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 constexpr int SkipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
67 constexpr 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 constexpr int RazorMargin[] = {0, 590, 604};
71 Value futility_margin(Depth d, bool improving) {
72 return Value((175 - 50 * improving) * d / ONE_PLY);
75 // Margin for pruning capturing moves: almost linear in depth
76 constexpr int CapturePruneMargin[] = { 0,
77 1 * PawnValueEg * 1055 / 1000,
78 2 * PawnValueEg * 1042 / 1000,
79 3 * PawnValueEg * 963 / 1000,
80 4 * PawnValueEg * 1038 / 1000,
81 5 * PawnValueEg * 950 / 1000,
82 6 * PawnValueEg * 930 / 1000
85 // Futility and reductions lookup tables, initialized at startup
86 int FutilityMoveCounts[2][16]; // [improving][depth]
87 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
89 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
90 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
93 // History and stats update bonus, based on depth
94 int stat_bonus(Depth depth) {
95 int d = depth / ONE_PLY;
96 return d > 17 ? 0 : d * d + 2 * d - 2;
99 // Skill structure is used to implement strength limit
101 explicit Skill(int l) : level(l) {}
102 bool enabled() const { return level < 20; }
103 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
104 Move pick_best(size_t multiPV);
107 Move best = MOVE_NONE;
110 template <NodeType NT>
111 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning);
113 template <NodeType NT>
114 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
116 Value value_to_tt(Value v, int ply);
117 Value value_from_tt(Value v, int ply);
118 void update_pv(Move* pv, Move move, Move* childPv);
119 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
120 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
121 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCnt, int bonus);
123 inline bool gives_check(const Position& pos, Move move) {
124 Color us = pos.side_to_move();
125 return type_of(move) == NORMAL && !(pos.blockers_for_king(~us) & pos.pieces(us))
126 ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
127 : pos.gives_check(move);
130 // perft() is our utility to verify move generation. All the leaf nodes up
131 // to the given depth are generated and counted, and the sum is returned.
133 uint64_t perft(Position& pos, Depth depth) {
136 uint64_t cnt, nodes = 0;
137 const bool leaf = (depth == 2 * ONE_PLY);
139 for (const auto& m : MoveList<LEGAL>(pos))
141 if (Root && depth <= ONE_PLY)
146 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
151 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
159 /// Search::init() is called at startup to initialize various lookup tables
161 void Search::init() {
163 for (int imp = 0; imp <= 1; ++imp)
164 for (int d = 1; d < 64; ++d)
165 for (int mc = 1; mc < 64; ++mc)
167 double r = log(d) * log(mc) / 1.95;
169 Reductions[NonPV][imp][d][mc] = int(std::round(r));
170 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
172 // Increase reduction for non-PV nodes when eval is not improving
174 Reductions[NonPV][imp][d][mc]++;
177 for (int d = 0; d < 16; ++d)
179 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
180 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
185 /// Search::clear() resets search state to its initial value
187 void Search::clear() {
189 Threads.main()->wait_for_search_finished();
191 Time.availableNodes = 0;
197 /// MainThread::search() is called by the main thread when the program receives
198 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
200 void MainThread::search() {
204 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
205 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
209 Color us = rootPos.side_to_move();
210 Time.init(Limits, us, rootPos.game_ply());
213 if (rootMoves.empty())
215 rootMoves.emplace_back(MOVE_NONE);
216 sync_cout << "info depth 0 score "
217 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
222 for (Thread* th : Threads)
224 th->start_searching();
226 Thread::search(); // Let's start searching!
229 // When we reach the maximum depth, we can arrive here without a raise of
230 // Threads.stop. However, if we are pondering or in an infinite search,
231 // the UCI protocol states that we shouldn't print the best move before the
232 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
233 // until the GUI sends one of those commands (which also raises Threads.stop).
234 Threads.stopOnPonderhit = true;
236 while (!Threads.stop && (Threads.ponder || Limits.infinite))
237 {} // Busy wait for a stop or a ponder reset
239 // Stop the threads if not already stopped (also raise the stop if
240 // "ponderhit" just reset Threads.ponder).
243 // Wait until all threads have finished
244 for (Thread* th : Threads)
246 th->wait_for_search_finished();
248 // When playing in 'nodes as time' mode, subtract the searched nodes from
249 // the available ones before exiting.
251 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
253 // Check if there are threads with a better score than main thread
254 Thread* bestThread = this;
255 if ( Options["MultiPV"] == 1
257 && !Skill(Options["Skill Level"]).enabled()
258 && rootMoves[0].pv[0] != MOVE_NONE)
260 for (Thread* th : Threads)
262 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
263 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
265 // Select the thread with the best score, always if it is a mate
267 && (depthDiff >= 0 || th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY))
272 previousScore = bestThread->rootMoves[0].score;
274 // Send again PV info if we have a new best thread
275 if (bestThread != this)
276 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
278 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
280 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
281 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
283 std::cout << sync_endl;
287 /// Thread::search() is the main iterative deepening loop. It calls search()
288 /// repeatedly with increasing depth until the allocated thinking time has been
289 /// consumed, the user stops the search, or the maximum search depth is reached.
291 void Thread::search() {
293 Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
294 Value bestValue, alpha, beta, delta;
295 Move lastBestMove = MOVE_NONE;
296 Depth lastBestMoveDepth = DEPTH_ZERO;
297 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
298 double timeReduction = 1.0;
299 Color us = rootPos.side_to_move();
301 std::memset(ss-4, 0, 7 * sizeof(Stack));
302 for (int i = 4; i > 0; i--)
303 (ss-i)->contHistory = this->contHistory[NO_PIECE][0].get(); // Use as sentinel
305 bestValue = delta = alpha = -VALUE_INFINITE;
306 beta = VALUE_INFINITE;
309 mainThread->bestMoveChanges = 0, mainThread->failedLow = false;
311 size_t multiPV = Options["MultiPV"];
312 Skill skill(Options["Skill Level"]);
314 // When playing with strength handicap enable MultiPV search that we will
315 // use behind the scenes to retrieve a set of possible moves.
317 multiPV = std::max(multiPV, (size_t)4);
319 multiPV = std::min(multiPV, rootMoves.size());
321 int ct = Options["Contempt"] * PawnValueEg / 100; // From centipawns
322 contempt = (us == WHITE ? make_score(ct, ct / 2)
323 : -make_score(ct, ct / 2));
325 // Iterative deepening loop until requested to stop or the target depth is reached
326 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
328 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
330 // Distribute search depths across the helper threads
333 int i = (idx - 1) % 20;
334 if (((rootDepth / ONE_PLY + rootPos.game_ply() + SkipPhase[i]) / SkipSize[i]) % 2)
335 continue; // Retry with an incremented rootDepth
338 // Age out PV variability metric
340 mainThread->bestMoveChanges *= 0.517, mainThread->failedLow = false;
342 // Save the last iteration's scores before first PV line is searched and
343 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
344 for (RootMove& rm : rootMoves)
345 rm.previousScore = rm.score;
347 // MultiPV loop. We perform a full root search for each PV line
348 for (PVIdx = 0; PVIdx < multiPV && !Threads.stop; ++PVIdx)
350 // Reset UCI info selDepth for each depth and each PV line
353 // Reset aspiration window starting size
354 if (rootDepth >= 5 * ONE_PLY)
356 Value previousScore = rootMoves[PVIdx].previousScore;
358 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
359 beta = std::min(previousScore + delta, VALUE_INFINITE);
361 ct = Options["Contempt"] * PawnValueEg / 100; // From centipawns
363 // Adjust contempt based on root move's previousScore (dynamic contempt)
364 ct += int(std::round(48 * atan(float(previousScore) / 128)));
366 contempt = (us == WHITE ? make_score(ct, ct / 2)
367 : -make_score(ct, ct / 2));
370 // Start with a small aspiration window and, in the case of a fail
371 // high/low, re-search with a bigger window until we don't fail
375 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
377 // Bring the best move to the front. It is critical that sorting
378 // is done with a stable algorithm because all the values but the
379 // first and eventually the new best one are set to -VALUE_INFINITE
380 // and we want to keep the same order for all the moves except the
381 // new PV that goes to the front. Note that in case of MultiPV
382 // search the already searched PV lines are preserved.
383 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
385 // If search has been stopped, we break immediately. Sorting is
386 // safe because RootMoves is still valid, although it refers to
387 // the previous iteration.
391 // When failing high/low give some update (without cluttering
392 // the UI) before a re-search.
395 && (bestValue <= alpha || bestValue >= beta)
396 && Time.elapsed() > 3000)
397 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
399 // In case of failing low/high increase aspiration window and
400 // re-search, otherwise exit the loop.
401 if (bestValue <= alpha)
403 beta = (alpha + beta) / 2;
404 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
408 mainThread->failedLow = true;
409 Threads.stopOnPonderhit = false;
412 else if (bestValue >= beta)
413 beta = std::min(bestValue + delta, VALUE_INFINITE);
417 delta += delta / 4 + 5;
419 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
422 // Sort the PV lines searched so far and update the GUI
423 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
426 && (Threads.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000))
427 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
431 completedDepth = rootDepth;
433 if (rootMoves[0].pv[0] != lastBestMove) {
434 lastBestMove = rootMoves[0].pv[0];
435 lastBestMoveDepth = rootDepth;
438 // Have we found a "mate in x"?
440 && bestValue >= VALUE_MATE_IN_MAX_PLY
441 && VALUE_MATE - bestValue <= 2 * Limits.mate)
447 // If skill level is enabled and time is up, pick a sub-optimal best move
448 if (skill.enabled() && skill.time_to_pick(rootDepth))
449 skill.pick_best(multiPV);
451 // Do we have time for the next iteration? Can we stop searching now?
452 if ( Limits.use_time_management()
454 && !Threads.stopOnPonderhit)
456 const int F[] = { mainThread->failedLow,
457 bestValue - mainThread->previousScore };
459 int improvingFactor = std::max(246, std::min(832, 306 + 119 * F[0] - 6 * F[1]));
461 // If the bestMove is stable over several iterations, reduce time accordingly
463 for (int i : {3, 4, 5})
464 if (lastBestMoveDepth * i < completedDepth)
465 timeReduction *= 1.25;
467 // Use part of the gained time from a previous stable move for the current move
468 double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
469 bestMoveInstability *= std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
471 // Stop the search if we have only one legal move, or if available time elapsed
472 if ( rootMoves.size() == 1
473 || Time.elapsed() > Time.optimum() * bestMoveInstability * improvingFactor / 581)
475 // If we are allowed to ponder do not stop the search now but
476 // keep pondering until the GUI sends "ponderhit" or "stop".
478 Threads.stopOnPonderhit = true;
488 mainThread->previousTimeReduction = timeReduction;
490 // If skill level is enabled, swap best PV line with the sub-optimal one
492 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
493 skill.best ? skill.best : skill.pick_best(multiPV)));
499 // search<>() is the main search function for both PV and non-PV nodes
501 template <NodeType NT>
502 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
504 // Use quiescence search when needed
506 return qsearch<NT>(pos, ss, alpha, beta);
508 constexpr bool PvNode = NT == PV;
509 const bool rootNode = PvNode && ss->ply == 0;
511 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
512 assert(PvNode || (alpha == beta - 1));
513 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
514 assert(!(PvNode && cutNode));
515 assert(depth / ONE_PLY * ONE_PLY == depth);
517 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
521 Move ttMove, move, excludedMove, bestMove;
522 Depth extension, newDepth;
523 Value bestValue, value, ttValue, eval, maxValue;
524 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
525 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
527 int moveCount, captureCount, quietCount;
529 // Step 1. Initialize node
530 Thread* thisThread = pos.this_thread();
531 inCheck = pos.checkers();
532 moveCount = captureCount = quietCount = ss->moveCount = 0;
533 bestValue = -VALUE_INFINITE;
534 maxValue = VALUE_INFINITE;
536 // Check for the available remaining time
537 if (thisThread == Threads.main())
538 static_cast<MainThread*>(thisThread)->check_time();
540 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
541 if (PvNode && thisThread->selDepth < ss->ply + 1)
542 thisThread->selDepth = ss->ply + 1;
546 // Step 2. Check for aborted search and immediate draw
547 if ( Threads.stop.load(std::memory_order_relaxed)
548 || pos.is_draw(ss->ply)
549 || ss->ply >= MAX_PLY)
550 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
552 // Step 3. Mate distance pruning. Even if we mate at the next move our score
553 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
554 // a shorter mate was found upward in the tree then there is no need to search
555 // because we will never beat the current alpha. Same logic but with reversed
556 // signs applies also in the opposite condition of being mated instead of giving
557 // mate. In this case return a fail-high score.
558 alpha = std::max(mated_in(ss->ply), alpha);
559 beta = std::min(mate_in(ss->ply+1), beta);
564 assert(0 <= ss->ply && ss->ply < MAX_PLY);
566 (ss+1)->ply = ss->ply + 1;
567 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
568 ss->contHistory = thisThread->contHistory[NO_PIECE][0].get();
569 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
570 Square prevSq = to_sq((ss-1)->currentMove);
572 // Initialize statScore to zero for the grandchildren of the current position.
573 // So statScore is shared between all grandchildren and only the first grandchild
574 // starts with statScore = 0. Later grandchildren start with the last calculated
575 // statScore of the previous grandchild. This influences the reduction rules in
576 // LMR which are based on the statScore of parent position.
577 (ss+2)->statScore = 0;
579 // Step 4. Transposition table lookup. We don't want the score of a partial
580 // search to overwrite a previous full search TT value, so we use a different
581 // position key in case of an excluded move.
582 excludedMove = ss->excludedMove;
583 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
584 tte = TT.probe(posKey, ttHit);
585 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
586 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
587 : ttHit ? tte->move() : MOVE_NONE;
589 // At non-PV nodes we check for an early TT cutoff
592 && tte->depth() >= depth
593 && ttValue != VALUE_NONE // Possible in case of TT access race
594 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
595 : (tte->bound() & BOUND_UPPER)))
597 // If ttMove is quiet, update move sorting heuristics on TT hit
602 if (!pos.capture_or_promotion(ttMove))
603 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
605 // Extra penalty for a quiet TT move in previous ply when it gets refuted
606 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
607 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
609 // Penalty for a quiet ttMove that fails low
610 else if (!pos.capture_or_promotion(ttMove))
612 int penalty = -stat_bonus(depth);
613 thisThread->mainHistory[pos.side_to_move()][from_to(ttMove)] << penalty;
614 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
620 // Step 5. Tablebases probe
621 if (!rootNode && TB::Cardinality)
623 int piecesCount = pos.count<ALL_PIECES>();
625 if ( piecesCount <= TB::Cardinality
626 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
627 && pos.rule50_count() == 0
628 && !pos.can_castle(ANY_CASTLING))
631 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
633 if (err != TB::ProbeState::FAIL)
635 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
637 int drawScore = TB::UseRule50 ? 1 : 0;
639 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
640 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
641 : VALUE_DRAW + 2 * wdl * drawScore;
643 Bound b = wdl < -drawScore ? BOUND_UPPER
644 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
646 if ( b == BOUND_EXACT
647 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
649 tte->save(posKey, value_to_tt(value, ss->ply), b,
650 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
651 MOVE_NONE, VALUE_NONE, TT.generation());
658 if (b == BOUND_LOWER)
659 bestValue = value, alpha = std::max(alpha, bestValue);
667 // Step 6. Evaluate the position statically
670 ss->staticEval = eval = VALUE_NONE;
676 // Never assume anything on values stored in TT
677 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
678 eval = ss->staticEval = evaluate(pos);
680 // Can ttValue be used as a better position evaluation?
681 if ( ttValue != VALUE_NONE
682 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
687 ss->staticEval = eval =
688 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
689 : -(ss-1)->staticEval + 2 * Eval::Tempo;
691 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
692 ss->staticEval, TT.generation());
695 improving = ss->staticEval >= (ss-2)->staticEval
696 ||(ss-2)->staticEval == VALUE_NONE;
698 if (skipEarlyPruning || !pos.non_pawn_material(pos.side_to_move()))
701 // Step 7. Razoring (skipped when in check, ~2 Elo)
703 && depth < 3 * ONE_PLY
704 && eval <= alpha - RazorMargin[depth / ONE_PLY])
706 Value ralpha = alpha - (depth >= 2 * ONE_PLY) * RazorMargin[depth / ONE_PLY];
707 Value v = qsearch<NonPV>(pos, ss, ralpha, ralpha+1);
708 if (depth < 2 * ONE_PLY || v <= ralpha)
712 // Step 8. Futility pruning: child node (skipped when in check, ~30 Elo)
714 && depth < 7 * ONE_PLY
715 && eval - futility_margin(depth, improving) >= beta
716 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
719 // Step 9. Null move search with verification search (~40 Elo)
722 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
723 && (ss->ply >= thisThread->nmp_ply || ss->ply % 2 != thisThread->nmp_odd))
725 assert(eval - beta >= 0);
727 // Null move dynamic reduction based on depth and value
728 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
730 ss->currentMove = MOVE_NULL;
731 ss->contHistory = thisThread->contHistory[NO_PIECE][0].get();
733 pos.do_null_move(st);
735 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
737 pos.undo_null_move();
739 if (nullValue >= beta)
741 // Do not return unproven mate scores
742 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
745 if (abs(beta) < VALUE_KNOWN_WIN && (depth < 12 * ONE_PLY || thisThread->nmp_ply))
748 // Do verification search at high depths. Disable null move pruning
749 // for side to move for the first part of the remaining search tree.
750 thisThread->nmp_ply = ss->ply + 3 * (depth-R) / 4;
751 thisThread->nmp_odd = ss->ply % 2;
753 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
755 thisThread->nmp_odd = thisThread->nmp_ply = 0;
762 // Step 10. ProbCut (skipped when in check, ~10 Elo)
763 // If we have a good enough capture and a reduced search returns a value
764 // much above beta, we can (almost) safely prune the previous move.
766 && depth >= 5 * ONE_PLY
767 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
769 assert(is_ok((ss-1)->currentMove));
771 Value rbeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
772 MovePicker mp(pos, ttMove, rbeta - ss->staticEval, &thisThread->captureHistory);
773 int probCutCount = 0;
775 while ( (move = mp.next_move()) != MOVE_NONE
781 ss->currentMove = move;
782 ss->contHistory = thisThread->contHistory[pos.moved_piece(move)][to_sq(move)].get();
784 assert(depth >= 5 * ONE_PLY);
786 pos.do_move(move, st);
788 // Perform a preliminary qsearch to verify that the move holds
789 value = -qsearch<NonPV>(pos, ss+1, -rbeta, -rbeta+1);
791 // If the qsearch held perform the regular search
793 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode, false);
802 // Step 11. Internal iterative deepening (skipped when in check, ~2 Elo)
803 if ( depth >= 6 * ONE_PLY
805 && (PvNode || ss->staticEval + 128 >= beta))
807 Depth d = 3 * depth / 4 - 2 * ONE_PLY;
808 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
810 tte = TT.probe(posKey, ttHit);
811 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
812 ttMove = ttHit ? tte->move() : MOVE_NONE;
815 moves_loop: // When in check, search starts from here
817 const PieceToHistory* contHist[] = { (ss-1)->contHistory, (ss-2)->contHistory, nullptr, (ss-4)->contHistory };
818 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
820 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory, contHist, countermove, ss->killers);
821 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
823 singularExtensionNode = !rootNode
824 && depth >= 8 * ONE_PLY
825 && ttMove != MOVE_NONE
826 && ttValue != VALUE_NONE
827 && !excludedMove // Recursive singular search is not allowed
828 && (tte->bound() & BOUND_LOWER)
829 && tte->depth() >= depth - 3 * ONE_PLY;
832 pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
834 // Step 12. Loop through all pseudo-legal moves until no moves remain
835 // or a beta cutoff occurs.
836 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
840 if (move == excludedMove)
843 // At root obey the "searchmoves" option and skip moves not listed in Root
844 // Move List. As a consequence any illegal move is also skipped. In MultiPV
845 // mode we also skip PV moves which have been already searched.
846 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
847 thisThread->rootMoves.end(), move))
850 ss->moveCount = ++moveCount;
852 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
853 sync_cout << "info depth " << depth / ONE_PLY
854 << " currmove " << UCI::move(move, pos.is_chess960())
855 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
857 (ss+1)->pv = nullptr;
859 extension = DEPTH_ZERO;
860 captureOrPromotion = pos.capture_or_promotion(move);
861 movedPiece = pos.moved_piece(move);
862 givesCheck = gives_check(pos, move);
864 moveCountPruning = depth < 16 * ONE_PLY
865 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
867 // Step 13. Extensions (~70 Elo)
869 // Singular extension search (~60 Elo). If all moves but one fail low on a search
870 // of (alpha-s, beta-s), and just one fails high on (alpha, beta), then
871 // that move is singular and should be extended. To verify this we do a
872 // reduced search on on all the other moves but the ttMove and if the
873 // result is lower than ttValue minus a margin then we will extend the ttMove.
874 if ( singularExtensionNode
878 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
879 ss->excludedMove = move;
880 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode, true);
881 ss->excludedMove = MOVE_NONE;
886 else if ( givesCheck // Check extension (~2 Elo)
891 // Calculate new depth for this move
892 newDepth = depth - ONE_PLY + extension;
894 // Step 14. Pruning at shallow depth (~170 Elo)
896 && pos.non_pawn_material(pos.side_to_move())
897 && bestValue > VALUE_MATED_IN_MAX_PLY)
899 if ( !captureOrPromotion
901 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
903 // Move count based pruning (~30 Elo)
904 if (moveCountPruning)
910 // Reduced depth of the next LMR search
911 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
913 // Countermoves based pruning (~20 Elo)
915 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
916 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
919 // Futility pruning: parent node (~2 Elo)
922 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
925 // Prune moves with negative SEE (~10 Elo)
927 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
930 else if ( depth < 7 * ONE_PLY // (~20 Elo)
932 && !pos.see_ge(move, -Value(CapturePruneMargin[depth / ONE_PLY])))
936 // Speculative prefetch as early as possible
937 prefetch(TT.first_entry(pos.key_after(move)));
939 // Check for legality just before making the move
940 if (!rootNode && !pos.legal(move))
942 ss->moveCount = --moveCount;
946 if (move == ttMove && captureOrPromotion)
949 // Update the current move (this must be done after singular extension search)
950 ss->currentMove = move;
951 ss->contHistory = thisThread->contHistory[movedPiece][to_sq(move)].get();
953 // Step 15. Make the move
954 pos.do_move(move, st, givesCheck);
956 // Step 16. Reduced depth search (LMR). If the move fails high it will be
957 // re-searched at full depth.
958 if ( depth >= 3 * ONE_PLY
960 && (!captureOrPromotion || moveCountPruning))
962 Depth r = reduction<PvNode>(improving, depth, moveCount);
964 if (captureOrPromotion)
965 r -= r ? ONE_PLY : DEPTH_ZERO;
968 // Decrease reduction if opponent's move count is high
969 if ((ss-1)->moveCount > 15)
972 // Decrease reduction for exact PV nodes
976 // Increase reduction if ttMove is a capture
980 // Increase reduction for cut nodes
984 // Decrease reduction for moves that escape a capture. Filter out
985 // castling moves, because they are coded as "king captures rook" and
986 // hence break make_move().
987 else if ( type_of(move) == NORMAL
988 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
991 ss->statScore = thisThread->mainHistory[~pos.side_to_move()][from_to(move)]
992 + (*contHist[0])[movedPiece][to_sq(move)]
993 + (*contHist[1])[movedPiece][to_sq(move)]
994 + (*contHist[3])[movedPiece][to_sq(move)]
997 // Decrease/increase reduction by comparing opponent's stat score
998 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1001 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1004 // Decrease/increase reduction for moves with a good/bad history
1005 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->statScore / 20000) * ONE_PLY);
1008 Depth d = std::max(newDepth - r, ONE_PLY);
1010 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
1012 doFullDepthSearch = (value > alpha && d != newDepth);
1015 doFullDepthSearch = !PvNode || moveCount > 1;
1017 // Step 17. Full depth search when LMR is skipped or fails high
1018 if (doFullDepthSearch)
1019 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
1021 // For PV nodes only, do a full PV search on the first move or after a fail
1022 // high (in the latter case search only if value < beta), otherwise let the
1023 // parent node fail low with value <= alpha and try another move.
1024 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1027 (ss+1)->pv[0] = MOVE_NONE;
1029 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1032 // Step 18. Undo move
1033 pos.undo_move(move);
1035 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1037 // Step 19. Check for a new best move
1038 // Finished searching the move. If a stop occurred, the return value of
1039 // the search cannot be trusted, and we return immediately without
1040 // updating best move, PV and TT.
1041 if (Threads.stop.load(std::memory_order_relaxed))
1046 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1047 thisThread->rootMoves.end(), move);
1049 // PV move or new best move?
1050 if (moveCount == 1 || value > alpha)
1053 rm.selDepth = thisThread->selDepth;
1058 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1059 rm.pv.push_back(*m);
1061 // We record how often the best move has been changed in each
1062 // iteration. This information is used for time management: When
1063 // the best move changes frequently, we allocate some more time.
1064 if (moveCount > 1 && thisThread == Threads.main())
1065 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1068 // All other moves but the PV are set to the lowest value: this
1069 // is not a problem when sorting because the sort is stable and the
1070 // move position in the list is preserved - just the PV is pushed up.
1071 rm.score = -VALUE_INFINITE;
1074 if (value > bestValue)
1082 if (PvNode && !rootNode) // Update pv even in fail-high case
1083 update_pv(ss->pv, move, (ss+1)->pv);
1085 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1089 assert(value >= beta); // Fail high
1095 if (move != bestMove)
1097 if (captureOrPromotion && captureCount < 32)
1098 capturesSearched[captureCount++] = move;
1100 else if (!captureOrPromotion && quietCount < 64)
1101 quietsSearched[quietCount++] = move;
1105 // The following condition would detect a stop only after move loop has been
1106 // completed. But in this case bestValue is valid because we have fully
1107 // searched our subtree, and we can anyhow save the result in TT.
1113 // Step 20. Check for mate and stalemate
1114 // All legal moves have been searched and if there are no legal moves, it
1115 // must be a mate or a stalemate. If we are in a singular extension search then
1116 // return a fail low score.
1118 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1121 bestValue = excludedMove ? alpha
1122 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1125 // Quiet best move: update move sorting heuristics
1126 if (!pos.capture_or_promotion(bestMove))
1127 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1129 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth));
1131 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1132 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1133 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1135 // Bonus for prior countermove that caused the fail low
1136 else if ( depth >= 3 * ONE_PLY
1137 && !pos.captured_piece()
1138 && is_ok((ss-1)->currentMove))
1139 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1142 bestValue = std::min(bestValue, maxValue);
1145 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1146 bestValue >= beta ? BOUND_LOWER :
1147 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1148 depth, bestMove, ss->staticEval, TT.generation());
1150 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1156 // qsearch() is the quiescence search function, which is called by the main
1157 // search function with depth zero, or recursively with depth less than ONE_PLY.
1158 template <NodeType NT>
1159 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1161 constexpr bool PvNode = NT == PV;
1163 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1164 assert(PvNode || (alpha == beta - 1));
1165 assert(depth <= DEPTH_ZERO);
1166 assert(depth / ONE_PLY * ONE_PLY == depth);
1172 Move ttMove, move, bestMove;
1174 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1175 bool ttHit, inCheck, givesCheck, evasionPrunable;
1180 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1182 ss->pv[0] = MOVE_NONE;
1185 (ss+1)->ply = ss->ply + 1;
1186 ss->currentMove = bestMove = MOVE_NONE;
1187 inCheck = pos.checkers();
1190 // Check for an immediate draw or maximum ply reached
1191 if ( pos.is_draw(ss->ply)
1192 || ss->ply >= MAX_PLY)
1193 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1195 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1197 // Decide whether or not to include checks: this fixes also the type of
1198 // TT entry depth that we are going to use. Note that in qsearch we use
1199 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1200 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1201 : DEPTH_QS_NO_CHECKS;
1202 // Transposition table lookup
1204 tte = TT.probe(posKey, ttHit);
1205 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1206 ttMove = ttHit ? tte->move() : MOVE_NONE;
1210 && tte->depth() >= ttDepth
1211 && ttValue != VALUE_NONE // Only in case of TT access race
1212 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1213 : (tte->bound() & BOUND_UPPER)))
1216 // Evaluate the position statically
1219 ss->staticEval = VALUE_NONE;
1220 bestValue = futilityBase = -VALUE_INFINITE;
1226 // Never assume anything on values stored in TT
1227 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1228 ss->staticEval = bestValue = evaluate(pos);
1230 // Can ttValue be used as a better position evaluation?
1231 if ( ttValue != VALUE_NONE
1232 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1233 bestValue = ttValue;
1236 ss->staticEval = bestValue =
1237 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1238 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1240 // Stand pat. Return immediately if static value is at least beta
1241 if (bestValue >= beta)
1244 tte->save(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1245 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1250 if (PvNode && bestValue > alpha)
1253 futilityBase = bestValue + 128;
1256 // Initialize a MovePicker object for the current position, and prepare
1257 // to search the moves. Because the depth is <= 0 here, only captures,
1258 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1260 MovePicker mp(pos, ttMove, depth, &pos.this_thread()->mainHistory, &pos.this_thread()->captureHistory, to_sq((ss-1)->currentMove));
1262 // Loop through the moves until no moves remain or a beta cutoff occurs
1263 while ((move = mp.next_move()) != MOVE_NONE)
1265 assert(is_ok(move));
1267 givesCheck = gives_check(pos, move);
1274 && futilityBase > -VALUE_KNOWN_WIN
1275 && !pos.advanced_pawn_push(move))
1277 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1279 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1281 if (futilityValue <= alpha)
1283 bestValue = std::max(bestValue, futilityValue);
1287 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1289 bestValue = std::max(bestValue, futilityBase);
1294 // Detect non-capture evasions that are candidates to be pruned
1295 evasionPrunable = inCheck
1296 && (depth != DEPTH_ZERO || moveCount > 2)
1297 && bestValue > VALUE_MATED_IN_MAX_PLY
1298 && !pos.capture(move);
1300 // Don't search moves with negative SEE values
1301 if ( (!inCheck || evasionPrunable)
1302 && !pos.see_ge(move))
1305 // Speculative prefetch as early as possible
1306 prefetch(TT.first_entry(pos.key_after(move)));
1308 // Check for legality just before making the move
1309 if (!pos.legal(move))
1315 ss->currentMove = move;
1317 // Make and search the move
1318 pos.do_move(move, st, givesCheck);
1319 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1320 pos.undo_move(move);
1322 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1324 // Check for a new best move
1325 if (value > bestValue)
1331 if (PvNode) // Update pv even in fail-high case
1332 update_pv(ss->pv, move, (ss+1)->pv);
1334 if (PvNode && value < beta) // Update alpha here!
1341 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1342 ttDepth, move, ss->staticEval, TT.generation());
1350 // All legal moves have been searched. A special case: If we're in check
1351 // and no legal moves were found, it is checkmate.
1352 if (inCheck && bestValue == -VALUE_INFINITE)
1353 return mated_in(ss->ply); // Plies to mate from the root
1355 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1356 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1357 ttDepth, bestMove, ss->staticEval, TT.generation());
1359 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1365 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1366 // "plies to mate from the current position". Non-mate scores are unchanged.
1367 // The function is called before storing a value in the transposition table.
1369 Value value_to_tt(Value v, int ply) {
1371 assert(v != VALUE_NONE);
1373 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1374 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1378 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1379 // from the transposition table (which refers to the plies to mate/be mated
1380 // from current position) to "plies to mate/be mated from the root".
1382 Value value_from_tt(Value v, int ply) {
1384 return v == VALUE_NONE ? VALUE_NONE
1385 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1386 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1390 // update_pv() adds current move and appends child pv[]
1392 void update_pv(Move* pv, Move move, Move* childPv) {
1394 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1400 // update_continuation_histories() updates histories of the move pairs formed
1401 // by moves at ply -1, -2, and -4 with current move.
1403 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1405 for (int i : {1, 2, 4})
1406 if (is_ok((ss-i)->currentMove))
1407 (*(ss-i)->contHistory)[pc][to] << bonus;
1411 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1413 void update_capture_stats(const Position& pos, Move move,
1414 Move* captures, int captureCnt, int bonus) {
1416 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1417 Piece moved_piece = pos.moved_piece(move);
1418 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1419 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1421 // Decrease all the other played capture moves
1422 for (int i = 0; i < captureCnt; ++i)
1424 moved_piece = pos.moved_piece(captures[i]);
1425 captured = type_of(pos.piece_on(to_sq(captures[i])));
1426 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1431 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1433 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1434 Move* quiets, int quietsCnt, int bonus) {
1436 if (ss->killers[0] != move)
1438 ss->killers[1] = ss->killers[0];
1439 ss->killers[0] = move;
1442 Color us = pos.side_to_move();
1443 Thread* thisThread = pos.this_thread();
1444 thisThread->mainHistory[us][from_to(move)] << bonus;
1445 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1447 if (is_ok((ss-1)->currentMove))
1449 Square prevSq = to_sq((ss-1)->currentMove);
1450 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1453 // Decrease all the other played quiet moves
1454 for (int i = 0; i < quietsCnt; ++i)
1456 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1457 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1461 // When playing with strength handicap, choose best move among a set of RootMoves
1462 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1464 Move Skill::pick_best(size_t multiPV) {
1466 const RootMoves& rootMoves = Threads.main()->rootMoves;
1467 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1469 // RootMoves are already sorted by score in descending order
1470 Value topScore = rootMoves[0].score;
1471 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1472 int weakness = 120 - 2 * level;
1473 int maxScore = -VALUE_INFINITE;
1475 // Choose best move. For each move score we add two terms, both dependent on
1476 // weakness. One is deterministic and bigger for weaker levels, and one is
1477 // random. Then we choose the move with the resulting highest score.
1478 for (size_t i = 0; i < multiPV; ++i)
1480 // This is our magic formula
1481 int push = ( weakness * int(topScore - rootMoves[i].score)
1482 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1484 if (rootMoves[i].score + push >= maxScore)
1486 maxScore = rootMoves[i].score + push;
1487 best = rootMoves[i].pv[0];
1496 /// MainThread::check_time() is used to print debug info and, more importantly,
1497 /// to detect when we are out of available time and thus stop the search.
1499 void MainThread::check_time() {
1504 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1505 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1507 static TimePoint lastInfoTime = now();
1509 TimePoint elapsed = Time.elapsed();
1510 TimePoint tick = Limits.startTime + elapsed;
1512 if (tick - lastInfoTime >= 1000)
1514 lastInfoTime = tick;
1518 // We should not stop pondering until told so by the GUI
1522 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1523 || (Limits.movetime && elapsed >= Limits.movetime)
1524 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1525 Threads.stop = true;
1529 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1530 /// that all (if any) unsearched PV lines are sent using a previous search score.
1532 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1534 std::stringstream ss;
1535 TimePoint elapsed = Time.elapsed() + 1;
1536 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1537 size_t PVIdx = pos.this_thread()->PVIdx;
1538 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1539 uint64_t nodesSearched = Threads.nodes_searched();
1540 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1542 for (size_t i = 0; i < multiPV; ++i)
1544 bool updated = (i <= PVIdx && rootMoves[i].score != -VALUE_INFINITE);
1546 if (depth == ONE_PLY && !updated)
1549 Depth d = updated ? depth : depth - ONE_PLY;
1550 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1552 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1553 v = tb ? TB::Score : v;
1555 if (ss.rdbuf()->in_avail()) // Not at first line
1559 << " depth " << d / ONE_PLY
1560 << " seldepth " << rootMoves[i].selDepth
1561 << " multipv " << i + 1
1562 << " score " << UCI::value(v);
1564 if (!tb && i == PVIdx)
1565 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1567 ss << " nodes " << nodesSearched
1568 << " nps " << nodesSearched * 1000 / elapsed;
1570 if (elapsed > 1000) // Earlier makes little sense
1571 ss << " hashfull " << TT.hashfull();
1573 ss << " tbhits " << tbHits
1574 << " time " << elapsed
1577 for (Move m : rootMoves[i].pv)
1578 ss << " " << UCI::move(m, pos.is_chess960());
1585 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1586 /// before exiting the search, for instance, in case we stop the search during a
1587 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1588 /// otherwise in case of 'ponder on' we have nothing to think on.
1590 bool RootMove::extract_ponder_from_tt(Position& pos) {
1595 assert(pv.size() == 1);
1600 pos.do_move(pv[0], st);
1601 TTEntry* tte = TT.probe(pos.key(), ttHit);
1605 Move m = tte->move(); // Local copy to be SMP safe
1606 if (MoveList<LEGAL>(pos).contains(m))
1610 pos.undo_move(pv[0]);
1611 return pv.size() > 1;
1615 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1618 UseRule50 = Options["Syzygy50MoveRule"];
1619 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1620 Cardinality = Options["SyzygyProbeLimit"];
1622 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1623 if (Cardinality > MaxCardinality)
1625 Cardinality = MaxCardinality;
1626 ProbeDepth = DEPTH_ZERO;
1629 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1632 // Don't filter any moves if the user requested analysis on multiple
1633 if (Options["MultiPV"] != 1)
1636 // If the current root position is in the tablebases, then RootMoves
1637 // contains only moves that preserve the draw or the win.
1638 RootInTB = root_probe(pos, rootMoves, TB::Score);
1641 Cardinality = 0; // Do not probe tablebases during the search
1643 else // If DTZ tables are missing, use WDL tables as a fallback
1645 // Filter out moves that do not preserve the draw or the win.
1646 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1648 // Only probe during search if winning
1649 if (RootInTB && TB::Score <= VALUE_DRAW)
1653 if (RootInTB && !UseRule50)
1654 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1655 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
1658 // Since root_probe() and root_probe_wdl() dirty the root move scores,
1659 // we reset them to -VALUE_INFINITE
1660 for (RootMove& rm : rootMoves)
1661 rm.score = -VALUE_INFINITE;