nodes_searched();
1610 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1612 for (size_t i = 0; i < multiPV; ++i)
1614 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1616 if (depth == ONE_PLY && !updated)
1619 Depth d = updated ? depth : depth - ONE_PLY;
1620 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1622 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1623 v = tb ? rootMoves[i].tbScore : v;
1625 if (ss.rdbuf()->in_avail()) // Not at first line
1629 << " depth " << d / ONE_PLY
1630 << " seldepth " << rootMoves[i].selDepth
1631 << " multipv " << i + 1
1632 << " score " << UCI::value(v);
1634 if (!tb && i == pvIdx)
1635 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1637 ss << " nodes " << nodesSearched
1638 << " nps " << nodesSearched * 1000 / elapsed;
1640 if (elapsed > 1000) // Earlier makes little sense
1641 ss << " hashfull " << TT.hashfull();
1643 ss << " tbhits " << tbHits
1644 << " time " << elapsed
1647 for (Move m : rootMoves[i].pv)
1648 ss << " " << UCI::move(m, pos.is_chess960());
1655 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1656 /// before exiting the search, for instance, in case we stop the search during a
1657 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1658 /// otherwise in case of 'ponder on' we have nothing to think on.
1660 bool RootMove::extract_ponder_from_tt(Position& pos) {
1665 assert(pv.size() == 1);
1667 if (pv[0] == MOVE_NONE)
1670 pos.do_move(pv[0], st);
1671 TTEntry* tte = TT.probe(pos.key(), ttHit);
1675 Move m = tte->move(); // Local copy to be SMP safe
1676 if (MoveList<LEGAL>(pos).contains(m))
1680 pos.undo_move(pv[0]);
1681 return pv.size() > 1;
1684 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1687 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1688 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1689 Cardinality = int(Options["SyzygyProbeLimit"]);
1690 bool dtz_available = true;
1692 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1693 // ProbeDepth == DEPTH_ZERO
1694 if (Cardinality > MaxCardinality)
1696 Cardinality = MaxCardinality;
1697 ProbeDepth = DEPTH_ZERO;
1700 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1702 // Rank moves using DTZ tables
1703 RootInTB = root_probe(pos, rootMoves);
1707 // DTZ tables are missing; try to rank moves using WDL tables
1708 dtz_available = false;
1709 RootInTB = root_probe_wdl(pos, rootMoves);
1715 // Sort moves according to TB rank
1716 std::sort(rootMoves.begin(), rootMoves.end(),
1717 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1719 // Probe during search only if DTZ is not available and we are winning
1720 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1725 // Clean up if root_probe() and root_probe_wdl() have failed
1726 for (auto& m : rootMoves)