2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2019 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
23 #include <cstring> // For std::memset
37 #include "syzygy/tbprobe.h"
44 namespace Tablebases {
52 namespace TB = Tablebases;
56 using namespace Search;
60 // Different node types, used as a template parameter
61 enum NodeType { NonPV, PV };
63 // Razor and futility margins
64 constexpr int RazorMargin = 600;
65 Value futility_margin(Depth d, bool improving) {
66 return Value((175 - 50 * improving) * d / ONE_PLY);
69 // Reductions lookup table, initialized at startup
70 int Reductions[64]; // [depth or moveNumber]
72 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
73 int r = Reductions[std::min(d / ONE_PLY, 63)] * Reductions[std::min(mn, 63)] / 1024;
74 return ((r + 512) / 1024 + (!i && r > 1024) - PvNode) * ONE_PLY;
77 constexpr int futility_move_count(bool improving, int depth) {
78 return (5 + depth * depth) * (1 + improving) / 2;
81 // History and stats update bonus, based on depth
82 int stat_bonus(Depth depth) {
83 int d = depth / ONE_PLY;
84 return d > 17 ? 0 : 29 * d * d + 138 * d - 134;
87 // Add a small random component to draw evaluations to keep search dynamic
88 // and to avoid 3fold-blindness.
89 Value value_draw(Depth depth, Thread* thisThread) {
90 return depth < 4 ? VALUE_DRAW
91 : VALUE_DRAW + Value(2 * (thisThread->nodes.load(std::memory_order_relaxed) % 2) - 1);
94 // Skill structure is used to implement strength limit
96 explicit Skill(int l) : level(l) {}
97 bool enabled() const { return level < 20; }
98 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
99 Move pick_best(size_t multiPV);
102 Move best = MOVE_NONE;
105 template <NodeType NT>
106 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
108 template <NodeType NT>
109 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
111 Value value_to_tt(Value v, int ply);
112 Value value_from_tt(Value v, int ply);
113 void update_pv(Move* pv, Move move, Move* childPv);
114 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
115 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietCount, int bonus);
116 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCount, int bonus);
118 // perft() is our utility to verify move generation. All the leaf nodes up
119 // to the given depth are generated and counted, and the sum is returned.
121 uint64_t perft(Position& pos, Depth depth) {
124 uint64_t cnt, nodes = 0;
125 const bool leaf = (depth == 2 * ONE_PLY);
127 for (const auto& m : MoveList<LEGAL>(pos))
129 if (Root && depth <= ONE_PLY)
134 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
139 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
147 /// Search::init() is called at startup to initialize various lookup tables
149 void Search::init() {
151 for (int i = 1; i < 64; ++i)
152 Reductions[i] = int(1024 * std::log(i) / std::sqrt(1.95));
156 /// Search::clear() resets search state to its initial value
158 void Search::clear() {
160 Threads.main()->wait_for_search_finished();
162 Time.availableNodes = 0;
165 Tablebases::init(Options["SyzygyPath"]); // Free up mapped files
169 /// MainThread::search() is called by the main thread when the program receives
170 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
172 void MainThread::search() {
176 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
177 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
181 Color us = rootPos.side_to_move();
182 Time.init(Limits, us, rootPos.game_ply());
185 if (rootMoves.empty())
187 rootMoves.emplace_back(MOVE_NONE);
188 sync_cout << "info depth 0 score "
189 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
194 for (Thread* th : Threads)
196 th->start_searching();
198 Thread::search(); // Let's start searching!
201 // When we reach the maximum depth, we can arrive here without a raise of
202 // Threads.stop. However, if we are pondering or in an infinite search,
203 // the UCI protocol states that we shouldn't print the best move before the
204 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
205 // until the GUI sends one of those commands.
207 while (!Threads.stop && (ponder || Limits.infinite))
208 {} // Busy wait for a stop or a ponder reset
210 // Stop the threads if not already stopped (also raise the stop if
211 // "ponderhit" just reset Threads.ponder).
214 // Wait until all threads have finished
215 for (Thread* th : Threads)
217 th->wait_for_search_finished();
219 // When playing in 'nodes as time' mode, subtract the searched nodes from
220 // the available ones before exiting.
222 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
224 // Check if there are threads with a better score than main thread
225 Thread* bestThread = this;
226 if ( Options["MultiPV"] == 1
228 && !Skill(Options["Skill Level"]).enabled()
229 && rootMoves[0].pv[0] != MOVE_NONE)
231 std::map<Move, int64_t> votes;
232 Value minScore = this->rootMoves[0].score;
234 // Find out minimum score and reset votes for moves which can be voted
235 for (Thread* th: Threads)
236 minScore = std::min(minScore, th->rootMoves[0].score);
238 // Vote according to score and depth
239 for (Thread* th : Threads)
241 int64_t s = th->rootMoves[0].score - minScore + 1;
242 votes[th->rootMoves[0].pv[0]] += 200 + s * s * int(th->completedDepth);
245 // Select best thread
246 auto bestVote = votes[this->rootMoves[0].pv[0]];
247 for (Thread* th : Threads)
248 if (votes[th->rootMoves[0].pv[0]] > bestVote)
250 bestVote = votes[th->rootMoves[0].pv[0]];
255 previousScore = bestThread->rootMoves[0].score;
257 // Send again PV info if we have a new best thread
258 if (bestThread != this)
259 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
261 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
263 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
264 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
266 std::cout << sync_endl;
270 /// Thread::search() is the main iterative deepening loop. It calls search()
271 /// repeatedly with increasing depth until the allocated thinking time has been
272 /// consumed, the user stops the search, or the maximum search depth is reached.
274 void Thread::search() {
276 // To allow access to (ss-5) up to (ss+2), the stack must be oversized.
277 // The former is needed to allow update_continuation_histories(ss-1, ...),
278 // which accesses its argument at ss-4, also near the root.
279 // The latter is needed for statScores and killer initialization.
280 Stack stack[MAX_PLY+10], *ss = stack+7;
282 Value bestValue, alpha, beta, delta;
283 Move lastBestMove = MOVE_NONE;
284 Depth lastBestMoveDepth = DEPTH_ZERO;
285 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
286 double timeReduction = 1.0;
287 Color us = rootPos.side_to_move();
289 std::memset(ss-7, 0, 10 * sizeof(Stack));
290 for (int i = 7; i > 0; i--)
291 (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
294 bestValue = delta = alpha = -VALUE_INFINITE;
295 beta = VALUE_INFINITE;
298 mainThread->bestMoveChanges = 0;
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 = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
312 // In analysis mode, adjust contempt in accordance with user preference
313 if (Limits.infinite || Options["UCI_AnalyseMode"])
314 ct = Options["Analysis Contempt"] == "Off" ? 0
315 : Options["Analysis Contempt"] == "Both" ? ct
316 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
317 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
320 // In evaluate.cpp the evaluation is from the white point of view
321 contempt = (us == WHITE ? make_score(ct, ct / 2)
322 : -make_score(ct, ct / 2));
324 // Iterative deepening loop until requested to stop or the target depth is reached
325 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
327 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
329 // Age out PV variability metric
331 mainThread->bestMoveChanges *= 0.517;
333 // Save the last iteration's scores before first PV line is searched and
334 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
335 for (RootMove& rm : rootMoves)
336 rm.previousScore = rm.score;
341 // MultiPV loop. We perform a full root search for each PV line
342 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
347 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
348 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
352 // Reset UCI info selDepth for each depth and each PV line
355 // Reset aspiration window starting size
356 if (rootDepth >= 5 * ONE_PLY)
358 Value previousScore = rootMoves[pvIdx].previousScore;
360 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
361 beta = std::min(previousScore + delta, VALUE_INFINITE);
363 // Adjust contempt based on root move's previousScore (dynamic contempt)
364 int dct = ct + 88 * previousScore / (abs(previousScore) + 200);
366 contempt = (us == WHITE ? make_score(dct, dct / 2)
367 : -make_score(dct, dct / 2));
370 // Start with a small aspiration window and, in the case of a fail
371 // high/low, re-search with a bigger window until we don't fail
373 int failedHighCnt = 0;
376 Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
377 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
379 // Bring the best move to the front. It is critical that sorting
380 // is done with a stable algorithm because all the values but the
381 // first and eventually the new best one are set to -VALUE_INFINITE
382 // and we want to keep the same order for all the moves except the
383 // new PV that goes to the front. Note that in case of MultiPV
384 // search the already searched PV lines are preserved.
385 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
387 // If search has been stopped, we break immediately. Sorting is
388 // safe because RootMoves is still valid, although it refers to
389 // the previous iteration.
393 // When failing high/low give some update (without cluttering
394 // the UI) before a re-search.
397 && (bestValue <= alpha || bestValue >= beta)
398 && Time.elapsed() > 3000)
399 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
401 // In case of failing low/high increase aspiration window and
402 // re-search, otherwise exit the loop.
403 if (bestValue <= alpha)
405 beta = (alpha + beta) / 2;
406 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
411 mainThread->stopOnPonderhit = false;
414 else if (bestValue >= beta)
416 beta = std::min(bestValue + delta, VALUE_INFINITE);
423 delta += delta / 4 + 5;
425 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
428 // Sort the PV lines searched so far and update the GUI
429 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
432 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
433 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
437 completedDepth = rootDepth;
439 if (rootMoves[0].pv[0] != lastBestMove) {
440 lastBestMove = rootMoves[0].pv[0];
441 lastBestMoveDepth = rootDepth;
444 // Have we found a "mate in x"?
446 && bestValue >= VALUE_MATE_IN_MAX_PLY
447 && VALUE_MATE - bestValue <= 2 * Limits.mate)
453 // If skill level is enabled and time is up, pick a sub-optimal best move
454 if (skill.enabled() && skill.time_to_pick(rootDepth))
455 skill.pick_best(multiPV);
457 // Do we have time for the next iteration? Can we stop searching now?
458 if ( Limits.use_time_management()
460 && !mainThread->stopOnPonderhit)
462 double fallingEval = (306 + 9 * (mainThread->previousScore - bestValue)) / 581.0;
463 fallingEval = clamp(fallingEval, 0.5, 1.5);
465 // If the bestMove is stable over several iterations, reduce time accordingly
466 timeReduction = lastBestMoveDepth + 10 * ONE_PLY < completedDepth ? 1.95 : 1.0;
467 double reduction = std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
469 // Use part of the gained time from a previous stable move for the current move
470 double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
472 // Stop the search if we have only one legal move, or if available time elapsed
473 if ( rootMoves.size() == 1
474 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
476 // If we are allowed to ponder do not stop the search now but
477 // keep pondering until the GUI sends "ponderhit" or "stop".
478 if (mainThread->ponder)
479 mainThread->stopOnPonderhit = true;
489 mainThread->previousTimeReduction = timeReduction;
491 // If skill level is enabled, swap best PV line with the sub-optimal one
493 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
494 skill.best ? skill.best : skill.pick_best(multiPV)));
500 // search<>() is the main search function for both PV and non-PV nodes
502 template <NodeType NT>
503 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
505 constexpr bool PvNode = NT == PV;
506 const bool rootNode = PvNode && ss->ply == 0;
508 // Check if we have an upcoming move which draws by repetition, or
509 // if the opponent had an alternative move earlier to this position.
510 if ( pos.rule50_count() >= 3
511 && alpha < VALUE_DRAW
513 && pos.has_game_cycle(ss->ply))
515 alpha = value_draw(depth, pos.this_thread());
520 // Dive into quiescence search when the depth reaches zero
522 return qsearch<NT>(pos, ss, alpha, beta);
524 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
525 assert(PvNode || (alpha == beta - 1));
526 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
527 assert(!(PvNode && cutNode));
528 assert(depth / ONE_PLY * ONE_PLY == depth);
530 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
534 Move ttMove, move, excludedMove, bestMove;
535 Depth extension, newDepth;
536 Value bestValue, value, ttValue, eval, maxValue, pureStaticEval;
537 bool ttHit, ttPv, inCheck, givesCheck, improving;
538 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
540 int moveCount, captureCount, quietCount;
542 // Step 1. Initialize node
543 Thread* thisThread = pos.this_thread();
544 inCheck = pos.checkers();
545 Color us = pos.side_to_move();
546 moveCount = captureCount = quietCount = ss->moveCount = 0;
547 bestValue = -VALUE_INFINITE;
548 maxValue = VALUE_INFINITE;
550 // Check for the available remaining time
551 if (thisThread == Threads.main())
552 static_cast<MainThread*>(thisThread)->check_time();
554 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
555 if (PvNode && thisThread->selDepth < ss->ply + 1)
556 thisThread->selDepth = ss->ply + 1;
560 // Step 2. Check for aborted search and immediate draw
561 if ( Threads.stop.load(std::memory_order_relaxed)
562 || pos.is_draw(ss->ply)
563 || ss->ply >= MAX_PLY)
564 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
565 : value_draw(depth, pos.this_thread());
567 // Step 3. Mate distance pruning. Even if we mate at the next move our score
568 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
569 // a shorter mate was found upward in the tree then there is no need to search
570 // because we will never beat the current alpha. Same logic but with reversed
571 // signs applies also in the opposite condition of being mated instead of giving
572 // mate. In this case return a fail-high score.
573 alpha = std::max(mated_in(ss->ply), alpha);
574 beta = std::min(mate_in(ss->ply+1), beta);
579 assert(0 <= ss->ply && ss->ply < MAX_PLY);
581 (ss+1)->ply = ss->ply + 1;
582 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
583 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
584 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
585 Square prevSq = to_sq((ss-1)->currentMove);
587 // Initialize statScore to zero for the grandchildren of the current position.
588 // So statScore is shared between all grandchildren and only the first grandchild
589 // starts with statScore = 0. Later grandchildren start with the last calculated
590 // statScore of the previous grandchild. This influences the reduction rules in
591 // LMR which are based on the statScore of parent position.
592 (ss+2)->statScore = 0;
594 // Step 4. Transposition table lookup. We don't want the score of a partial
595 // search to overwrite a previous full search TT value, so we use a different
596 // position key in case of an excluded move.
597 excludedMove = ss->excludedMove;
598 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
599 tte = TT.probe(posKey, ttHit);
600 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
601 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
602 : ttHit ? tte->move() : MOVE_NONE;
603 ttPv = (ttHit && tte->is_pv()) || (PvNode && depth > 4 * ONE_PLY);
605 // At non-PV nodes we check for an early TT cutoff
608 && tte->depth() >= depth
609 && ttValue != VALUE_NONE // Possible in case of TT access race
610 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
611 : (tte->bound() & BOUND_UPPER)))
613 // If ttMove is quiet, update move sorting heuristics on TT hit
618 if (!pos.capture_or_promotion(ttMove))
619 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
621 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
622 if ( ((ss-1)->moveCount == 1 || (ss-1)->currentMove == (ss-1)->killers[0])
623 && !pos.captured_piece())
624 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
626 // Penalty for a quiet ttMove that fails low
627 else if (!pos.capture_or_promotion(ttMove))
629 int penalty = -stat_bonus(depth);
630 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
631 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
637 // Step 5. Tablebases probe
638 if (!rootNode && TB::Cardinality)
640 int piecesCount = pos.count<ALL_PIECES>();
642 if ( piecesCount <= TB::Cardinality
643 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
644 && pos.rule50_count() == 0
645 && !pos.can_castle(ANY_CASTLING))
648 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
650 // Force check of time on the next occasion
651 if (thisThread == Threads.main())
652 static_cast<MainThread*>(thisThread)->callsCnt = 0;
654 if (err != TB::ProbeState::FAIL)
656 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
658 int drawScore = TB::UseRule50 ? 1 : 0;
660 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
661 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
662 : VALUE_DRAW + 2 * wdl * drawScore;
664 Bound b = wdl < -drawScore ? BOUND_UPPER
665 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
667 if ( b == BOUND_EXACT
668 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
670 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
671 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
672 MOVE_NONE, VALUE_NONE);
679 if (b == BOUND_LOWER)
680 bestValue = value, alpha = std::max(alpha, bestValue);
688 // Step 6. Static evaluation of the position
691 ss->staticEval = eval = pureStaticEval = VALUE_NONE;
693 goto moves_loop; // Skip early pruning when in check
697 // Never assume anything on values stored in TT
698 ss->staticEval = eval = pureStaticEval = tte->eval();
699 if (eval == VALUE_NONE)
700 ss->staticEval = eval = pureStaticEval = 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 if ((ss-1)->currentMove != MOVE_NULL)
711 int bonus = -(ss-1)->statScore / 512;
713 pureStaticEval = evaluate(pos);
714 ss->staticEval = eval = pureStaticEval + bonus;
717 ss->staticEval = eval = pureStaticEval = -(ss-1)->staticEval + 2 * Eval::Tempo;
719 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, pureStaticEval);
722 // Step 7. Razoring (~2 Elo)
723 if ( !rootNode // The required rootNode PV handling is not available in qsearch
724 && depth < 2 * ONE_PLY
725 && eval <= alpha - RazorMargin)
726 return qsearch<NT>(pos, ss, alpha, beta);
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 < 23200
743 && pureStaticEval >= 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(int(eval - beta) / 200, 3)) * ONE_PLY;
753 ss->currentMove = MOVE_NULL;
754 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
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 raisedBeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
795 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
796 int probCutCount = 0;
798 while ( (move = mp.next_move()) != MOVE_NONE
799 && probCutCount < 2 + 2 * cutNode)
800 if (move != excludedMove && pos.legal(move))
804 ss->currentMove = move;
805 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
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, -raisedBeta, -raisedBeta+1);
814 // If the qsearch held, perform the regular search
815 if (value >= raisedBeta)
816 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
820 if (value >= raisedBeta)
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,
839 nullptr, (ss-4)->continuationHistory,
840 nullptr, (ss-6)->continuationHistory };
841 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
843 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
844 &thisThread->captureHistory,
848 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
850 moveCountPruning = false;
851 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
853 // Step 12. Loop through all pseudo-legal moves until no moves remain
854 // or a beta cutoff occurs.
855 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
859 if (move == excludedMove)
862 // At root obey the "searchmoves" option and skip moves not listed in Root
863 // Move List. As a consequence any illegal move is also skipped. In MultiPV
864 // mode we also skip PV moves which have been already searched and those
865 // of lower "TB rank" if we are in a TB root position.
866 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
867 thisThread->rootMoves.begin() + thisThread->pvLast, move))
870 ss->moveCount = ++moveCount;
872 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
873 sync_cout << "info depth " << depth / ONE_PLY
874 << " currmove " << UCI::move(move, pos.is_chess960())
875 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
877 (ss+1)->pv = nullptr;
879 extension = DEPTH_ZERO;
880 captureOrPromotion = pos.capture_or_promotion(move);
881 movedPiece = pos.moved_piece(move);
882 givesCheck = pos.gives_check(move);
884 // Step 13. Extensions (~70 Elo)
886 // Singular extension search (~60 Elo). If all moves but one fail low on a
887 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
888 // then that move is singular and should be extended. To verify this we do
889 // a reduced search on all the other moves but the ttMove and if the
890 // result is lower than ttValue minus a margin then we will extend the ttMove.
891 if ( depth >= 8 * ONE_PLY
894 && !excludedMove // Avoid recursive singular search
895 /* && ttValue != VALUE_NONE Already implicit in the next condition */
896 && abs(ttValue) < VALUE_KNOWN_WIN
897 && (tte->bound() & BOUND_LOWER)
898 && tte->depth() >= depth - 3 * ONE_PLY
901 Value singularBeta = ttValue - 2 * depth / ONE_PLY;
902 ss->excludedMove = move;
903 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, depth / 2, cutNode);
904 ss->excludedMove = MOVE_NONE;
906 if (value < singularBeta)
910 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
911 // search without the ttMove. So we assume this expected Cut-node is not singular,
912 // that is multiple moves fail high, and we can prune the whole subtree by returning
913 // the hard beta bound.
914 else if (cutNode && singularBeta > beta)
918 // Check extension (~2 Elo)
920 && (pos.blockers_for_king(~us) & from_sq(move) || pos.see_ge(move)))
923 // Castling extension
924 else if (type_of(move) == CASTLING)
927 // Calculate new depth for this move
928 newDepth = depth - ONE_PLY + extension;
930 // Step 14. Pruning at shallow depth (~170 Elo)
932 && pos.non_pawn_material(us)
933 && bestValue > VALUE_MATED_IN_MAX_PLY)
935 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
936 moveCountPruning = moveCount >= futility_move_count(improving,depth / ONE_PLY);
938 if ( !captureOrPromotion
940 && !pos.advanced_pawn_push(move))
942 // Move count based pruning (~30 Elo)
943 if (moveCountPruning)
946 // Reduced depth of the next LMR search
947 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
949 // Countermoves based pruning (~20 Elo)
950 if ( lmrDepth < 3 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
951 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
952 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
955 // Futility pruning: parent node (~2 Elo)
958 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
961 // Prune moves with negative SEE (~10 Elo)
962 if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
965 else if (!pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY))) // (~20 Elo)
969 // Speculative prefetch as early as possible
970 prefetch(TT.first_entry(pos.key_after(move)));
972 // Check for legality just before making the move
973 if (!rootNode && !pos.legal(move))
975 ss->moveCount = --moveCount;
979 // Update the current move (this must be done after singular extension search)
980 ss->currentMove = move;
981 ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
983 // Step 15. Make the move
984 pos.do_move(move, st, givesCheck);
986 // Step 16. Reduced depth search (LMR). If the move fails high it will be
987 // re-searched at full depth.
988 if ( depth >= 3 * ONE_PLY
990 && (!captureOrPromotion || moveCountPruning))
992 Depth r = reduction<PvNode>(improving, depth, moveCount);
994 // Decrease reduction if position is or has been on the PV
998 // Decrease reduction if opponent's move count is high (~10 Elo)
999 if ((ss-1)->moveCount > 15)
1002 if (!captureOrPromotion)
1004 // Increase reduction if ttMove is a capture (~0 Elo)
1008 // Increase reduction for cut nodes (~5 Elo)
1012 // Decrease reduction for moves that escape a capture. Filter out
1013 // castling moves, because they are coded as "king captures rook" and
1014 // hence break make_move(). (~5 Elo)
1015 else if ( type_of(move) == NORMAL
1016 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1019 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1020 + (*contHist[0])[movedPiece][to_sq(move)]
1021 + (*contHist[1])[movedPiece][to_sq(move)]
1022 + (*contHist[3])[movedPiece][to_sq(move)]
1025 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1026 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1029 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1032 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1033 r -= ss->statScore / 20000 * ONE_PLY;
1036 Depth d = std::max(newDepth - std::max(r, DEPTH_ZERO), ONE_PLY);
1038 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1040 doFullDepthSearch = (value > alpha && d != newDepth);
1043 doFullDepthSearch = !PvNode || moveCount > 1;
1045 // Step 17. Full depth search when LMR is skipped or fails high
1046 if (doFullDepthSearch)
1047 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1049 // For PV nodes only, do a full PV search on the first move or after a fail
1050 // high (in the latter case search only if value < beta), otherwise let the
1051 // parent node fail low with value <= alpha and try another move.
1052 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1055 (ss+1)->pv[0] = MOVE_NONE;
1057 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1060 // Step 18. Undo move
1061 pos.undo_move(move);
1063 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1065 // Step 19. Check for a new best move
1066 // Finished searching the move. If a stop occurred, the return value of
1067 // the search cannot be trusted, and we return immediately without
1068 // updating best move, PV and TT.
1069 if (Threads.stop.load(std::memory_order_relaxed))
1074 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1075 thisThread->rootMoves.end(), move);
1077 // PV move or new best move?
1078 if (moveCount == 1 || value > alpha)
1081 rm.selDepth = thisThread->selDepth;
1086 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1087 rm.pv.push_back(*m);
1089 // We record how often the best move has been changed in each
1090 // iteration. This information is used for time management: When
1091 // the best move changes frequently, we allocate some more time.
1092 if (moveCount > 1 && thisThread == Threads.main())
1093 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1096 // All other moves but the PV are set to the lowest value: this
1097 // is not a problem when sorting because the sort is stable and the
1098 // move position in the list is preserved - just the PV is pushed up.
1099 rm.score = -VALUE_INFINITE;
1102 if (value > bestValue)
1110 if (PvNode && !rootNode) // Update pv even in fail-high case
1111 update_pv(ss->pv, move, (ss+1)->pv);
1113 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1117 assert(value >= beta); // Fail high
1124 if (move != bestMove)
1126 if (captureOrPromotion && captureCount < 32)
1127 capturesSearched[captureCount++] = move;
1129 else if (!captureOrPromotion && quietCount < 64)
1130 quietsSearched[quietCount++] = move;
1134 // The following condition would detect a stop only after move loop has been
1135 // completed. But in this case bestValue is valid because we have fully
1136 // searched our subtree, and we can anyhow save the result in TT.
1142 // Step 20. Check for mate and stalemate
1143 // All legal moves have been searched and if there are no legal moves, it
1144 // must be a mate or a stalemate. If we are in a singular extension search then
1145 // return a fail low score.
1147 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1150 bestValue = excludedMove ? alpha
1151 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1154 // Quiet best move: update move sorting heuristics
1155 if (!pos.capture_or_promotion(bestMove))
1156 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1157 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1159 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1161 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1162 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1163 && !pos.captured_piece())
1164 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1167 // Bonus for prior countermove that caused the fail low
1168 else if ( (depth >= 3 * ONE_PLY || PvNode)
1169 && !pos.captured_piece())
1170 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1173 bestValue = std::min(bestValue, maxValue);
1176 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1177 bestValue >= beta ? BOUND_LOWER :
1178 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1179 depth, bestMove, pureStaticEval);
1181 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1187 // qsearch() is the quiescence search function, which is called by the main
1188 // search function with depth zero, or recursively with depth less than ONE_PLY.
1189 template <NodeType NT>
1190 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1192 constexpr bool PvNode = NT == PV;
1194 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1195 assert(PvNode || (alpha == beta - 1));
1196 assert(depth <= DEPTH_ZERO);
1197 assert(depth / ONE_PLY * ONE_PLY == depth);
1203 Move ttMove, move, bestMove;
1205 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1206 bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
1211 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1213 ss->pv[0] = MOVE_NONE;
1216 Thread* thisThread = pos.this_thread();
1217 (ss+1)->ply = ss->ply + 1;
1218 ss->currentMove = bestMove = MOVE_NONE;
1219 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
1220 inCheck = pos.checkers();
1223 // Check for an immediate draw or maximum ply reached
1224 if ( pos.is_draw(ss->ply)
1225 || ss->ply >= MAX_PLY)
1226 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1228 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1230 // Decide whether or not to include checks: this fixes also the type of
1231 // TT entry depth that we are going to use. Note that in qsearch we use
1232 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1233 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1234 : DEPTH_QS_NO_CHECKS;
1235 // Transposition table lookup
1237 tte = TT.probe(posKey, ttHit);
1238 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1239 ttMove = ttHit ? tte->move() : MOVE_NONE;
1240 pvHit = ttHit && tte->is_pv();
1244 && tte->depth() >= ttDepth
1245 && ttValue != VALUE_NONE // Only in case of TT access race
1246 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1247 : (tte->bound() & BOUND_UPPER)))
1250 // Evaluate the position statically
1253 ss->staticEval = VALUE_NONE;
1254 bestValue = futilityBase = -VALUE_INFINITE;
1260 // Never assume anything on values stored in TT
1261 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1262 ss->staticEval = bestValue = evaluate(pos);
1264 // Can ttValue be used as a better position evaluation?
1265 if ( ttValue != VALUE_NONE
1266 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1267 bestValue = ttValue;
1270 ss->staticEval = bestValue =
1271 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1272 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1274 // Stand pat. Return immediately if static value is at least beta
1275 if (bestValue >= beta)
1278 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1279 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1284 if (PvNode && bestValue > alpha)
1287 futilityBase = bestValue + 128;
1290 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1291 nullptr, (ss-4)->continuationHistory,
1292 nullptr, (ss-6)->continuationHistory };
1294 // Initialize a MovePicker object for the current position, and prepare
1295 // to search the moves. Because the depth is <= 0 here, only captures,
1296 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1298 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1299 &thisThread->captureHistory,
1301 to_sq((ss-1)->currentMove));
1303 // Loop through the moves until no moves remain or a beta cutoff occurs
1304 while ((move = mp.next_move()) != MOVE_NONE)
1306 assert(is_ok(move));
1308 givesCheck = pos.gives_check(move);
1315 && futilityBase > -VALUE_KNOWN_WIN
1316 && !pos.advanced_pawn_push(move))
1318 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1320 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1322 if (futilityValue <= alpha)
1324 bestValue = std::max(bestValue, futilityValue);
1328 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1330 bestValue = std::max(bestValue, futilityBase);
1335 // Detect non-capture evasions that are candidates to be pruned
1336 evasionPrunable = inCheck
1337 && (depth != DEPTH_ZERO || moveCount > 2)
1338 && bestValue > VALUE_MATED_IN_MAX_PLY
1339 && !pos.capture(move);
1341 // Don't search moves with negative SEE values
1342 if ( (!inCheck || evasionPrunable)
1343 && !pos.see_ge(move))
1346 // Speculative prefetch as early as possible
1347 prefetch(TT.first_entry(pos.key_after(move)));
1349 // Check for legality just before making the move
1350 if (!pos.legal(move))
1356 ss->currentMove = move;
1357 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
1359 // Make and search the move
1360 pos.do_move(move, st, givesCheck);
1361 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1362 pos.undo_move(move);
1364 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1366 // Check for a new best move
1367 if (value > bestValue)
1375 if (PvNode) // Update pv even in fail-high case
1376 update_pv(ss->pv, move, (ss+1)->pv);
1378 if (PvNode && value < beta) // Update alpha here!
1386 // All legal moves have been searched. A special case: If we're in check
1387 // and no legal moves were found, it is checkmate.
1388 if (inCheck && bestValue == -VALUE_INFINITE)
1389 return mated_in(ss->ply); // Plies to mate from the root
1391 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1392 bestValue >= beta ? BOUND_LOWER :
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, 6})
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 captureCount, 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 < captureCount; ++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 quietCount, 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 < quietCount; ++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 || stopOnPonderhit))
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);
1636 if (pv[0] == MOVE_NONE)
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)