2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2018 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
54 namespace TB = Tablebases;
58 using namespace Search;
62 // Different node types, used as a template parameter
63 enum NodeType { NonPV, PV };
65 // Sizes and phases of the skip-blocks, used for distributing search depths across the threads
66 const int SkipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
67 const int SkipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
69 // Razor and futility margins
70 const int RazorMargin1 = 590;
71 const int RazorMargin2 = 604;
72 Value futility_margin(Depth d, bool improving) {
73 return Value((175 - 50 * improving) * d / ONE_PLY);
76 // Futility and reductions lookup tables, initialized at startup
77 int FutilityMoveCounts[2][16]; // [improving][depth]
78 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
80 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
81 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
84 // History and stats update bonus, based on depth
85 int stat_bonus(Depth depth) {
86 int d = depth / ONE_PLY;
87 return d > 17 ? 0 : d * d + 2 * d - 2;
90 // Skill structure is used to implement strength limit
92 explicit Skill(int l) : level(l) {}
93 bool enabled() const { return level < 20; }
94 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
95 Move pick_best(size_t multiPV);
98 Move best = MOVE_NONE;
101 template <NodeType NT>
102 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning);
104 template <NodeType NT>
105 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
107 Value value_to_tt(Value v, int ply);
108 Value value_from_tt(Value v, int ply);
109 void update_pv(Move* pv, Move move, Move* childPv);
110 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
111 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
112 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCnt, int bonus);
114 inline bool gives_check(const Position& pos, Move move) {
115 Color us = pos.side_to_move();
116 return type_of(move) == NORMAL && !(pos.blockers_for_king(~us) & pos.pieces(us))
117 ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
118 : pos.gives_check(move);
121 // perft() is our utility to verify move generation. All the leaf nodes up
122 // to the given depth are generated and counted, and the sum is returned.
124 uint64_t perft(Position& pos, Depth depth) {
127 uint64_t cnt, nodes = 0;
128 const bool leaf = (depth == 2 * ONE_PLY);
130 for (const auto& m : MoveList<LEGAL>(pos))
132 if (Root && depth <= ONE_PLY)
137 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
142 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
150 /// Search::init() is called at startup to initialize various lookup tables
152 void Search::init() {
154 for (int imp = 0; imp <= 1; ++imp)
155 for (int d = 1; d < 64; ++d)
156 for (int mc = 1; mc < 64; ++mc)
158 double r = log(d) * log(mc) / 1.95;
160 Reductions[NonPV][imp][d][mc] = int(std::round(r));
161 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
163 // Increase reduction for non-PV nodes when eval is not improving
164 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
165 Reductions[NonPV][imp][d][mc]++;
168 for (int d = 0; d < 16; ++d)
170 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
171 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
176 /// Search::clear() resets search state to its initial value
178 void Search::clear() {
180 Threads.main()->wait_for_search_finished();
182 Time.availableNodes = 0;
188 /// MainThread::search() is called by the main thread when the program receives
189 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
191 void MainThread::search() {
195 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
196 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
200 Color us = rootPos.side_to_move();
201 Time.init(Limits, us, rootPos.game_ply());
204 if (rootMoves.empty())
206 rootMoves.emplace_back(MOVE_NONE);
207 sync_cout << "info depth 0 score "
208 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
213 for (Thread* th : Threads)
215 th->start_searching();
217 Thread::search(); // Let's start searching!
220 // When we reach the maximum depth, we can arrive here without a raise of
221 // Threads.stop. However, if we are pondering or in an infinite search,
222 // the UCI protocol states that we shouldn't print the best move before the
223 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
224 // until the GUI sends one of those commands (which also raises Threads.stop).
225 Threads.stopOnPonderhit = true;
227 while (!Threads.stop && (Threads.ponder || Limits.infinite))
228 {} // Busy wait for a stop or a ponder reset
230 // Stop the threads if not already stopped (also raise the stop if
231 // "ponderhit" just reset Threads.ponder).
234 // Wait until all threads have finished
235 for (Thread* th : Threads)
237 th->wait_for_search_finished();
239 // When playing in 'nodes as time' mode, subtract the searched nodes from
240 // the available ones before exiting.
242 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
244 // Check if there are threads with a better score than main thread
245 Thread* bestThread = this;
246 if ( Options["MultiPV"] == 1
248 && !Skill(Options["Skill Level"]).enabled()
249 && rootMoves[0].pv[0] != MOVE_NONE)
251 for (Thread* th : Threads)
253 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
254 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
256 // Select the thread with the best score, always if it is a mate
258 && (depthDiff >= 0 || th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY))
263 previousScore = bestThread->rootMoves[0].score;
265 // Send again PV info if we have a new best thread
266 if (bestThread != this)
267 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
269 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
271 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
272 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
274 std::cout << sync_endl;
278 /// Thread::search() is the main iterative deepening loop. It calls search()
279 /// repeatedly with increasing depth until the allocated thinking time has been
280 /// consumed, the user stops the search, or the maximum search depth is reached.
282 void Thread::search() {
284 Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
285 Value bestValue, alpha, beta, delta;
286 Move lastBestMove = MOVE_NONE;
287 Depth lastBestMoveDepth = DEPTH_ZERO;
288 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
289 double timeReduction = 1.0;
290 Color us = rootPos.side_to_move();
292 std::memset(ss-4, 0, 7 * sizeof(Stack));
293 for (int i = 4; i > 0; i--)
294 (ss-i)->contHistory = this->contHistory[NO_PIECE][0].get(); // Use as sentinel
296 bestValue = delta = alpha = -VALUE_INFINITE;
297 beta = VALUE_INFINITE;
300 mainThread->bestMoveChanges = 0, mainThread->failedLow = false;
302 size_t multiPV = Options["MultiPV"];
303 Skill skill(Options["Skill Level"]);
305 // When playing with strength handicap enable MultiPV search that we will
306 // use behind the scenes to retrieve a set of possible moves.
308 multiPV = std::max(multiPV, (size_t)4);
310 multiPV = std::min(multiPV, rootMoves.size());
312 int ct = Options["Contempt"] * PawnValueEg / 100; // From centipawns
313 Eval::Contempt = (us == WHITE ? make_score(ct, ct / 2)
314 : -make_score(ct, ct / 2));
316 // Iterative deepening loop until requested to stop or the target depth is reached
317 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
319 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
321 // Distribute search depths across the helper threads
324 int i = (idx - 1) % 20;
325 if (((rootDepth / ONE_PLY + rootPos.game_ply() + SkipPhase[i]) / SkipSize[i]) % 2)
326 continue; // Retry with an incremented rootDepth
329 // Age out PV variability metric
331 mainThread->bestMoveChanges *= 0.517, mainThread->failedLow = false;
333 // Save the last iteration's scores before first PV line is searched and
334 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
335 for (RootMove& rm : rootMoves)
336 rm.previousScore = rm.score;
338 // MultiPV loop. We perform a full root search for each PV line
339 for (PVIdx = 0; PVIdx < multiPV && !Threads.stop; ++PVIdx)
341 // Reset UCI info selDepth for each depth and each PV line
344 // Reset aspiration window starting size
345 if (rootDepth >= 5 * ONE_PLY)
348 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
349 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
351 ct = Options["Contempt"] * PawnValueEg / 100; // From centipawns
353 // Adjust contempt based on current bestValue (dynamic contempt)
354 ct += int(std::round(48 * atan(float(bestValue) / 128)));
356 Eval::Contempt = (us == WHITE ? make_score(ct, ct / 2)
357 : -make_score(ct, ct / 2));
360 // Start with a small aspiration window and, in the case of a fail
361 // high/low, re-search with a bigger window until we don't fail
365 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
367 // Bring the best move to the front. It is critical that sorting
368 // is done with a stable algorithm because all the values but the
369 // first and eventually the new best one are set to -VALUE_INFINITE
370 // and we want to keep the same order for all the moves except the
371 // new PV that goes to the front. Note that in case of MultiPV
372 // search the already searched PV lines are preserved.
373 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
375 // If search has been stopped, we break immediately. Sorting is
376 // safe because RootMoves is still valid, although it refers to
377 // the previous iteration.
381 // When failing high/low give some update (without cluttering
382 // the UI) before a re-search.
385 && (bestValue <= alpha || bestValue >= beta)
386 && Time.elapsed() > 3000)
387 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
389 // In case of failing low/high increase aspiration window and
390 // re-search, otherwise exit the loop.
391 if (bestValue <= alpha)
393 beta = (alpha + beta) / 2;
394 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
398 mainThread->failedLow = true;
399 Threads.stopOnPonderhit = false;
402 else if (bestValue >= beta)
403 beta = std::min(bestValue + delta, VALUE_INFINITE);
407 delta += delta / 4 + 5;
409 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
412 // Sort the PV lines searched so far and update the GUI
413 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
416 && (Threads.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000))
417 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
421 completedDepth = rootDepth;
423 if (rootMoves[0].pv[0] != lastBestMove) {
424 lastBestMove = rootMoves[0].pv[0];
425 lastBestMoveDepth = rootDepth;
428 // Have we found a "mate in x"?
430 && bestValue >= VALUE_MATE_IN_MAX_PLY
431 && VALUE_MATE - bestValue <= 2 * Limits.mate)
437 // If skill level is enabled and time is up, pick a sub-optimal best move
438 if (skill.enabled() && skill.time_to_pick(rootDepth))
439 skill.pick_best(multiPV);
441 // Do we have time for the next iteration? Can we stop searching now?
442 if ( Limits.use_time_management()
444 && !Threads.stopOnPonderhit)
446 const int F[] = { mainThread->failedLow,
447 bestValue - mainThread->previousScore };
449 int improvingFactor = std::max(246, std::min(832, 306 + 119 * F[0] - 6 * F[1]));
451 // If the bestMove is stable over several iterations, reduce time accordingly
453 for (int i : {3, 4, 5})
454 if (lastBestMoveDepth * i < completedDepth)
455 timeReduction *= 1.25;
457 // Use part of the gained time from a previous stable move for the current move
458 double unstablePvFactor = 1.0 + mainThread->bestMoveChanges;
459 unstablePvFactor *= std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
461 // Stop the search if we have only one legal move, or if available time elapsed
462 if ( rootMoves.size() == 1
463 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 581)
465 // If we are allowed to ponder do not stop the search now but
466 // keep pondering until the GUI sends "ponderhit" or "stop".
468 Threads.stopOnPonderhit = true;
478 mainThread->previousTimeReduction = timeReduction;
480 // If skill level is enabled, swap best PV line with the sub-optimal one
482 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
483 skill.best ? skill.best : skill.pick_best(multiPV)));
489 // search<>() is the main search function for both PV and non-PV nodes
491 template <NodeType NT>
492 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
494 const bool PvNode = NT == PV;
495 const bool rootNode = PvNode && ss->ply == 0;
497 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
498 assert(PvNode || (alpha == beta - 1));
499 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
500 assert(!(PvNode && cutNode));
501 assert(depth / ONE_PLY * ONE_PLY == depth);
503 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
507 Move ttMove, move, excludedMove, bestMove;
508 Depth extension, newDepth;
509 Value bestValue, value, ttValue, eval, maxValue;
510 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
511 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
513 int moveCount, captureCount, quietCount;
515 // Step 1. Initialize node
516 Thread* thisThread = pos.this_thread();
517 inCheck = pos.checkers();
518 moveCount = captureCount = quietCount = ss->moveCount = 0;
519 bestValue = -VALUE_INFINITE;
520 maxValue = VALUE_INFINITE;
522 // Check for the available remaining time
523 if (thisThread == Threads.main())
524 static_cast<MainThread*>(thisThread)->check_time();
526 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
527 if (PvNode && thisThread->selDepth < ss->ply + 1)
528 thisThread->selDepth = ss->ply + 1;
532 // Step 2. Check for aborted search and immediate draw
533 if ( Threads.stop.load(std::memory_order_relaxed)
534 || pos.is_draw(ss->ply)
535 || ss->ply >= MAX_PLY)
536 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
538 // Step 3. Mate distance pruning. Even if we mate at the next move our score
539 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
540 // a shorter mate was found upward in the tree then there is no need to search
541 // because we will never beat the current alpha. Same logic but with reversed
542 // signs applies also in the opposite condition of being mated instead of giving
543 // mate. In this case return a fail-high score.
544 alpha = std::max(mated_in(ss->ply), alpha);
545 beta = std::min(mate_in(ss->ply+1), beta);
550 assert(0 <= ss->ply && ss->ply < MAX_PLY);
552 (ss+1)->ply = ss->ply + 1;
553 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
554 ss->contHistory = thisThread->contHistory[NO_PIECE][0].get();
555 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
556 Square prevSq = to_sq((ss-1)->currentMove);
558 // Initialize statScore to zero for the grandchildren of the current position.
559 // So statScore is shared between all grandchildren and only the first grandchild
560 // starts with statScore = 0. Later grandchildren start with the last calculated
561 // statScore of the previous grandchild. This influences the reduction rules in
562 // LMR which are based on the statScore of parent position.
563 (ss+2)->statScore = 0;
565 // Step 4. Transposition table lookup. We don't want the score of a partial
566 // search to overwrite a previous full search TT value, so we use a different
567 // position key in case of an excluded move.
568 excludedMove = ss->excludedMove;
569 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
570 tte = TT.probe(posKey, ttHit);
571 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
572 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
573 : ttHit ? tte->move() : MOVE_NONE;
575 // At non-PV nodes we check for an early TT cutoff
578 && tte->depth() >= depth
579 && ttValue != VALUE_NONE // Possible in case of TT access race
580 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
581 : (tte->bound() & BOUND_UPPER)))
583 // If ttMove is quiet, update move sorting heuristics on TT hit
588 if (!pos.capture_or_promotion(ttMove))
589 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
591 // Extra penalty for a quiet TT move in previous ply when it gets refuted
592 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
593 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
595 // Penalty for a quiet ttMove that fails low
596 else if (!pos.capture_or_promotion(ttMove))
598 int penalty = -stat_bonus(depth);
599 thisThread->mainHistory[pos.side_to_move()][from_to(ttMove)] << penalty;
600 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
606 // Step 5. Tablebases probe
607 if (!rootNode && TB::Cardinality)
609 int piecesCount = pos.count<ALL_PIECES>();
611 if ( piecesCount <= TB::Cardinality
612 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
613 && pos.rule50_count() == 0
614 && !pos.can_castle(ANY_CASTLING))
617 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
619 if (err != TB::ProbeState::FAIL)
621 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
623 int drawScore = TB::UseRule50 ? 1 : 0;
625 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
626 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
627 : VALUE_DRAW + 2 * wdl * drawScore;
629 Bound b = wdl < -drawScore ? BOUND_UPPER
630 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
632 if ( b == BOUND_EXACT
633 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
635 tte->save(posKey, value_to_tt(value, ss->ply), b,
636 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
637 MOVE_NONE, VALUE_NONE, TT.generation());
644 if (b == BOUND_LOWER)
645 bestValue = value, alpha = std::max(alpha, bestValue);
653 // Step 6. Evaluate the position statically
656 ss->staticEval = eval = VALUE_NONE;
662 // Never assume anything on values stored in TT
663 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
664 eval = ss->staticEval = evaluate(pos);
666 // Can ttValue be used as a better position evaluation?
667 if ( ttValue != VALUE_NONE
668 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
673 ss->staticEval = eval =
674 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
675 : -(ss-1)->staticEval + 2 * Eval::Tempo;
677 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
678 ss->staticEval, TT.generation());
681 improving = ss->staticEval >= (ss-2)->staticEval
682 ||(ss-2)->staticEval == VALUE_NONE;
684 if (skipEarlyPruning || !pos.non_pawn_material(pos.side_to_move()))
687 // Step 7. Razoring (skipped when in check)
689 && depth <= 2 * ONE_PLY)
691 if ( depth == ONE_PLY
692 && eval + RazorMargin1 <= alpha)
693 return qsearch<NonPV>(pos, ss, alpha, alpha+1);
695 else if (eval + RazorMargin2 <= alpha)
697 Value ralpha = alpha - RazorMargin2;
698 Value v = qsearch<NonPV>(pos, ss, ralpha, ralpha+1);
705 // Step 8. Futility pruning: child node (skipped when in check)
707 && depth < 7 * ONE_PLY
708 && eval - futility_margin(depth, improving) >= beta
709 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
712 // Step 9. Null move search with verification search
715 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
716 && (ss->ply >= thisThread->nmp_ply || ss->ply % 2 != thisThread->nmp_odd))
718 assert(eval - beta >= 0);
720 // Null move dynamic reduction based on depth and value
721 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
723 ss->currentMove = MOVE_NULL;
724 ss->contHistory = thisThread->contHistory[NO_PIECE][0].get();
726 pos.do_null_move(st);
727 Value nullValue = depth-R < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -beta+1)
728 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
729 pos.undo_null_move();
731 if (nullValue >= beta)
733 // Do not return unproven mate scores
734 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
737 if (abs(beta) < VALUE_KNOWN_WIN && (depth < 12 * ONE_PLY || thisThread->nmp_ply))
740 // Do verification search at high depths. Disable null move pruning
741 // for side to move for the first part of the remaining search tree.
742 thisThread->nmp_ply = ss->ply + 3 * (depth-R) / 4;
743 thisThread->nmp_odd = ss->ply % 2;
745 Value v = depth-R < ONE_PLY ? qsearch<NonPV>(pos, ss, beta-1, beta)
746 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
748 thisThread->nmp_odd = thisThread->nmp_ply = 0;
755 // Step 10. ProbCut (skipped when in check)
756 // If we have a good enough capture and a reduced search returns a value
757 // much above beta, we can (almost) safely prune the previous move.
759 && depth >= 5 * ONE_PLY
760 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
762 assert(is_ok((ss-1)->currentMove));
764 Value rbeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
765 MovePicker mp(pos, ttMove, rbeta - ss->staticEval, &thisThread->captureHistory);
766 int probCutCount = 0;
768 while ( (move = mp.next_move()) != MOVE_NONE
769 && probCutCount < depth / ONE_PLY - 3)
774 ss->currentMove = move;
775 ss->contHistory = thisThread->contHistory[pos.moved_piece(move)][to_sq(move)].get();
777 assert(depth >= 5 * ONE_PLY);
779 pos.do_move(move, st);
781 // Perform a preliminary qsearch to verify that the move holds
782 value = -qsearch<NonPV>(pos, ss+1, -rbeta, -rbeta+1);
784 // If the qsearch held perform the regular search
786 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode, false);
795 // Step 11. Internal iterative deepening (skipped when in check)
796 if ( depth >= 6 * ONE_PLY
798 && (PvNode || ss->staticEval + 256 >= beta))
800 Depth d = 3 * depth / 4 - 2 * ONE_PLY;
801 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
803 tte = TT.probe(posKey, ttHit);
804 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
805 ttMove = ttHit ? tte->move() : MOVE_NONE;
808 moves_loop: // When in check, search starts from here
810 const PieceToHistory* contHist[] = { (ss-1)->contHistory, (ss-2)->contHistory, nullptr, (ss-4)->contHistory };
811 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
813 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory, contHist, countermove, ss->killers);
814 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
816 singularExtensionNode = !rootNode
817 && depth >= 8 * ONE_PLY
818 && ttMove != MOVE_NONE
819 && ttValue != VALUE_NONE
820 && !excludedMove // Recursive singular search is not allowed
821 && (tte->bound() & BOUND_LOWER)
822 && tte->depth() >= depth - 3 * ONE_PLY;
825 pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
827 // Step 12. Loop through all pseudo-legal moves until no moves remain
828 // or a beta cutoff occurs.
829 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
833 if (move == excludedMove)
836 // At root obey the "searchmoves" option and skip moves not listed in Root
837 // Move List. As a consequence any illegal move is also skipped. In MultiPV
838 // mode we also skip PV moves which have been already searched.
839 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
840 thisThread->rootMoves.end(), move))
843 ss->moveCount = ++moveCount;
845 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
846 sync_cout << "info depth " << depth / ONE_PLY
847 << " currmove " << UCI::move(move, pos.is_chess960())
848 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
850 (ss+1)->pv = nullptr;
852 extension = DEPTH_ZERO;
853 captureOrPromotion = pos.capture_or_promotion(move);
854 movedPiece = pos.moved_piece(move);
855 givesCheck = gives_check(pos, move);
857 moveCountPruning = depth < 16 * ONE_PLY
858 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
860 // Step 13. Extensions
862 // Singular extension search. If all moves but one fail low on a search
863 // of (alpha-s, beta-s), and just one fails high on (alpha, beta), then
864 // that move is singular and should be extended. To verify this we do a
865 // reduced search on on all the other moves but the ttMove and if the
866 // result is lower than ttValue minus a margin then we will extend the ttMove.
867 if ( singularExtensionNode
871 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
872 ss->excludedMove = move;
873 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode, true);
874 ss->excludedMove = MOVE_NONE;
879 else if ( givesCheck // Check extension
884 // Calculate new depth for this move
885 newDepth = depth - ONE_PLY + extension;
887 // Step 14. Pruning at shallow depth
889 && pos.non_pawn_material(pos.side_to_move())
890 && bestValue > VALUE_MATED_IN_MAX_PLY)
892 if ( !captureOrPromotion
894 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
896 // Move count based pruning
897 if (moveCountPruning)
903 // Reduced depth of the next LMR search
904 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
906 // Countermoves based pruning
908 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
909 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
912 // Futility pruning: parent node
915 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
918 // Prune moves with negative SEE
920 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
923 else if ( depth < 7 * ONE_PLY
925 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
929 // Speculative prefetch as early as possible
930 prefetch(TT.first_entry(pos.key_after(move)));
932 // Check for legality just before making the move
933 if (!rootNode && !pos.legal(move))
935 ss->moveCount = --moveCount;
939 if (move == ttMove && captureOrPromotion)
942 // Update the current move (this must be done after singular extension search)
943 ss->currentMove = move;
944 ss->contHistory = thisThread->contHistory[movedPiece][to_sq(move)].get();
946 // Step 15. Make the move
947 pos.do_move(move, st, givesCheck);
949 // Step 16. Reduced depth search (LMR). If the move fails high it will be
950 // re-searched at full depth.
951 if ( depth >= 3 * ONE_PLY
953 && (!captureOrPromotion || moveCountPruning))
955 Depth r = reduction<PvNode>(improving, depth, moveCount);
957 if (captureOrPromotion)
958 r -= r ? ONE_PLY : DEPTH_ZERO;
961 // Decrease reduction if opponent's move count is high
962 if ((ss-1)->moveCount > 15)
965 // Decrease reduction for exact PV nodes
969 // Increase reduction if ttMove is a capture
973 // Increase reduction for cut nodes
977 // Decrease reduction for moves that escape a capture. Filter out
978 // castling moves, because they are coded as "king captures rook" and
979 // hence break make_move().
980 else if ( type_of(move) == NORMAL
981 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
984 ss->statScore = thisThread->mainHistory[~pos.side_to_move()][from_to(move)]
985 + (*contHist[0])[movedPiece][to_sq(move)]
986 + (*contHist[1])[movedPiece][to_sq(move)]
987 + (*contHist[3])[movedPiece][to_sq(move)]
990 // Decrease/increase reduction by comparing opponent's stat score
991 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
994 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
997 // Decrease/increase reduction for moves with a good/bad history
998 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->statScore / 20000) * ONE_PLY);
1001 Depth d = std::max(newDepth - r, ONE_PLY);
1003 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
1005 doFullDepthSearch = (value > alpha && d != newDepth);
1008 doFullDepthSearch = !PvNode || moveCount > 1;
1010 // Step 17. Full depth search when LMR is skipped or fails high
1011 if (doFullDepthSearch)
1012 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha)
1013 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
1015 // For PV nodes only, do a full PV search on the first move or after a fail
1016 // high (in the latter case search only if value < beta), otherwise let the
1017 // parent node fail low with value <= alpha and try another move.
1018 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1021 (ss+1)->pv[0] = MOVE_NONE;
1023 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha)
1024 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1027 // Step 18. Undo move
1028 pos.undo_move(move);
1030 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1032 // Step 19. Check for a new best move
1033 // Finished searching the move. If a stop occurred, the return value of
1034 // the search cannot be trusted, and we return immediately without
1035 // updating best move, PV and TT.
1036 if (Threads.stop.load(std::memory_order_relaxed))
1041 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1042 thisThread->rootMoves.end(), move);
1044 // PV move or new best move?
1045 if (moveCount == 1 || value > alpha)
1048 rm.selDepth = thisThread->selDepth;
1053 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1054 rm.pv.push_back(*m);
1056 // We record how often the best move has been changed in each
1057 // iteration. This information is used for time management: When
1058 // the best move changes frequently, we allocate some more time.
1059 if (moveCount > 1 && thisThread == Threads.main())
1060 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1063 // All other moves but the PV are set to the lowest value: this
1064 // is not a problem when sorting because the sort is stable and the
1065 // move position in the list is preserved - just the PV is pushed up.
1066 rm.score = -VALUE_INFINITE;
1069 if (value > bestValue)
1077 if (PvNode && !rootNode) // Update pv even in fail-high case
1078 update_pv(ss->pv, move, (ss+1)->pv);
1080 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1084 assert(value >= beta); // Fail high
1090 if (move != bestMove)
1092 if (captureOrPromotion && captureCount < 32)
1093 capturesSearched[captureCount++] = move;
1095 else if (!captureOrPromotion && quietCount < 64)
1096 quietsSearched[quietCount++] = move;
1100 // The following condition would detect a stop only after move loop has been
1101 // completed. But in this case bestValue is valid because we have fully
1102 // searched our subtree, and we can anyhow save the result in TT.
1108 // Step 20. Check for mate and stalemate
1109 // All legal moves have been searched and if there are no legal moves, it
1110 // must be a mate or a stalemate. If we are in a singular extension search then
1111 // return a fail low score.
1113 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1116 bestValue = excludedMove ? alpha
1117 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1120 // Quiet best move: update move sorting heuristics
1121 if (!pos.capture_or_promotion(bestMove))
1122 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1124 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth));
1126 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1127 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1128 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1130 // Bonus for prior countermove that caused the fail low
1131 else if ( depth >= 3 * ONE_PLY
1132 && !pos.captured_piece()
1133 && is_ok((ss-1)->currentMove))
1134 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1137 bestValue = std::min(bestValue, maxValue);
1140 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1141 bestValue >= beta ? BOUND_LOWER :
1142 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1143 depth, bestMove, ss->staticEval, TT.generation());
1145 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1151 // qsearch() is the quiescence search function, which is called by the main
1152 // search function with depth zero, or recursively with depth less than ONE_PLY.
1153 template <NodeType NT>
1154 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1156 const bool PvNode = NT == PV;
1157 const bool inCheck = bool(pos.checkers());
1159 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1160 assert(PvNode || (alpha == beta - 1));
1161 assert(depth <= DEPTH_ZERO);
1162 assert(depth / ONE_PLY * ONE_PLY == depth);
1168 Move ttMove, move, bestMove;
1170 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1171 bool ttHit, givesCheck, evasionPrunable;
1176 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1178 ss->pv[0] = MOVE_NONE;
1181 (ss+1)->ply = ss->ply + 1;
1182 ss->currentMove = bestMove = MOVE_NONE;
1185 // Check for an immediate draw or maximum ply reached
1186 if ( pos.is_draw(ss->ply)
1187 || ss->ply >= MAX_PLY)
1188 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1190 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1192 // Decide whether or not to include checks: this fixes also the type of
1193 // TT entry depth that we are going to use. Note that in qsearch we use
1194 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1195 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1196 : DEPTH_QS_NO_CHECKS;
1197 // Transposition table lookup
1199 tte = TT.probe(posKey, ttHit);
1200 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1201 ttMove = ttHit ? tte->move() : MOVE_NONE;
1205 && tte->depth() >= ttDepth
1206 && ttValue != VALUE_NONE // Only in case of TT access race
1207 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1208 : (tte->bound() & BOUND_UPPER)))
1211 // Evaluate the position statically
1214 ss->staticEval = VALUE_NONE;
1215 bestValue = futilityBase = -VALUE_INFINITE;
1221 // Never assume anything on values stored in TT
1222 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1223 ss->staticEval = bestValue = evaluate(pos);
1225 // Can ttValue be used as a better position evaluation?
1226 if ( ttValue != VALUE_NONE
1227 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1228 bestValue = ttValue;
1231 ss->staticEval = bestValue =
1232 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1233 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1235 // Stand pat. Return immediately if static value is at least beta
1236 if (bestValue >= beta)
1239 tte->save(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1240 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1245 if (PvNode && bestValue > alpha)
1248 futilityBase = bestValue + 128;
1251 // Initialize a MovePicker object for the current position, and prepare
1252 // to search the moves. Because the depth is <= 0 here, only captures,
1253 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1255 MovePicker mp(pos, ttMove, depth, &pos.this_thread()->mainHistory, &pos.this_thread()->captureHistory, to_sq((ss-1)->currentMove));
1257 // Loop through the moves until no moves remain or a beta cutoff occurs
1258 while ((move = mp.next_move()) != MOVE_NONE)
1260 assert(is_ok(move));
1262 givesCheck = gives_check(pos, move);
1269 && futilityBase > -VALUE_KNOWN_WIN
1270 && !pos.advanced_pawn_push(move))
1272 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1274 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1276 if (futilityValue <= alpha)
1278 bestValue = std::max(bestValue, futilityValue);
1282 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1284 bestValue = std::max(bestValue, futilityBase);
1289 // Detect non-capture evasions that are candidates to be pruned
1290 evasionPrunable = inCheck
1291 && (depth != DEPTH_ZERO || moveCount > 2)
1292 && bestValue > VALUE_MATED_IN_MAX_PLY
1293 && !pos.capture(move);
1295 // Don't search moves with negative SEE values
1296 if ( (!inCheck || evasionPrunable)
1297 && !pos.see_ge(move))
1300 // Speculative prefetch as early as possible
1301 prefetch(TT.first_entry(pos.key_after(move)));
1303 // Check for legality just before making the move
1304 if (!pos.legal(move))
1310 ss->currentMove = move;
1312 // Make and search the move
1313 pos.do_move(move, st, givesCheck);
1314 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1315 pos.undo_move(move);
1317 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1319 // Check for a new best move
1320 if (value > bestValue)
1326 if (PvNode) // Update pv even in fail-high case
1327 update_pv(ss->pv, move, (ss+1)->pv);
1329 if (PvNode && value < beta) // Update alpha here!
1336 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1337 ttDepth, move, ss->staticEval, TT.generation());
1345 // All legal moves have been searched. A special case: If we're in check
1346 // and no legal moves were found, it is checkmate.
1347 if (inCheck && bestValue == -VALUE_INFINITE)
1348 return mated_in(ss->ply); // Plies to mate from the root
1350 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1351 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1352 ttDepth, bestMove, ss->staticEval, TT.generation());
1354 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1360 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1361 // "plies to mate from the current position". Non-mate scores are unchanged.
1362 // The function is called before storing a value in the transposition table.
1364 Value value_to_tt(Value v, int ply) {
1366 assert(v != VALUE_NONE);
1368 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1369 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1373 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1374 // from the transposition table (which refers to the plies to mate/be mated
1375 // from current position) to "plies to mate/be mated from the root".
1377 Value value_from_tt(Value v, int ply) {
1379 return v == VALUE_NONE ? VALUE_NONE
1380 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1381 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1385 // update_pv() adds current move and appends child pv[]
1387 void update_pv(Move* pv, Move move, Move* childPv) {
1389 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1395 // update_continuation_histories() updates histories of the move pairs formed
1396 // by moves at ply -1, -2, and -4 with current move.
1398 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1400 for (int i : {1, 2, 4})
1401 if (is_ok((ss-i)->currentMove))
1402 (*(ss-i)->contHistory)[pc][to] << bonus;
1406 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1408 void update_capture_stats(const Position& pos, Move move,
1409 Move* captures, int captureCnt, int bonus) {
1411 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1412 Piece moved_piece = pos.moved_piece(move);
1413 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1414 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1416 // Decrease all the other played capture moves
1417 for (int i = 0; i < captureCnt; ++i)
1419 moved_piece = pos.moved_piece(captures[i]);
1420 captured = type_of(pos.piece_on(to_sq(captures[i])));
1421 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1426 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1428 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1429 Move* quiets, int quietsCnt, int bonus) {
1431 if (ss->killers[0] != move)
1433 ss->killers[1] = ss->killers[0];
1434 ss->killers[0] = move;
1437 Color us = pos.side_to_move();
1438 Thread* thisThread = pos.this_thread();
1439 thisThread->mainHistory[us][from_to(move)] << bonus;
1440 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1442 if (is_ok((ss-1)->currentMove))
1444 Square prevSq = to_sq((ss-1)->currentMove);
1445 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1448 // Decrease all the other played quiet moves
1449 for (int i = 0; i < quietsCnt; ++i)
1451 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1452 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1456 // When playing with strength handicap, choose best move among a set of RootMoves
1457 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1459 Move Skill::pick_best(size_t multiPV) {
1461 const RootMoves& rootMoves = Threads.main()->rootMoves;
1462 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1464 // RootMoves are already sorted by score in descending order
1465 Value topScore = rootMoves[0].score;
1466 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1467 int weakness = 120 - 2 * level;
1468 int maxScore = -VALUE_INFINITE;
1470 // Choose best move. For each move score we add two terms, both dependent on
1471 // weakness. One is deterministic and bigger for weaker levels, and one is
1472 // random. Then we choose the move with the resulting highest score.
1473 for (size_t i = 0; i < multiPV; ++i)
1475 // This is our magic formula
1476 int push = ( weakness * int(topScore - rootMoves[i].score)
1477 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1479 if (rootMoves[i].score + push >= maxScore)
1481 maxScore = rootMoves[i].score + push;
1482 best = rootMoves[i].pv[0];
1491 /// MainThread::check_time() is used to print debug info and, more importantly,
1492 /// to detect when we are out of available time and thus stop the search.
1494 void MainThread::check_time() {
1499 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1500 callsCnt = Limits.nodes ? std::min(4096, int(Limits.nodes / 1024)) : 4096;
1502 static TimePoint lastInfoTime = now();
1504 int elapsed = Time.elapsed();
1505 TimePoint tick = Limits.startTime + elapsed;
1507 if (tick - lastInfoTime >= 1000)
1509 lastInfoTime = tick;
1513 // We should not stop pondering until told so by the GUI
1517 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1518 || (Limits.movetime && elapsed >= Limits.movetime)
1519 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1520 Threads.stop = true;
1524 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1525 /// that all (if any) unsearched PV lines are sent using a previous search score.
1527 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1529 std::stringstream ss;
1530 int elapsed = Time.elapsed() + 1;
1531 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1532 size_t PVIdx = pos.this_thread()->PVIdx;
1533 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1534 uint64_t nodesSearched = Threads.nodes_searched();
1535 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1537 for (size_t i = 0; i < multiPV; ++i)
1539 bool updated = (i <= PVIdx && rootMoves[i].score != -VALUE_INFINITE);
1541 if (depth == ONE_PLY && !updated)
1544 Depth d = updated ? depth : depth - ONE_PLY;
1545 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1547 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1548 v = tb ? TB::Score : v;
1550 if (ss.rdbuf()->in_avail()) // Not at first line
1554 << " depth " << d / ONE_PLY
1555 << " seldepth " << rootMoves[i].selDepth
1556 << " multipv " << i + 1
1557 << " score " << UCI::value(v);
1559 if (!tb && i == PVIdx)
1560 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1562 ss << " nodes " << nodesSearched
1563 << " nps " << nodesSearched * 1000 / elapsed;
1565 if (elapsed > 1000) // Earlier makes little sense
1566 ss << " hashfull " << TT.hashfull();
1568 ss << " tbhits " << tbHits
1569 << " time " << elapsed
1572 for (Move m : rootMoves[i].pv)
1573 ss << " " << UCI::move(m, pos.is_chess960());
1580 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1581 /// before exiting the search, for instance, in case we stop the search during a
1582 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1583 /// otherwise in case of 'ponder on' we have nothing to think on.
1585 bool RootMove::extract_ponder_from_tt(Position& pos) {
1590 assert(pv.size() == 1);
1595 pos.do_move(pv[0], st);
1596 TTEntry* tte = TT.probe(pos.key(), ttHit);
1600 Move m = tte->move(); // Local copy to be SMP safe
1601 if (MoveList<LEGAL>(pos).contains(m))
1605 pos.undo_move(pv[0]);
1606 return pv.size() > 1;
1610 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1613 UseRule50 = Options["Syzygy50MoveRule"];
1614 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1615 Cardinality = Options["SyzygyProbeLimit"];
1617 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1618 if (Cardinality > MaxCardinality)
1620 Cardinality = MaxCardinality;
1621 ProbeDepth = DEPTH_ZERO;
1624 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1627 // Don't filter any moves if the user requested analysis on multiple
1628 if (Options["MultiPV"] != 1)
1631 // If the current root position is in the tablebases, then RootMoves
1632 // contains only moves that preserve the draw or the win.
1633 RootInTB = root_probe(pos, rootMoves, TB::Score);
1636 Cardinality = 0; // Do not probe tablebases during the search
1638 else // If DTZ tables are missing, use WDL tables as a fallback
1640 // Filter out moves that do not preserve the draw or the win.
1641 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1643 // Only probe during search if winning
1644 if (RootInTB && TB::Score <= VALUE_DRAW)
1648 if (RootInTB && !UseRule50)
1649 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1650 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
1653 // Since root_probe() and root_probe_wdl() dirty the root move scores,
1654 // we reset them to -VALUE_INFINITE
1655 for (RootMove& rm : rootMoves)
1656 rm.score = -VALUE_INFINITE;