2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2018 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
53 namespace TB = Tablebases;
57 using namespace Search;
61 // Different node types, used as a template parameter
62 enum NodeType { NonPV, PV };
64 // Sizes and phases of the skip-blocks, used for distributing search depths across the threads
65 constexpr int SkipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
66 constexpr int SkipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
68 // Razor and futility margins
69 constexpr int RazorMargin[] = {0, 590, 604};
70 Value futility_margin(Depth d, bool improving) {
71 return Value((175 - 50 * improving) * d / ONE_PLY);
74 // Futility and reductions lookup tables, initialized at startup
75 int FutilityMoveCounts[2][16]; // [improving][depth]
76 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
78 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
79 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
82 // History and stats update bonus, based on depth
83 int stat_bonus(Depth depth) {
84 int d = depth / ONE_PLY;
85 return d > 17 ? 0 : 33 * d * d + 66 * d - 66;
88 // Skill structure is used to implement strength limit
90 explicit Skill(int l) : level(l) {}
91 bool enabled() const { return level < 20; }
92 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
93 Move pick_best(size_t multiPV);
96 Move best = MOVE_NONE;
99 template <NodeType NT>
100 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
102 template <NodeType NT>
103 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
105 Value value_to_tt(Value v, int ply);
106 Value value_from_tt(Value v, int ply);
107 void update_pv(Move* pv, Move move, Move* childPv);
108 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
109 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
110 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCnt, int bonus);
112 inline bool gives_check(const Position& pos, Move move) {
113 Color us = pos.side_to_move();
114 return type_of(move) == NORMAL && !(pos.blockers_for_king(~us) & pos.pieces(us))
115 ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
116 : pos.gives_check(move);
119 // perft() is our utility to verify move generation. All the leaf nodes up
120 // to the given depth are generated and counted, and the sum is returned.
122 uint64_t perft(Position& pos, Depth depth) {
125 uint64_t cnt, nodes = 0;
126 const bool leaf = (depth == 2 * ONE_PLY);
128 for (const auto& m : MoveList<LEGAL>(pos))
130 if (Root && depth <= ONE_PLY)
135 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
140 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
148 /// Search::init() is called at startup to initialize various lookup tables
150 void Search::init() {
152 for (int imp = 0; imp <= 1; ++imp)
153 for (int d = 1; d < 64; ++d)
154 for (int mc = 1; mc < 64; ++mc)
156 double r = log(d) * log(mc) / 1.95;
158 Reductions[NonPV][imp][d][mc] = int(std::round(r));
159 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
161 // Increase reduction for non-PV nodes when eval is not improving
163 Reductions[NonPV][imp][d][mc]++;
166 for (int d = 0; d < 16; ++d)
168 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
169 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
174 /// Search::clear() resets search state to its initial value
176 void Search::clear() {
178 Threads.main()->wait_for_search_finished();
180 Time.availableNodes = 0;
186 /// MainThread::search() is called by the main thread when the program receives
187 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
189 void MainThread::search() {
193 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
194 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
198 Color us = rootPos.side_to_move();
199 Time.init(Limits, us, rootPos.game_ply());
202 if (rootMoves.empty())
204 rootMoves.emplace_back(MOVE_NONE);
205 sync_cout << "info depth 0 score "
206 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
211 for (Thread* th : Threads)
213 th->start_searching();
215 Thread::search(); // Let's start searching!
218 // When we reach the maximum depth, we can arrive here without a raise of
219 // Threads.stop. However, if we are pondering or in an infinite search,
220 // the UCI protocol states that we shouldn't print the best move before the
221 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
222 // until the GUI sends one of those commands (which also raises Threads.stop).
223 Threads.stopOnPonderhit = true;
225 while (!Threads.stop && (Threads.ponder || Limits.infinite))
226 {} // Busy wait for a stop or a ponder reset
228 // Stop the threads if not already stopped (also raise the stop if
229 // "ponderhit" just reset Threads.ponder).
232 // Wait until all threads have finished
233 for (Thread* th : Threads)
235 th->wait_for_search_finished();
237 // When playing in 'nodes as time' mode, subtract the searched nodes from
238 // the available ones before exiting.
240 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
242 // Check if there are threads with a better score than main thread
243 Thread* bestThread = this;
244 if ( Options["MultiPV"] == 1
246 && !Skill(Options["Skill Level"]).enabled()
247 && rootMoves[0].pv[0] != MOVE_NONE)
249 for (Thread* th : Threads)
251 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
252 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
254 // Select the thread with the best score, always if it is a mate
256 && (depthDiff >= 0 || th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY))
261 previousScore = bestThread->rootMoves[0].score;
263 // Send again PV info if we have a new best thread
264 if (bestThread != this)
265 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
267 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
269 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
270 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
272 std::cout << sync_endl;
276 /// Thread::search() is the main iterative deepening loop. It calls search()
277 /// repeatedly with increasing depth until the allocated thinking time has been
278 /// consumed, the user stops the search, or the maximum search depth is reached.
280 void Thread::search() {
282 Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
283 Value bestValue, alpha, beta, delta;
284 Move lastBestMove = MOVE_NONE;
285 Depth lastBestMoveDepth = DEPTH_ZERO;
286 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
287 double timeReduction = 1.0;
288 Color us = rootPos.side_to_move();
291 std::memset(ss-4, 0, 7 * sizeof(Stack));
292 for (int i = 4; i > 0; i--)
293 (ss-i)->continuationHistory = this->continuationHistory[NO_PIECE][0].get(); // Use as sentinel
295 bestValue = delta = alpha = -VALUE_INFINITE;
296 beta = VALUE_INFINITE;
299 mainThread->bestMoveChanges = 0, failedLow = false;
301 size_t multiPV = Options["MultiPV"];
302 Skill skill(Options["Skill Level"]);
304 // When playing with strength handicap enable MultiPV search that we will
305 // use behind the scenes to retrieve a set of possible moves.
307 multiPV = std::max(multiPV, (size_t)4);
309 multiPV = std::min(multiPV, rootMoves.size());
311 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
313 // In analysis mode, adjust contempt in accordance with user preference
314 if (Limits.infinite || Options["UCI_AnalyseMode"])
315 ct = Options["Analysis Contempt"] == "Off" ? 0
316 : Options["Analysis Contempt"] == "Both" ? ct
317 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
318 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
321 // In evaluate.cpp the evaluation is from the white point of view
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 + SkipPhase[i]) / SkipSize[i]) % 2)
335 continue; // Retry with an incremented rootDepth
338 // Age out PV variability metric
340 mainThread->bestMoveChanges *= 0.517, 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;
350 // MultiPV loop. We perform a full root search for each PV line
351 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
356 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
357 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
361 // Reset UCI info selDepth for each depth and each PV line
364 // Reset aspiration window starting size
365 if (rootDepth >= 5 * ONE_PLY)
367 Value previousScore = rootMoves[pvIdx].previousScore;
369 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
370 beta = std::min(previousScore + delta, VALUE_INFINITE);
372 // Adjust contempt based on root move's previousScore (dynamic contempt)
373 int dct = ct + 88 * previousScore / (abs(previousScore) + 200);
375 contempt = (us == WHITE ? make_score(dct, dct / 2)
376 : -make_score(dct, dct / 2));
379 // Start with a small aspiration window and, in the case of a fail
380 // high/low, re-search with a bigger window until we don't fail
384 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false);
386 // Bring the best move to the front. It is critical that sorting
387 // is done with a stable algorithm because all the values but the
388 // first and eventually the new best one are set to -VALUE_INFINITE
389 // and we want to keep the same order for all the moves except the
390 // new PV that goes to the front. Note that in case of MultiPV
391 // search the already searched PV lines are preserved.
392 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
394 // If search has been stopped, we break immediately. Sorting is
395 // safe because RootMoves is still valid, although it refers to
396 // the previous iteration.
400 // When failing high/low give some update (without cluttering
401 // the UI) before a re-search.
404 && (bestValue <= alpha || bestValue >= beta)
405 && Time.elapsed() > 3000)
406 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
408 // In case of failing low/high increase aspiration window and
409 // re-search, otherwise exit the loop.
410 if (bestValue <= alpha)
412 beta = (alpha + beta) / 2;
413 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
418 Threads.stopOnPonderhit = false;
421 else if (bestValue >= beta)
422 beta = std::min(bestValue + delta, VALUE_INFINITE);
426 delta += delta / 4 + 5;
428 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
431 // Sort the PV lines searched so far and update the GUI
432 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
435 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
436 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
440 completedDepth = rootDepth;
442 if (rootMoves[0].pv[0] != lastBestMove) {
443 lastBestMove = rootMoves[0].pv[0];
444 lastBestMoveDepth = rootDepth;
447 // Have we found a "mate in x"?
449 && bestValue >= VALUE_MATE_IN_MAX_PLY
450 && VALUE_MATE - bestValue <= 2 * Limits.mate)
456 // If skill level is enabled and time is up, pick a sub-optimal best move
457 if (skill.enabled() && skill.time_to_pick(rootDepth))
458 skill.pick_best(multiPV);
460 // Do we have time for the next iteration? Can we stop searching now?
461 if ( Limits.use_time_management()
463 && !Threads.stopOnPonderhit)
465 const int F[] = { failedLow,
466 bestValue - mainThread->previousScore };
468 int improvingFactor = std::max(246, std::min(832, 306 + 119 * F[0] - 6 * F[1]));
470 // If the bestMove is stable over several iterations, reduce time accordingly
472 for (int i : {3, 4, 5})
473 if (lastBestMoveDepth * i < completedDepth)
474 timeReduction *= 1.25;
476 // Use part of the gained time from a previous stable move for the current move
477 double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
478 bestMoveInstability *= std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
480 // Stop the search if we have only one legal move, or if available time elapsed
481 if ( rootMoves.size() == 1
482 || Time.elapsed() > Time.optimum() * bestMoveInstability * improvingFactor / 581)
484 // If we are allowed to ponder do not stop the search now but
485 // keep pondering until the GUI sends "ponderhit" or "stop".
487 Threads.stopOnPonderhit = true;
497 mainThread->previousTimeReduction = timeReduction;
499 // If skill level is enabled, swap best PV line with the sub-optimal one
501 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
502 skill.best ? skill.best : skill.pick_best(multiPV)));
508 // search<>() is the main search function for both PV and non-PV nodes
510 template <NodeType NT>
511 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
513 constexpr bool PvNode = NT == PV;
514 const bool rootNode = PvNode && ss->ply == 0;
516 // Check if we have an upcoming move which draws by repetition, or
517 // if the opponent had an alternative move earlier to this position.
518 if ( pos.rule50_count() >= 3
519 && alpha < VALUE_DRAW
521 && pos.has_game_cycle(ss->ply))
528 // Dive into quiescence search when the depth reaches zero
530 return qsearch<NT>(pos, ss, alpha, beta);
532 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
533 assert(PvNode || (alpha == beta - 1));
534 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
535 assert(!(PvNode && cutNode));
536 assert(depth / ONE_PLY * ONE_PLY == depth);
538 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
542 Move ttMove, move, excludedMove, bestMove;
543 Depth extension, newDepth;
544 Value bestValue, value, ttValue, eval, maxValue;
545 bool ttHit, inCheck, givesCheck, improving;
546 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
548 int moveCount, captureCount, quietCount;
550 // Step 1. Initialize node
551 Thread* thisThread = pos.this_thread();
552 inCheck = pos.checkers();
553 Color us = pos.side_to_move();
554 moveCount = captureCount = quietCount = ss->moveCount = 0;
555 bestValue = -VALUE_INFINITE;
556 maxValue = VALUE_INFINITE;
558 // Check for the available remaining time
559 if (thisThread == Threads.main())
560 static_cast<MainThread*>(thisThread)->check_time();
562 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
563 if (PvNode && thisThread->selDepth < ss->ply + 1)
564 thisThread->selDepth = ss->ply + 1;
568 // Step 2. Check for aborted search and immediate draw
569 if ( Threads.stop.load(std::memory_order_relaxed)
570 || pos.is_draw(ss->ply)
571 || ss->ply >= MAX_PLY)
572 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
574 // Step 3. Mate distance pruning. Even if we mate at the next move our score
575 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
576 // a shorter mate was found upward in the tree then there is no need to search
577 // because we will never beat the current alpha. Same logic but with reversed
578 // signs applies also in the opposite condition of being mated instead of giving
579 // mate. In this case return a fail-high score.
580 alpha = std::max(mated_in(ss->ply), alpha);
581 beta = std::min(mate_in(ss->ply+1), beta);
586 assert(0 <= ss->ply && ss->ply < MAX_PLY);
588 (ss+1)->ply = ss->ply + 1;
589 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
590 ss->continuationHistory = thisThread->continuationHistory[NO_PIECE][0].get();
591 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
592 Square prevSq = to_sq((ss-1)->currentMove);
594 // Initialize statScore to zero for the grandchildren of the current position.
595 // So statScore is shared between all grandchildren and only the first grandchild
596 // starts with statScore = 0. Later grandchildren start with the last calculated
597 // statScore of the previous grandchild. This influences the reduction rules in
598 // LMR which are based on the statScore of parent position.
599 (ss+2)->statScore = 0;
601 // Step 4. Transposition table lookup. We don't want the score of a partial
602 // search to overwrite a previous full search TT value, so we use a different
603 // position key in case of an excluded move.
604 excludedMove = ss->excludedMove;
605 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
606 tte = TT.probe(posKey, ttHit);
607 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
608 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
609 : ttHit ? tte->move() : MOVE_NONE;
611 // At non-PV nodes we check for an early TT cutoff
614 && tte->depth() >= depth
615 && ttValue != VALUE_NONE // Possible in case of TT access race
616 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
617 : (tte->bound() & BOUND_UPPER)))
619 // If ttMove is quiet, update move sorting heuristics on TT hit
624 if (!pos.capture_or_promotion(ttMove))
625 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
627 // Extra penalty for a quiet TT move in previous ply when it gets refuted
628 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
629 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
631 // Penalty for a quiet ttMove that fails low
632 else if (!pos.capture_or_promotion(ttMove))
634 int penalty = -stat_bonus(depth);
635 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
636 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
642 // Step 5. Tablebases probe
643 if (!rootNode && TB::Cardinality)
645 int piecesCount = pos.count<ALL_PIECES>();
647 if ( piecesCount <= TB::Cardinality
648 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
649 && pos.rule50_count() == 0
650 && !pos.can_castle(ANY_CASTLING))
653 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
655 if (err != TB::ProbeState::FAIL)
657 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
659 int drawScore = TB::UseRule50 ? 1 : 0;
661 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
662 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
663 : VALUE_DRAW + 2 * wdl * drawScore;
665 Bound b = wdl < -drawScore ? BOUND_UPPER
666 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
668 if ( b == BOUND_EXACT
669 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
671 tte->save(posKey, value_to_tt(value, ss->ply), b,
672 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
673 MOVE_NONE, VALUE_NONE);
680 if (b == BOUND_LOWER)
681 bestValue = value, alpha = std::max(alpha, bestValue);
689 // Step 6. Static evaluation of the position
692 ss->staticEval = eval = VALUE_NONE;
694 goto moves_loop; // Skip early pruning when in check
698 // Never assume anything on values stored in TT
699 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
700 eval = ss->staticEval = evaluate(pos);
702 // Can ttValue be used as a better position evaluation?
703 if ( ttValue != VALUE_NONE
704 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
709 ss->staticEval = eval =
710 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
711 : -(ss-1)->staticEval + 2 * Eval::Tempo;
713 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
717 // Step 7. Razoring (~2 Elo)
719 && depth < 3 * ONE_PLY
720 && eval <= alpha - RazorMargin[depth / ONE_PLY])
722 Value ralpha = alpha - (depth >= 2 * ONE_PLY) * RazorMargin[depth / ONE_PLY];
723 Value v = qsearch<NonPV>(pos, ss, ralpha, ralpha+1);
724 if (depth < 2 * ONE_PLY || v <= ralpha)
728 improving = ss->staticEval >= (ss-2)->staticEval
729 || (ss-2)->staticEval == VALUE_NONE;
731 // Step 8. Futility pruning: child node (~30 Elo)
733 && depth < 7 * ONE_PLY
734 && eval - futility_margin(depth, improving) >= beta
735 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
738 // Step 9. Null move search with verification search (~40 Elo)
740 && (ss-1)->currentMove != MOVE_NULL
741 && (ss-1)->statScore < 22500
743 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
745 && pos.non_pawn_material(us)
746 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
748 assert(eval - beta >= 0);
750 // Null move dynamic reduction based on depth and value
751 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
753 ss->currentMove = MOVE_NULL;
754 ss->continuationHistory = thisThread->continuationHistory[NO_PIECE][0].get();
756 pos.do_null_move(st);
758 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
760 pos.undo_null_move();
762 if (nullValue >= beta)
764 // Do not return unproven mate scores
765 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
768 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 12 * ONE_PLY))
771 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
773 // Do verification search at high depths, with null move pruning disabled
774 // for us, until ply exceeds nmpMinPly.
775 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
776 thisThread->nmpColor = us;
778 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
780 thisThread->nmpMinPly = 0;
787 // Step 10. ProbCut (~10 Elo)
788 // If we have a good enough capture and a reduced search returns a value
789 // much above beta, we can (almost) safely prune the previous move.
791 && depth >= 5 * ONE_PLY
792 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
794 Value rbeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
795 MovePicker mp(pos, ttMove, rbeta - ss->staticEval, &thisThread->captureHistory);
796 int probCutCount = 0;
798 while ( (move = mp.next_move()) != MOVE_NONE
804 ss->currentMove = move;
805 ss->continuationHistory = thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)].get();
807 assert(depth >= 5 * ONE_PLY);
809 pos.do_move(move, st);
811 // Perform a preliminary qsearch to verify that the move holds
812 value = -qsearch<NonPV>(pos, ss+1, -rbeta, -rbeta+1);
814 // If the qsearch held perform the regular search
816 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode);
825 // Step 11. Internal iterative deepening (~2 Elo)
826 if ( depth >= 8 * ONE_PLY
829 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
831 tte = TT.probe(posKey, ttHit);
832 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
833 ttMove = ttHit ? tte->move() : MOVE_NONE;
836 moves_loop: // When in check, search starts from here
838 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, nullptr, (ss-4)->continuationHistory };
839 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
841 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
842 &thisThread->captureHistory,
846 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
850 pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
852 // Step 12. Loop through all pseudo-legal moves until no moves remain
853 // or a beta cutoff occurs.
854 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
858 if (move == excludedMove)
861 // At root obey the "searchmoves" option and skip moves not listed in Root
862 // Move List. As a consequence any illegal move is also skipped. In MultiPV
863 // mode we also skip PV moves which have been already searched and those
864 // of lower "TB rank" if we are in a TB root position.
865 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
866 thisThread->rootMoves.begin() + thisThread->pvLast, move))
869 ss->moveCount = ++moveCount;
871 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
872 sync_cout << "info depth " << depth / ONE_PLY
873 << " currmove " << UCI::move(move, pos.is_chess960())
874 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
876 (ss+1)->pv = nullptr;
878 extension = DEPTH_ZERO;
879 captureOrPromotion = pos.capture_or_promotion(move);
880 movedPiece = pos.moved_piece(move);
881 givesCheck = gives_check(pos, move);
883 moveCountPruning = depth < 16 * ONE_PLY
884 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
886 // Step 13. Extensions (~70 Elo)
888 // Singular extension search (~60 Elo). If all moves but one fail low on a
889 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
890 // then that move is singular and should be extended. To verify this we do
891 // a reduced search on on all the other moves but the ttMove and if the
892 // result is lower than ttValue minus a margin then we will extend the ttMove.
893 if ( depth >= 8 * ONE_PLY
896 && !excludedMove // Recursive singular search is not allowed
897 && ttValue != VALUE_NONE
898 && (tte->bound() & BOUND_LOWER)
899 && tte->depth() >= depth - 3 * ONE_PLY
902 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
903 ss->excludedMove = move;
904 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
905 ss->excludedMove = MOVE_NONE;
910 else if ( givesCheck // Check extension (~2 Elo)
915 // Calculate new depth for this move
916 newDepth = depth - ONE_PLY + extension;
918 // Step 14. Pruning at shallow depth (~170 Elo)
920 && pos.non_pawn_material(us)
921 && bestValue > VALUE_MATED_IN_MAX_PLY)
923 if ( !captureOrPromotion
925 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
927 // Move count based pruning (~30 Elo)
928 if (moveCountPruning)
934 // Reduced depth of the next LMR search
935 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
937 // Countermoves based pruning (~20 Elo)
939 && (lmrDepth < 3 || ((ss - 1)->statScore > 0 && !PvNode))
940 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
941 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
944 // Futility pruning: parent node (~2 Elo)
947 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
950 // Prune moves with negative SEE (~10 Elo)
951 if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
954 else if ( !extension // (~20 Elo)
955 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
959 // Speculative prefetch as early as possible
960 prefetch(TT.first_entry(pos.key_after(move)));
962 // Check for legality just before making the move
963 if (!rootNode && !pos.legal(move))
965 ss->moveCount = --moveCount;
969 if (move == ttMove && captureOrPromotion)
972 // Update the current move (this must be done after singular extension search)
973 ss->currentMove = move;
974 ss->continuationHistory = thisThread->continuationHistory[movedPiece][to_sq(move)].get();
976 // Step 15. Make the move
977 pos.do_move(move, st, givesCheck);
979 // Step 16. Reduced depth search (LMR). If the move fails high it will be
980 // re-searched at full depth.
981 if ( depth >= 3 * ONE_PLY
983 && (!captureOrPromotion || moveCountPruning))
985 Depth r = reduction<PvNode>(improving, depth, moveCount);
987 if (captureOrPromotion) // (~5 Elo)
989 // Decrease reduction by comparing opponent's stat score
990 if ((ss-1)->statScore < 0)
995 // Decrease reduction if opponent's move count is high (~5 Elo)
996 if ((ss-1)->moveCount > 15)
999 // Decrease reduction for exact PV nodes (~0 Elo)
1003 // Increase reduction if ttMove is a capture (~0 Elo)
1007 // Increase reduction for cut nodes (~5 Elo)
1011 // Decrease reduction for moves that escape a capture. Filter out
1012 // castling moves, because they are coded as "king captures rook" and
1013 // hence break make_move(). (~5 Elo)
1014 else if ( type_of(move) == NORMAL
1015 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1018 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1019 + (*contHist[0])[movedPiece][to_sq(move)]
1020 + (*contHist[1])[movedPiece][to_sq(move)]
1021 + (*contHist[3])[movedPiece][to_sq(move)]
1024 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1025 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1028 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1031 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1032 r -= ss->statScore / 20000 * ONE_PLY;
1035 Depth d = std::max(newDepth - std::max(r, DEPTH_ZERO), ONE_PLY);
1037 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1039 doFullDepthSearch = (value > alpha && d != newDepth);
1042 doFullDepthSearch = !PvNode || moveCount > 1;
1044 // Step 17. Full depth search when LMR is skipped or fails high
1045 if (doFullDepthSearch)
1046 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1048 // For PV nodes only, do a full PV search on the first move or after a fail
1049 // high (in the latter case search only if value < beta), otherwise let the
1050 // parent node fail low with value <= alpha and try another move.
1051 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1054 (ss+1)->pv[0] = MOVE_NONE;
1056 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1059 // Step 18. Undo move
1060 pos.undo_move(move);
1062 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1064 // Step 19. Check for a new best move
1065 // Finished searching the move. If a stop occurred, the return value of
1066 // the search cannot be trusted, and we return immediately without
1067 // updating best move, PV and TT.
1068 if (Threads.stop.load(std::memory_order_relaxed))
1073 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1074 thisThread->rootMoves.end(), move);
1076 // PV move or new best move?
1077 if (moveCount == 1 || value > alpha)
1080 rm.selDepth = thisThread->selDepth;
1085 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1086 rm.pv.push_back(*m);
1088 // We record how often the best move has been changed in each
1089 // iteration. This information is used for time management: When
1090 // the best move changes frequently, we allocate some more time.
1091 if (moveCount > 1 && thisThread == Threads.main())
1092 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1095 // All other moves but the PV are set to the lowest value: this
1096 // is not a problem when sorting because the sort is stable and the
1097 // move position in the list is preserved - just the PV is pushed up.
1098 rm.score = -VALUE_INFINITE;
1101 if (value > bestValue)
1109 if (PvNode && !rootNode) // Update pv even in fail-high case
1110 update_pv(ss->pv, move, (ss+1)->pv);
1112 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1116 assert(value >= beta); // Fail high
1123 if (move != bestMove)
1125 if (captureOrPromotion && captureCount < 32)
1126 capturesSearched[captureCount++] = move;
1128 else if (!captureOrPromotion && quietCount < 64)
1129 quietsSearched[quietCount++] = move;
1133 // The following condition would detect a stop only after move loop has been
1134 // completed. But in this case bestValue is valid because we have fully
1135 // searched our subtree, and we can anyhow save the result in TT.
1141 // Step 20. Check for mate and stalemate
1142 // All legal moves have been searched and if there are no legal moves, it
1143 // must be a mate or a stalemate. If we are in a singular extension search then
1144 // return a fail low score.
1146 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1149 bestValue = excludedMove ? alpha
1150 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1153 // Quiet best move: update move sorting heuristics
1154 if (!pos.capture_or_promotion(bestMove))
1155 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1156 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1158 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1160 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1161 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1162 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1164 // Bonus for prior countermove that caused the fail low
1165 else if ( (depth >= 3 * ONE_PLY || PvNode)
1166 && !pos.captured_piece()
1167 && is_ok((ss-1)->currentMove))
1168 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1171 bestValue = std::min(bestValue, maxValue);
1174 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1175 bestValue >= beta ? BOUND_LOWER :
1176 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1177 depth, bestMove, ss->staticEval);
1179 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1185 // qsearch() is the quiescence search function, which is called by the main
1186 // search function with depth zero, or recursively with depth less than ONE_PLY.
1187 template <NodeType NT>
1188 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1190 constexpr bool PvNode = NT == PV;
1192 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1193 assert(PvNode || (alpha == beta - 1));
1194 assert(depth <= DEPTH_ZERO);
1195 assert(depth / ONE_PLY * ONE_PLY == depth);
1201 Move ttMove, move, bestMove;
1203 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1204 bool ttHit, inCheck, givesCheck, evasionPrunable;
1209 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1211 ss->pv[0] = MOVE_NONE;
1214 Thread* thisThread = pos.this_thread();
1215 (ss+1)->ply = ss->ply + 1;
1216 ss->currentMove = bestMove = MOVE_NONE;
1217 ss->continuationHistory = thisThread->continuationHistory[NO_PIECE][0].get();
1218 inCheck = pos.checkers();
1221 // Check for an immediate draw or maximum ply reached
1222 if ( pos.is_draw(ss->ply)
1223 || ss->ply >= MAX_PLY)
1224 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1226 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1228 // Decide whether or not to include checks: this fixes also the type of
1229 // TT entry depth that we are going to use. Note that in qsearch we use
1230 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1231 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1232 : DEPTH_QS_NO_CHECKS;
1233 // Transposition table lookup
1235 tte = TT.probe(posKey, ttHit);
1236 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1237 ttMove = ttHit ? tte->move() : MOVE_NONE;
1241 && tte->depth() >= ttDepth
1242 && ttValue != VALUE_NONE // Only in case of TT access race
1243 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1244 : (tte->bound() & BOUND_UPPER)))
1247 // Evaluate the position statically
1250 ss->staticEval = VALUE_NONE;
1251 bestValue = futilityBase = -VALUE_INFINITE;
1257 // Never assume anything on values stored in TT
1258 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1259 ss->staticEval = bestValue = evaluate(pos);
1261 // Can ttValue be used as a better position evaluation?
1262 if ( ttValue != VALUE_NONE
1263 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1264 bestValue = ttValue;
1267 ss->staticEval = bestValue =
1268 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1269 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1271 // Stand pat. Return immediately if static value is at least beta
1272 if (bestValue >= beta)
1275 tte->save(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1276 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1281 if (PvNode && bestValue > alpha)
1284 futilityBase = bestValue + 128;
1287 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, nullptr, (ss-4)->continuationHistory };
1289 // Initialize a MovePicker object for the current position, and prepare
1290 // to search the moves. Because the depth is <= 0 here, only captures,
1291 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1293 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1294 &thisThread->captureHistory,
1296 to_sq((ss-1)->currentMove));
1298 // Loop through the moves until no moves remain or a beta cutoff occurs
1299 while ((move = mp.next_move()) != MOVE_NONE)
1301 assert(is_ok(move));
1303 givesCheck = gives_check(pos, move);
1310 && futilityBase > -VALUE_KNOWN_WIN
1311 && !pos.advanced_pawn_push(move))
1313 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1315 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1317 if (futilityValue <= alpha)
1319 bestValue = std::max(bestValue, futilityValue);
1323 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1325 bestValue = std::max(bestValue, futilityBase);
1330 // Detect non-capture evasions that are candidates to be pruned
1331 evasionPrunable = inCheck
1332 && (depth != DEPTH_ZERO || moveCount > 2)
1333 && bestValue > VALUE_MATED_IN_MAX_PLY
1334 && !pos.capture(move);
1336 // Don't search moves with negative SEE values
1337 if ( (!inCheck || evasionPrunable)
1338 && !pos.see_ge(move))
1341 // Speculative prefetch as early as possible
1342 prefetch(TT.first_entry(pos.key_after(move)));
1344 // Check for legality just before making the move
1345 if (!pos.legal(move))
1351 ss->currentMove = move;
1352 ss->continuationHistory = thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)].get();
1354 // Make and search the move
1355 pos.do_move(move, st, givesCheck);
1356 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1357 pos.undo_move(move);
1359 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1361 // Check for a new best move
1362 if (value > bestValue)
1368 if (PvNode) // Update pv even in fail-high case
1369 update_pv(ss->pv, move, (ss+1)->pv);
1371 if (PvNode && value < beta) // Update alpha here!
1378 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1379 ttDepth, move, ss->staticEval);
1387 // All legal moves have been searched. A special case: If we're in check
1388 // and no legal moves were found, it is checkmate.
1389 if (inCheck && bestValue == -VALUE_INFINITE)
1390 return mated_in(ss->ply); // Plies to mate from the root
1392 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1393 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1394 ttDepth, bestMove, ss->staticEval);
1396 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1402 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1403 // "plies to mate from the current position". Non-mate scores are unchanged.
1404 // The function is called before storing a value in the transposition table.
1406 Value value_to_tt(Value v, int ply) {
1408 assert(v != VALUE_NONE);
1410 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1411 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1415 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1416 // from the transposition table (which refers to the plies to mate/be mated
1417 // from current position) to "plies to mate/be mated from the root".
1419 Value value_from_tt(Value v, int ply) {
1421 return v == VALUE_NONE ? VALUE_NONE
1422 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1423 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1427 // update_pv() adds current move and appends child pv[]
1429 void update_pv(Move* pv, Move move, Move* childPv) {
1431 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1437 // update_continuation_histories() updates histories of the move pairs formed
1438 // by moves at ply -1, -2, and -4 with current move.
1440 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1442 for (int i : {1, 2, 4})
1443 if (is_ok((ss-i)->currentMove))
1444 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1448 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1450 void update_capture_stats(const Position& pos, Move move,
1451 Move* captures, int captureCnt, int bonus) {
1453 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1454 Piece moved_piece = pos.moved_piece(move);
1455 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1457 if (pos.capture_or_promotion(move))
1458 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1460 // Decrease all the other played capture moves
1461 for (int i = 0; i < captureCnt; ++i)
1463 moved_piece = pos.moved_piece(captures[i]);
1464 captured = type_of(pos.piece_on(to_sq(captures[i])));
1465 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1470 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1472 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1473 Move* quiets, int quietsCnt, int bonus) {
1475 if (ss->killers[0] != move)
1477 ss->killers[1] = ss->killers[0];
1478 ss->killers[0] = move;
1481 Color us = pos.side_to_move();
1482 Thread* thisThread = pos.this_thread();
1483 thisThread->mainHistory[us][from_to(move)] << bonus;
1484 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1486 if (is_ok((ss-1)->currentMove))
1488 Square prevSq = to_sq((ss-1)->currentMove);
1489 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1492 // Decrease all the other played quiet moves
1493 for (int i = 0; i < quietsCnt; ++i)
1495 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1496 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1500 // When playing with strength handicap, choose best move among a set of RootMoves
1501 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1503 Move Skill::pick_best(size_t multiPV) {
1505 const RootMoves& rootMoves = Threads.main()->rootMoves;
1506 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1508 // RootMoves are already sorted by score in descending order
1509 Value topScore = rootMoves[0].score;
1510 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1511 int weakness = 120 - 2 * level;
1512 int maxScore = -VALUE_INFINITE;
1514 // Choose best move. For each move score we add two terms, both dependent on
1515 // weakness. One is deterministic and bigger for weaker levels, and one is
1516 // random. Then we choose the move with the resulting highest score.
1517 for (size_t i = 0; i < multiPV; ++i)
1519 // This is our magic formula
1520 int push = ( weakness * int(topScore - rootMoves[i].score)
1521 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1523 if (rootMoves[i].score + push >= maxScore)
1525 maxScore = rootMoves[i].score + push;
1526 best = rootMoves[i].pv[0];
1535 /// MainThread::check_time() is used to print debug info and, more importantly,
1536 /// to detect when we are out of available time and thus stop the search.
1538 void MainThread::check_time() {
1543 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1544 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1546 static TimePoint lastInfoTime = now();
1548 TimePoint elapsed = Time.elapsed();
1549 TimePoint tick = Limits.startTime + elapsed;
1551 if (tick - lastInfoTime >= 1000)
1553 lastInfoTime = tick;
1557 // We should not stop pondering until told so by the GUI
1561 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1562 || (Limits.movetime && elapsed >= Limits.movetime)
1563 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1564 Threads.stop = true;
1568 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1569 /// that all (if any) unsearched PV lines are sent using a previous search score.
1571 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1573 std::stringstream ss;
1574 TimePoint elapsed = Time.elapsed() + 1;
1575 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1576 size_t pvIdx = pos.this_thread()->pvIdx;
1577 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1578 uint64_t nodesSearched = Threads.nodes_searched();
1579 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1581 for (size_t i = 0; i < multiPV; ++i)
1583 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1585 if (depth == ONE_PLY && !updated)
1588 Depth d = updated ? depth : depth - ONE_PLY;
1589 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1591 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1592 v = tb ? rootMoves[i].tbScore : v;
1594 if (ss.rdbuf()->in_avail()) // Not at first line
1598 << " depth " << d / ONE_PLY
1599 << " seldepth " << rootMoves[i].selDepth
1600 << " multipv " << i + 1
1601 << " score " << UCI::value(v);
1603 if (!tb && i == pvIdx)
1604 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1606 ss << " nodes " << nodesSearched
1607 << " nps " << nodesSearched * 1000 / elapsed;
1609 if (elapsed > 1000) // Earlier makes little sense
1610 ss << " hashfull " << TT.hashfull();
1612 ss << " tbhits " << tbHits
1613 << " time " << elapsed
1616 for (Move m : rootMoves[i].pv)
1617 ss << " " << UCI::move(m, pos.is_chess960());
1624 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1625 /// before exiting the search, for instance, in case we stop the search during a
1626 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1627 /// otherwise in case of 'ponder on' we have nothing to think on.
1629 bool RootMove::extract_ponder_from_tt(Position& pos) {
1634 assert(pv.size() == 1);
1639 pos.do_move(pv[0], st);
1640 TTEntry* tte = TT.probe(pos.key(), ttHit);
1644 Move m = tte->move(); // Local copy to be SMP safe
1645 if (MoveList<LEGAL>(pos).contains(m))
1649 pos.undo_move(pv[0]);
1650 return pv.size() > 1;
1653 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1656 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1657 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1658 Cardinality = int(Options["SyzygyProbeLimit"]);
1659 bool dtz_available = true;
1661 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1662 // ProbeDepth == DEPTH_ZERO
1663 if (Cardinality > MaxCardinality)
1665 Cardinality = MaxCardinality;
1666 ProbeDepth = DEPTH_ZERO;
1669 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1671 // Rank moves using DTZ tables
1672 RootInTB = root_probe(pos, rootMoves);
1676 // DTZ tables are missing; try to rank moves using WDL tables
1677 dtz_available = false;
1678 RootInTB = root_probe_wdl(pos, rootMoves);
1684 // Sort moves according to TB rank
1685 std::sort(rootMoves.begin(), rootMoves.end(),
1686 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1688 // Probe during search only if DTZ is not available and we are winning
1689 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1694 // Assign the same rank to all moves
1695 for (auto& m : rootMoves)