2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2018 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
54 namespace TB = Tablebases;
58 using namespace Search;
62 // Different node types, used as a template parameter
63 enum NodeType { NonPV, PV };
65 // Sizes and phases of the skip-blocks, used for distributing search depths across the threads
66 const int SkipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
67 const int SkipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
69 // Razor and futility margins
70 const int RazorMargin1 = 590;
71 const int RazorMargin2 = 604;
72 Value futility_margin(Depth d) { return Value(150 * 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 : d * d + 2 * d - 2;
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, bool skipEarlyPruning);
102 template <NodeType NT, bool InCheck>
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
162 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
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();
290 std::memset(ss-4, 0, 7 * sizeof(Stack));
291 for (int i = 4; i > 0; i--)
292 (ss-i)->contHistory = this->contHistory[NO_PIECE][0].get(); // Use as sentinel
294 bestValue = delta = alpha = -VALUE_INFINITE;
295 beta = VALUE_INFINITE;
298 mainThread->bestMoveChanges = 0, mainThread->failedLow = false;
300 size_t multiPV = Options["MultiPV"];
301 Skill skill(Options["Skill Level"]);
303 // When playing with strength handicap enable MultiPV search that we will
304 // use behind the scenes to retrieve a set of possible moves.
306 multiPV = std::max(multiPV, (size_t)4);
308 multiPV = std::min(multiPV, rootMoves.size());
310 int ct = Options["Contempt"] * PawnValueEg / 100; // From centipawns
311 Eval::Contempt = (us == WHITE ? make_score(ct, ct / 2)
312 : -make_score(ct, ct / 2));
314 // Iterative deepening loop until requested to stop or the target depth is reached
315 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
317 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
319 // Distribute search depths across the helper threads
322 int i = (idx - 1) % 20;
323 if (((rootDepth / ONE_PLY + rootPos.game_ply() + SkipPhase[i]) / SkipSize[i]) % 2)
324 continue; // Retry with an incremented rootDepth
327 // Age out PV variability metric
329 mainThread->bestMoveChanges *= 0.517, mainThread->failedLow = false;
331 // Save the last iteration's scores before first PV line is searched and
332 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
333 for (RootMove& rm : rootMoves)
334 rm.previousScore = rm.score;
336 // MultiPV loop. We perform a full root search for each PV line
337 for (PVIdx = 0; PVIdx < multiPV && !Threads.stop; ++PVIdx)
339 // Reset UCI info selDepth for each depth and each PV line
342 // Reset aspiration window starting size
343 if (rootDepth >= 5 * ONE_PLY)
346 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
347 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
349 ct = Options["Contempt"] * PawnValueEg / 100; // From centipawns
351 // Adjust contempt based on current bestValue (dynamic contempt)
352 int sign = (bestValue > 0) - (bestValue < 0);
353 ct += bestValue > 500 ? 70 :
354 bestValue < -500 ? -70 :
355 bestValue / 10 + sign * int(std::round(3.22 * log(1 + abs(bestValue))));
357 Eval::Contempt = (us == WHITE ? make_score(ct, ct / 2)
358 : -make_score(ct, ct / 2));
361 // Start with a small aspiration window and, in the case of a fail
362 // high/low, re-search with a bigger window until we don't fail
366 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
368 // Bring the best move to the front. It is critical that sorting
369 // is done with a stable algorithm because all the values but the
370 // first and eventually the new best one are set to -VALUE_INFINITE
371 // and we want to keep the same order for all the moves except the
372 // new PV that goes to the front. Note that in case of MultiPV
373 // search the already searched PV lines are preserved.
374 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
376 // If search has been stopped, we break immediately. Sorting is
377 // safe because RootMoves is still valid, although it refers to
378 // the previous iteration.
382 // When failing high/low give some update (without cluttering
383 // the UI) before a re-search.
386 && (bestValue <= alpha || bestValue >= beta)
387 && Time.elapsed() > 3000)
388 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
390 // In case of failing low/high increase aspiration window and
391 // re-search, otherwise exit the loop.
392 if (bestValue <= alpha)
394 beta = (alpha + beta) / 2;
395 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
399 mainThread->failedLow = true;
400 Threads.stopOnPonderhit = false;
403 else if (bestValue >= beta)
404 beta = std::min(bestValue + delta, VALUE_INFINITE);
408 delta += delta / 4 + 5;
410 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
413 // Sort the PV lines searched so far and update the GUI
414 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
417 && (Threads.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000))
418 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
422 completedDepth = rootDepth;
424 if (rootMoves[0].pv[0] != lastBestMove) {
425 lastBestMove = rootMoves[0].pv[0];
426 lastBestMoveDepth = rootDepth;
429 // Have we found a "mate in x"?
431 && bestValue >= VALUE_MATE_IN_MAX_PLY
432 && VALUE_MATE - bestValue <= 2 * Limits.mate)
438 // If skill level is enabled and time is up, pick a sub-optimal best move
439 if (skill.enabled() && skill.time_to_pick(rootDepth))
440 skill.pick_best(multiPV);
442 // Do we have time for the next iteration? Can we stop searching now?
443 if ( Limits.use_time_management()
445 && !Threads.stopOnPonderhit)
447 const int F[] = { mainThread->failedLow,
448 bestValue - mainThread->previousScore };
450 int improvingFactor = std::max(246, std::min(832, 306 + 119 * F[0] - 6 * F[1]));
452 // If the bestMove is stable over several iterations, reduce time accordingly
454 for (int i : {3, 4, 5})
455 if (lastBestMoveDepth * i < completedDepth)
456 timeReduction *= 1.25;
458 // Use part of the gained time from a previous stable move for the current move
459 double unstablePvFactor = 1.0 + mainThread->bestMoveChanges;
460 unstablePvFactor *= std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
462 // Stop the search if we have only one legal move, or if available time elapsed
463 if ( rootMoves.size() == 1
464 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 581)
466 // If we are allowed to ponder do not stop the search now but
467 // keep pondering until the GUI sends "ponderhit" or "stop".
469 Threads.stopOnPonderhit = true;
479 mainThread->previousTimeReduction = timeReduction;
481 // If skill level is enabled, swap best PV line with the sub-optimal one
483 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
484 skill.best ? skill.best : skill.pick_best(multiPV)));
490 // search<>() is the main search function for both PV and non-PV nodes
492 template <NodeType NT>
493 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
495 const bool PvNode = NT == PV;
496 const bool rootNode = PvNode && ss->ply == 0;
498 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
499 assert(PvNode || (alpha == beta - 1));
500 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
501 assert(!(PvNode && cutNode));
502 assert(depth / ONE_PLY * ONE_PLY == depth);
504 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
508 Move ttMove, move, excludedMove, bestMove;
509 Depth extension, newDepth;
510 Value bestValue, value, ttValue, eval, maxValue;
511 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
512 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
514 int moveCount, captureCount, quietCount;
516 // Step 1. Initialize node
517 Thread* thisThread = pos.this_thread();
518 inCheck = pos.checkers();
519 moveCount = captureCount = quietCount = ss->moveCount = 0;
520 bestValue = -VALUE_INFINITE;
521 maxValue = VALUE_INFINITE;
523 // Check for the available remaining time
524 if (thisThread == Threads.main())
525 static_cast<MainThread*>(thisThread)->check_time();
527 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
528 if (PvNode && thisThread->selDepth < ss->ply + 1)
529 thisThread->selDepth = ss->ply + 1;
533 // Step 2. Check for aborted search and immediate draw
534 if ( Threads.stop.load(std::memory_order_relaxed)
535 || pos.is_draw(ss->ply)
536 || ss->ply >= MAX_PLY)
537 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
539 // Step 3. Mate distance pruning. Even if we mate at the next move our score
540 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
541 // a shorter mate was found upward in the tree then there is no need to search
542 // because we will never beat the current alpha. Same logic but with reversed
543 // signs applies also in the opposite condition of being mated instead of giving
544 // mate. In this case return a fail-high score.
545 alpha = std::max(mated_in(ss->ply), alpha);
546 beta = std::min(mate_in(ss->ply+1), beta);
551 assert(0 <= ss->ply && ss->ply < MAX_PLY);
553 (ss+1)->ply = ss->ply + 1;
554 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
555 ss->contHistory = thisThread->contHistory[NO_PIECE][0].get();
556 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
557 Square prevSq = to_sq((ss-1)->currentMove);
559 // Initialize statScore to zero for the grandchildren of the current position.
560 // So statScore is shared between all grandchildren and only the first grandchild
561 // starts with statScore = 0. Later grandchildren start with the last calculated
562 // statScore of the previous grandchild. This influences the reduction rules in
563 // LMR which are based on the statScore of parent position.
564 (ss+2)->statScore = 0;
566 // Step 4. Transposition table lookup. We don't want the score of a partial
567 // search to overwrite a previous full search TT value, so we use a different
568 // position key in case of an excluded move.
569 excludedMove = ss->excludedMove;
570 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
571 tte = TT.probe(posKey, ttHit);
572 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
573 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
574 : ttHit ? tte->move() : MOVE_NONE;
576 // At non-PV nodes we check for an early TT cutoff
579 && tte->depth() >= depth
580 && ttValue != VALUE_NONE // Possible in case of TT access race
581 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
582 : (tte->bound() & BOUND_UPPER)))
584 // If ttMove is quiet, update move sorting heuristics on TT hit
589 if (!pos.capture_or_promotion(ttMove))
590 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
592 // Extra penalty for a quiet TT move in previous ply when it gets refuted
593 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
594 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
596 // Penalty for a quiet ttMove that fails low
597 else if (!pos.capture_or_promotion(ttMove))
599 int penalty = -stat_bonus(depth);
600 thisThread->mainHistory[pos.side_to_move()][from_to(ttMove)] << penalty;
601 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
607 // Step 5. Tablebases probe
608 if (!rootNode && TB::Cardinality)
610 int piecesCount = pos.count<ALL_PIECES>();
612 if ( piecesCount <= TB::Cardinality
613 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
614 && pos.rule50_count() == 0
615 && !pos.can_castle(ANY_CASTLING))
618 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
620 if (err != TB::ProbeState::FAIL)
622 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
624 int drawScore = TB::UseRule50 ? 1 : 0;
626 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
627 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
628 : VALUE_DRAW + 2 * wdl * drawScore;
630 Bound b = wdl < -drawScore ? BOUND_UPPER
631 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
633 if ( b == BOUND_EXACT
634 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
636 tte->save(posKey, value_to_tt(value, ss->ply), b,
637 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
638 MOVE_NONE, VALUE_NONE, TT.generation());
645 if (b == BOUND_LOWER)
646 bestValue = value, alpha = std::max(alpha, bestValue);
654 // Step 6. Evaluate the position statically
657 ss->staticEval = eval = VALUE_NONE;
662 // Never assume anything on values stored in TT
663 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
664 eval = ss->staticEval = evaluate(pos);
666 // Can ttValue be used as a better position evaluation?
667 if ( ttValue != VALUE_NONE
668 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
673 ss->staticEval = eval =
674 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
675 : -(ss-1)->staticEval + 2 * Eval::Tempo;
677 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
678 ss->staticEval, TT.generation());
681 if (skipEarlyPruning || !pos.non_pawn_material(pos.side_to_move()))
684 // Step 7. Razoring (skipped when in check)
686 && depth <= 2 * ONE_PLY)
688 if ( depth == ONE_PLY
689 && eval + RazorMargin1 <= alpha)
690 return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
692 else if (eval + RazorMargin2 <= alpha)
694 Value ralpha = alpha - RazorMargin2;
695 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1);
702 // Step 8. Futility pruning: child node (skipped when in check)
704 && depth < 7 * ONE_PLY
705 && eval - futility_margin(depth) >= beta
706 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
709 // Step 9. Null move search with verification search
712 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
713 && (ss->ply >= thisThread->nmp_ply || ss->ply % 2 != thisThread->nmp_odd))
715 assert(eval - beta >= 0);
717 // Null move dynamic reduction based on depth and value
718 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
720 ss->currentMove = MOVE_NULL;
721 ss->contHistory = thisThread->contHistory[NO_PIECE][0].get();
723 pos.do_null_move(st);
724 Value nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1)
725 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
726 pos.undo_null_move();
728 if (nullValue >= beta)
730 // Do not return unproven mate scores
731 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
734 if (abs(beta) < VALUE_KNOWN_WIN && (depth < 12 * ONE_PLY || thisThread->nmp_ply))
737 // Do verification search at high depths. Disable null move pruning
738 // for side to move for the first part of the remaining search tree.
739 thisThread->nmp_ply = ss->ply + 3 * (depth-R) / 4;
740 thisThread->nmp_odd = ss->ply % 2;
742 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
743 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
745 thisThread->nmp_odd = thisThread->nmp_ply = 0;
752 // Step 10. ProbCut (skipped when in check)
753 // If we have a good enough capture and a reduced search returns a value
754 // much above beta, we can (almost) safely prune the previous move.
756 && depth >= 5 * ONE_PLY
757 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
759 assert(is_ok((ss-1)->currentMove));
761 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
762 MovePicker mp(pos, ttMove, rbeta - ss->staticEval, &thisThread->captureHistory);
763 int probCutCount = 0;
765 while ( (move = mp.next_move()) != MOVE_NONE
766 && probCutCount < depth / ONE_PLY - 3)
771 ss->currentMove = move;
772 ss->contHistory = thisThread->contHistory[pos.moved_piece(move)][to_sq(move)].get();
774 assert(depth >= 5 * ONE_PLY);
776 pos.do_move(move, st);
778 // Perform a preliminary search at depth 1 to verify that the move holds.
779 // We will only do this search if the depth is not 5, thus avoiding two
780 // searches at depth 1 in a row.
781 if (depth != 5 * ONE_PLY)
782 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, ONE_PLY, !cutNode, true);
784 // If the first search was skipped or was performed and held, perform
785 // the regular search.
786 if (depth == 5 * ONE_PLY || value >= rbeta)
787 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode, false);
796 // Step 11. Internal iterative deepening (skipped when in check)
797 if ( depth >= 6 * ONE_PLY
799 && (PvNode || ss->staticEval + 256 >= beta))
801 Depth d = 3 * depth / 4 - 2 * ONE_PLY;
802 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
804 tte = TT.probe(posKey, ttHit);
805 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
806 ttMove = ttHit ? tte->move() : MOVE_NONE;
809 moves_loop: // When in check, search starts from here
811 const PieceToHistory* contHist[] = { (ss-1)->contHistory, (ss-2)->contHistory, nullptr, (ss-4)->contHistory };
812 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
814 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory, contHist, countermove, ss->killers);
815 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
816 improving = ss->staticEval >= (ss-2)->staticEval
817 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
818 ||(ss-2)->staticEval == VALUE_NONE;
820 singularExtensionNode = !rootNode
821 && depth >= 8 * ONE_PLY
822 && ttMove != MOVE_NONE
823 && ttValue != VALUE_NONE
824 && !excludedMove // Recursive singular search is not allowed
825 && (tte->bound() & BOUND_LOWER)
826 && tte->depth() >= depth - 3 * ONE_PLY;
829 pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
831 // Step 12. Loop through all pseudo-legal moves until no moves remain
832 // or a beta cutoff occurs.
833 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
837 if (move == excludedMove)
840 // At root obey the "searchmoves" option and skip moves not listed in Root
841 // Move List. As a consequence any illegal move is also skipped. In MultiPV
842 // mode we also skip PV moves which have been already searched.
843 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
844 thisThread->rootMoves.end(), move))
847 ss->moveCount = ++moveCount;
849 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
850 sync_cout << "info depth " << depth / ONE_PLY
851 << " currmove " << UCI::move(move, pos.is_chess960())
852 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
854 (ss+1)->pv = nullptr;
856 extension = DEPTH_ZERO;
857 captureOrPromotion = pos.capture_or_promotion(move);
858 movedPiece = pos.moved_piece(move);
859 givesCheck = gives_check(pos, move);
861 moveCountPruning = depth < 16 * ONE_PLY
862 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
864 // Step 13. Extensions
866 // Singular extension search. If all moves but one fail low on a search
867 // of (alpha-s, beta-s), and just one fails high on (alpha, beta), then
868 // that move is singular and should be extended. To verify this we do a
869 // reduced search on on all the other moves but the ttMove and if the
870 // result is lower than ttValue minus a margin then we will extend the ttMove.
871 if ( singularExtensionNode
875 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
876 ss->excludedMove = move;
877 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode, true);
878 ss->excludedMove = MOVE_NONE;
883 else if ( givesCheck // Check extension
888 // Calculate new depth for this move
889 newDepth = depth - ONE_PLY + extension;
891 // Step 14. Pruning at shallow depth
893 && pos.non_pawn_material(pos.side_to_move())
894 && bestValue > VALUE_MATED_IN_MAX_PLY)
896 if ( !captureOrPromotion
898 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
900 // Move count based pruning
901 if (moveCountPruning)
907 // Reduced depth of the next LMR search
908 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
910 // Countermoves based pruning
912 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
913 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
916 // Futility pruning: parent node
919 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
922 // Prune moves with negative SEE
924 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
927 else if ( depth < 7 * ONE_PLY
929 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
933 // Speculative prefetch as early as possible
934 prefetch(TT.first_entry(pos.key_after(move)));
936 // Check for legality just before making the move
937 if (!rootNode && !pos.legal(move))
939 ss->moveCount = --moveCount;
943 if (move == ttMove && captureOrPromotion)
946 // Update the current move (this must be done after singular extension search)
947 ss->currentMove = move;
948 ss->contHistory = thisThread->contHistory[movedPiece][to_sq(move)].get();
950 // Step 15. Make the move
951 pos.do_move(move, st, givesCheck);
953 // Step 16. Reduced depth search (LMR). If the move fails high it will be
954 // re-searched at full depth.
955 if ( depth >= 3 * ONE_PLY
957 && (!captureOrPromotion || moveCountPruning))
959 Depth r = reduction<PvNode>(improving, depth, moveCount);
961 if (captureOrPromotion)
962 r -= r ? ONE_PLY : DEPTH_ZERO;
965 // Decrease reduction if opponent's move count is high
966 if ((ss-1)->moveCount > 15)
969 // Decrease reduction for exact PV nodes
973 // Increase reduction if ttMove is a capture
977 // Increase reduction for cut nodes
981 // Decrease reduction for moves that escape a capture. Filter out
982 // castling moves, because they are coded as "king captures rook" and
983 // hence break make_move().
984 else if ( type_of(move) == NORMAL
985 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
988 ss->statScore = thisThread->mainHistory[~pos.side_to_move()][from_to(move)]
989 + (*contHist[0])[movedPiece][to_sq(move)]
990 + (*contHist[1])[movedPiece][to_sq(move)]
991 + (*contHist[3])[movedPiece][to_sq(move)]
994 // Decrease/increase reduction by comparing opponent's stat score
995 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
998 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1001 // Decrease/increase reduction for moves with a good/bad history
1002 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->statScore / 20000) * ONE_PLY);
1005 Depth d = std::max(newDepth - r, ONE_PLY);
1007 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
1009 doFullDepthSearch = (value > alpha && d != newDepth);
1012 doFullDepthSearch = !PvNode || moveCount > 1;
1014 // Step 17. Full depth search when LMR is skipped or fails high
1015 if (doFullDepthSearch)
1016 value = newDepth < ONE_PLY ?
1017 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha)
1018 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha)
1019 : - 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 = newDepth < ONE_PLY ?
1030 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha)
1031 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha)
1032 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1035 // Step 18. Undo move
1036 pos.undo_move(move);
1038 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1040 // Step 19. Check for a new best move
1041 // Finished searching the move. If a stop occurred, the return value of
1042 // the search cannot be trusted, and we return immediately without
1043 // updating best move, PV and TT.
1044 if (Threads.stop.load(std::memory_order_relaxed))
1049 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1050 thisThread->rootMoves.end(), move);
1052 // PV move or new best move?
1053 if (moveCount == 1 || value > alpha)
1056 rm.selDepth = thisThread->selDepth;
1061 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1062 rm.pv.push_back(*m);
1064 // We record how often the best move has been changed in each
1065 // iteration. This information is used for time management: When
1066 // the best move changes frequently, we allocate some more time.
1067 if (moveCount > 1 && thisThread == Threads.main())
1068 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1071 // All other moves but the PV are set to the lowest value: this
1072 // is not a problem when sorting because the sort is stable and the
1073 // move position in the list is preserved - just the PV is pushed up.
1074 rm.score = -VALUE_INFINITE;
1077 if (value > bestValue)
1085 if (PvNode && !rootNode) // Update pv even in fail-high case
1086 update_pv(ss->pv, move, (ss+1)->pv);
1088 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1092 assert(value >= beta); // Fail high
1098 if (move != bestMove)
1100 if (captureOrPromotion && captureCount < 32)
1101 capturesSearched[captureCount++] = move;
1103 else if (!captureOrPromotion && quietCount < 64)
1104 quietsSearched[quietCount++] = move;
1108 // The following condition would detect a stop only after move loop has been
1109 // completed. But in this case bestValue is valid because we have fully
1110 // searched our subtree, and we can anyhow save the result in TT.
1116 // Step 20. Check for mate and stalemate
1117 // All legal moves have been searched and if there are no legal moves, it
1118 // must be a mate or a stalemate. If we are in a singular extension search then
1119 // return a fail low score.
1121 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1124 bestValue = excludedMove ? alpha
1125 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1128 // Quiet best move: update move sorting heuristics
1129 if (!pos.capture_or_promotion(bestMove))
1130 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1132 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth));
1134 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1135 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1136 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1138 // Bonus for prior countermove that caused the fail low
1139 else if ( depth >= 3 * ONE_PLY
1140 && !pos.captured_piece()
1141 && is_ok((ss-1)->currentMove))
1142 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1145 bestValue = std::min(bestValue, maxValue);
1148 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1149 bestValue >= beta ? BOUND_LOWER :
1150 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1151 depth, bestMove, ss->staticEval, TT.generation());
1153 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1159 // qsearch() is the quiescence search function, which is called by the main
1160 // search function with depth zero, or recursively with depth less than ONE_PLY.
1162 template <NodeType NT, bool InCheck>
1163 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1165 const bool PvNode = NT == PV;
1167 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1168 assert(PvNode || (alpha == beta - 1));
1169 assert(depth <= DEPTH_ZERO);
1170 assert(depth / ONE_PLY * ONE_PLY == depth);
1171 assert(InCheck == bool(pos.checkers()));
1177 Move ttMove, move, bestMove;
1179 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1180 bool ttHit, givesCheck, evasionPrunable;
1185 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1187 ss->pv[0] = MOVE_NONE;
1190 (ss+1)->ply = ss->ply + 1;
1191 ss->currentMove = bestMove = MOVE_NONE;
1194 // Check for an immediate draw or maximum ply reached
1195 if ( pos.is_draw(ss->ply)
1196 || ss->ply >= MAX_PLY)
1197 return (ss->ply >= MAX_PLY && !InCheck) ? evaluate(pos) : VALUE_DRAW;
1199 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1201 // Decide whether or not to include checks: this fixes also the type of
1202 // TT entry depth that we are going to use. Note that in qsearch we use
1203 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1204 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1205 : DEPTH_QS_NO_CHECKS;
1206 // Transposition table lookup
1208 tte = TT.probe(posKey, ttHit);
1209 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1210 ttMove = ttHit ? tte->move() : MOVE_NONE;
1214 && tte->depth() >= ttDepth
1215 && ttValue != VALUE_NONE // Only in case of TT access race
1216 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1217 : (tte->bound() & BOUND_UPPER)))
1220 // Evaluate the position statically
1223 ss->staticEval = VALUE_NONE;
1224 bestValue = futilityBase = -VALUE_INFINITE;
1230 // Never assume anything on values stored in TT
1231 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1232 ss->staticEval = bestValue = evaluate(pos);
1234 // Can ttValue be used as a better position evaluation?
1235 if ( ttValue != VALUE_NONE
1236 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1237 bestValue = ttValue;
1240 ss->staticEval = bestValue =
1241 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1242 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1244 // Stand pat. Return immediately if static value is at least beta
1245 if (bestValue >= beta)
1248 tte->save(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1249 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1254 if (PvNode && bestValue > alpha)
1257 futilityBase = bestValue + 128;
1260 // Initialize a MovePicker object for the current position, and prepare
1261 // to search the moves. Because the depth is <= 0 here, only captures,
1262 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1264 MovePicker mp(pos, ttMove, depth, &pos.this_thread()->mainHistory, &pos.this_thread()->captureHistory, to_sq((ss-1)->currentMove));
1266 // Loop through the moves until no moves remain or a beta cutoff occurs
1267 while ((move = mp.next_move()) != MOVE_NONE)
1269 assert(is_ok(move));
1271 givesCheck = gives_check(pos, move);
1278 && futilityBase > -VALUE_KNOWN_WIN
1279 && !pos.advanced_pawn_push(move))
1281 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1283 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1285 if (futilityValue <= alpha)
1287 bestValue = std::max(bestValue, futilityValue);
1291 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1293 bestValue = std::max(bestValue, futilityBase);
1298 // Detect non-capture evasions that are candidates to be pruned
1299 evasionPrunable = InCheck
1300 && (depth != DEPTH_ZERO || moveCount > 2)
1301 && bestValue > VALUE_MATED_IN_MAX_PLY
1302 && !pos.capture(move);
1304 // Don't search moves with negative SEE values
1305 if ( (!InCheck || evasionPrunable)
1306 && !pos.see_ge(move))
1309 // Speculative prefetch as early as possible
1310 prefetch(TT.first_entry(pos.key_after(move)));
1312 // Check for legality just before making the move
1313 if (!pos.legal(move))
1319 ss->currentMove = move;
1321 // Make and search the move
1322 pos.do_move(move, st, givesCheck);
1323 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1324 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1325 pos.undo_move(move);
1327 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1329 // Check for a new best move
1330 if (value > bestValue)
1336 if (PvNode) // Update pv even in fail-high case
1337 update_pv(ss->pv, move, (ss+1)->pv);
1339 if (PvNode && value < beta) // Update alpha here!
1346 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1347 ttDepth, move, ss->staticEval, TT.generation());
1355 // All legal moves have been searched. A special case: If we're in check
1356 // and no legal moves were found, it is checkmate.
1357 if (InCheck && bestValue == -VALUE_INFINITE)
1358 return mated_in(ss->ply); // Plies to mate from the root
1360 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1361 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1362 ttDepth, bestMove, ss->staticEval, TT.generation());
1364 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1370 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1371 // "plies to mate from the current position". Non-mate scores are unchanged.
1372 // The function is called before storing a value in the transposition table.
1374 Value value_to_tt(Value v, int ply) {
1376 assert(v != VALUE_NONE);
1378 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1379 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1383 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1384 // from the transposition table (which refers to the plies to mate/be mated
1385 // from current position) to "plies to mate/be mated from the root".
1387 Value value_from_tt(Value v, int ply) {
1389 return v == VALUE_NONE ? VALUE_NONE
1390 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1391 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1395 // update_pv() adds current move and appends child pv[]
1397 void update_pv(Move* pv, Move move, Move* childPv) {
1399 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1405 // update_continuation_histories() updates histories of the move pairs formed
1406 // by moves at ply -1, -2, and -4 with current move.
1408 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1410 for (int i : {1, 2, 4})
1411 if (is_ok((ss-i)->currentMove))
1412 (*(ss-i)->contHistory)[pc][to] << bonus;
1416 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1418 void update_capture_stats(const Position& pos, Move move,
1419 Move* captures, int captureCnt, int bonus) {
1421 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1422 Piece moved_piece = pos.moved_piece(move);
1423 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1424 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1426 // Decrease all the other played capture moves
1427 for (int i = 0; i < captureCnt; ++i)
1429 moved_piece = pos.moved_piece(captures[i]);
1430 captured = type_of(pos.piece_on(to_sq(captures[i])));
1431 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1436 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1438 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1439 Move* quiets, int quietsCnt, int bonus) {
1441 if (ss->killers[0] != move)
1443 ss->killers[1] = ss->killers[0];
1444 ss->killers[0] = move;
1447 Color us = pos.side_to_move();
1448 Thread* thisThread = pos.this_thread();
1449 thisThread->mainHistory[us][from_to(move)] << bonus;
1450 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1452 if (is_ok((ss-1)->currentMove))
1454 Square prevSq = to_sq((ss-1)->currentMove);
1455 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1458 // Decrease all the other played quiet moves
1459 for (int i = 0; i < quietsCnt; ++i)
1461 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1462 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1466 // When playing with strength handicap, choose best move among a set of RootMoves
1467 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1469 Move Skill::pick_best(size_t multiPV) {
1471 const RootMoves& rootMoves = Threads.main()->rootMoves;
1472 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1474 // RootMoves are already sorted by score in descending order
1475 Value topScore = rootMoves[0].score;
1476 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1477 int weakness = 120 - 2 * level;
1478 int maxScore = -VALUE_INFINITE;
1480 // Choose best move. For each move score we add two terms, both dependent on
1481 // weakness. One is deterministic and bigger for weaker levels, and one is
1482 // random. Then we choose the move with the resulting highest score.
1483 for (size_t i = 0; i < multiPV; ++i)
1485 // This is our magic formula
1486 int push = ( weakness * int(topScore - rootMoves[i].score)
1487 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1489 if (rootMoves[i].score + push >= maxScore)
1491 maxScore = rootMoves[i].score + push;
1492 best = rootMoves[i].pv[0];
1501 /// MainThread::check_time() is used to print debug info and, more importantly,
1502 /// to detect when we are out of available time and thus stop the search.
1504 void MainThread::check_time() {
1509 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1510 callsCnt = Limits.nodes ? std::min(4096, int(Limits.nodes / 1024)) : 4096;
1512 static TimePoint lastInfoTime = now();
1514 int elapsed = Time.elapsed();
1515 TimePoint tick = Limits.startTime + elapsed;
1517 if (tick - lastInfoTime >= 1000)
1519 lastInfoTime = tick;
1523 // We should not stop pondering until told so by the GUI
1527 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1528 || (Limits.movetime && elapsed >= Limits.movetime)
1529 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1530 Threads.stop = true;
1534 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1535 /// that all (if any) unsearched PV lines are sent using a previous search score.
1537 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1539 std::stringstream ss;
1540 int elapsed = Time.elapsed() + 1;
1541 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1542 size_t PVIdx = pos.this_thread()->PVIdx;
1543 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1544 uint64_t nodesSearched = Threads.nodes_searched();
1545 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1547 for (size_t i = 0; i < multiPV; ++i)
1549 bool updated = (i <= PVIdx && rootMoves[i].score != -VALUE_INFINITE);
1551 if (depth == ONE_PLY && !updated)
1554 Depth d = updated ? depth : depth - ONE_PLY;
1555 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1557 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1558 v = tb ? TB::Score : v;
1560 if (ss.rdbuf()->in_avail()) // Not at first line
1564 << " depth " << d / ONE_PLY
1565 << " seldepth " << rootMoves[i].selDepth
1566 << " multipv " << i + 1
1567 << " score " << UCI::value(v);
1569 if (!tb && i == PVIdx)
1570 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1572 ss << " nodes " << nodesSearched
1573 << " nps " << nodesSearched * 1000 / elapsed;
1575 if (elapsed > 1000) // Earlier makes little sense
1576 ss << " hashfull " << TT.hashfull();
1578 ss << " tbhits " << tbHits
1579 << " time " << elapsed
1582 for (Move m : rootMoves[i].pv)
1583 ss << " " << UCI::move(m, pos.is_chess960());
1590 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1591 /// before exiting the search, for instance, in case we stop the search during a
1592 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1593 /// otherwise in case of 'ponder on' we have nothing to think on.
1595 bool RootMove::extract_ponder_from_tt(Position& pos) {
1600 assert(pv.size() == 1);
1605 pos.do_move(pv[0], st);
1606 TTEntry* tte = TT.probe(pos.key(), ttHit);
1610 Move m = tte->move(); // Local copy to be SMP safe
1611 if (MoveList<LEGAL>(pos).contains(m))
1615 pos.undo_move(pv[0]);
1616 return pv.size() > 1;
1620 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1623 UseRule50 = Options["Syzygy50MoveRule"];
1624 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1625 Cardinality = Options["SyzygyProbeLimit"];
1627 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1628 if (Cardinality > MaxCardinality)
1630 Cardinality = MaxCardinality;
1631 ProbeDepth = DEPTH_ZERO;
1634 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1637 // Don't filter any moves if the user requested analysis on multiple
1638 if (Options["MultiPV"] != 1)
1641 // If the current root position is in the tablebases, then RootMoves
1642 // contains only moves that preserve the draw or the win.
1643 RootInTB = root_probe(pos, rootMoves, TB::Score);
1646 Cardinality = 0; // Do not probe tablebases during the search
1648 else // If DTZ tables are missing, use WDL tables as a fallback
1650 // Filter out moves that do not preserve the draw or the win.
1651 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1653 // Only probe during search if winning
1654 if (RootInTB && TB::Score <= VALUE_DRAW)
1658 if (RootInTB && !UseRule50)
1659 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1660 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
1663 // Since root_probe() and root_probe_wdl() dirty the root move scores,
1664 // we reset them to -VALUE_INFINITE
1665 for (RootMove& rm : rootMoves)
1666 rm.score = -VALUE_INFINITE;