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/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
53 namespace TB = Tablebases;
57 using namespace Search;
61 // Different node types, used as a template parameter
62 enum NodeType { NonPV, PV };
64 // Sizes and phases of the skip-blocks, used for distributing search depths across the threads
65 constexpr int SkipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
66 constexpr int SkipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
68 // Razor and futility margins
69 constexpr int RazorMargin = 600;
70 Value futility_margin(Depth d, bool improving) {
71 return Value((175 - 50 * improving) * d / ONE_PLY);
74 // Futility and reductions lookup tables, initialized at startup
75 int FutilityMoveCounts[2][16]; // [improving][depth]
76 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
78 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
79 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
82 // History and stats update bonus, based on depth
83 int stat_bonus(Depth depth) {
84 int d = depth / ONE_PLY;
85 return d > 17 ? 0 : 29 * d * d + 138 * d - 134;
88 // Add a small random component to draw evaluations to keep search dynamic
89 // and to avoid 3fold-blindness.
90 Value value_draw(Depth depth, Thread* thisThread) {
91 return depth < 4 ? VALUE_DRAW
92 : VALUE_DRAW + Value(2 * (thisThread->nodes.load(std::memory_order_relaxed) % 2) - 1);
95 // Skill structure is used to implement strength limit
97 explicit Skill(int l) : level(l) {}
98 bool enabled() const { return level < 20; }
99 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
100 Move pick_best(size_t multiPV);
103 Move best = MOVE_NONE;
106 template <NodeType NT>
107 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
109 template <NodeType NT>
110 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
112 Value value_to_tt(Value v, int ply);
113 Value value_from_tt(Value v, int ply);
114 void update_pv(Move* pv, Move move, Move* childPv);
115 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
116 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietCount, int bonus);
117 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCount, int bonus);
119 inline bool gives_check(const Position& pos, Move move) {
120 Color us = pos.side_to_move();
121 return type_of(move) == NORMAL && !(pos.blockers_for_king(~us) & pos.pieces(us))
122 ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
123 : pos.gives_check(move);
126 // perft() is our utility to verify move generation. All the leaf nodes up
127 // to the given depth are generated and counted, and the sum is returned.
129 uint64_t perft(Position& pos, Depth depth) {
132 uint64_t cnt, nodes = 0;
133 const bool leaf = (depth == 2 * ONE_PLY);
135 for (const auto& m : MoveList<LEGAL>(pos))
137 if (Root && depth <= ONE_PLY)
142 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
147 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
155 /// Search::init() is called at startup to initialize various lookup tables
157 void Search::init() {
159 for (int imp = 0; imp <= 1; ++imp)
160 for (int d = 1; d < 64; ++d)
161 for (int mc = 1; mc < 64; ++mc)
163 double r = log(d) * log(mc) / 1.95;
165 Reductions[NonPV][imp][d][mc] = int(std::round(r));
166 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
168 // Increase reduction for non-PV nodes when eval is not improving
170 Reductions[NonPV][imp][d][mc]++;
173 for (int d = 0; d < 16; ++d)
175 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
176 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
181 /// Search::clear() resets search state to its initial value
183 void Search::clear() {
185 Threads.main()->wait_for_search_finished();
187 Time.availableNodes = 0;
190 Tablebases::init(Options["SyzygyPath"]); // Free up mapped files
194 /// MainThread::search() is called by the main thread when the program receives
195 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
197 void MainThread::search() {
201 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
202 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
206 Color us = rootPos.side_to_move();
207 Time.init(Limits, us, rootPos.game_ply());
210 if (rootMoves.empty())
212 rootMoves.emplace_back(MOVE_NONE);
213 sync_cout << "info depth 0 score "
214 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
219 for (Thread* th : Threads)
221 th->start_searching();
223 Thread::search(); // Let's start searching!
226 // When we reach the maximum depth, we can arrive here without a raise of
227 // Threads.stop. However, if we are pondering or in an infinite search,
228 // the UCI protocol states that we shouldn't print the best move before the
229 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
230 // until the GUI sends one of those commands.
232 while (!Threads.stop && (ponder || Limits.infinite))
233 {} // Busy wait for a stop or a ponder reset
235 // Stop the threads if not already stopped (also raise the stop if
236 // "ponderhit" just reset Threads.ponder).
239 // Wait until all threads have finished
240 for (Thread* th : Threads)
242 th->wait_for_search_finished();
244 // When playing in 'nodes as time' mode, subtract the searched nodes from
245 // the available ones before exiting.
247 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
249 // Check if there are threads with a better score than main thread
250 Thread* bestThread = this;
251 if ( Options["MultiPV"] == 1
253 && !Skill(Options["Skill Level"]).enabled()
254 && rootMoves[0].pv[0] != MOVE_NONE)
256 std::map<Move, int64_t> votes;
257 Value minScore = this->rootMoves[0].score;
259 // Find out minimum score and reset votes for moves which can be voted
260 for (Thread* th: Threads)
261 minScore = std::min(minScore, th->rootMoves[0].score);
263 // Vote according to score and depth
264 for (Thread* th : Threads)
266 int64_t s = th->rootMoves[0].score - minScore + 1;
267 votes[th->rootMoves[0].pv[0]] += 200 + s * s * int(th->completedDepth);
270 // Select best thread
271 auto bestVote = votes[this->rootMoves[0].pv[0]];
272 for (Thread* th : Threads)
273 if (votes[th->rootMoves[0].pv[0]] > bestVote)
275 bestVote = votes[th->rootMoves[0].pv[0]];
280 previousScore = bestThread->rootMoves[0].score;
282 // Send again PV info if we have a new best thread
283 if (bestThread != this)
284 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
286 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
288 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
289 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
291 std::cout << sync_endl;
295 /// Thread::search() is the main iterative deepening loop. It calls search()
296 /// repeatedly with increasing depth until the allocated thinking time has been
297 /// consumed, the user stops the search, or the maximum search depth is reached.
299 void Thread::search() {
301 // To allow access to (ss-5) up to (ss+2), the stack must be oversized.
302 // The former is needed to allow update_continuation_histories(ss-1, ...),
303 // which accesses its argument at ss-4, also near the root.
304 // The latter is needed for statScores and killer initialization.
305 Stack stack[MAX_PLY+8], *ss = stack+5;
307 Value bestValue, alpha, beta, delta;
308 Move lastBestMove = MOVE_NONE;
309 Depth lastBestMoveDepth = DEPTH_ZERO;
310 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
311 double timeReduction = 1.0;
312 Color us = rootPos.side_to_move();
315 std::memset(ss-5, 0, 8 * sizeof(Stack));
316 for (int i = 5; i > 0; i--)
317 (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
320 bestValue = delta = alpha = -VALUE_INFINITE;
321 beta = VALUE_INFINITE;
324 mainThread->bestMoveChanges = 0, failedLow = false;
326 size_t multiPV = Options["MultiPV"];
327 Skill skill(Options["Skill Level"]);
329 // When playing with strength handicap enable MultiPV search that we will
330 // use behind the scenes to retrieve a set of possible moves.
332 multiPV = std::max(multiPV, (size_t)4);
334 multiPV = std::min(multiPV, rootMoves.size());
336 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
338 // In analysis mode, adjust contempt in accordance with user preference
339 if (Limits.infinite || Options["UCI_AnalyseMode"])
340 ct = Options["Analysis Contempt"] == "Off" ? 0
341 : Options["Analysis Contempt"] == "Both" ? ct
342 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
343 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
346 // In evaluate.cpp the evaluation is from the white point of view
347 contempt = (us == WHITE ? make_score(ct, ct / 2)
348 : -make_score(ct, ct / 2));
350 // Iterative deepening loop until requested to stop or the target depth is reached
351 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
353 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
355 // Distribute search depths across the helper threads
358 int i = (idx - 1) % 20;
359 if (((rootDepth / ONE_PLY + SkipPhase[i]) / SkipSize[i]) % 2)
360 continue; // Retry with an incremented rootDepth
363 // Age out PV variability metric
365 mainThread->bestMoveChanges *= 0.517, failedLow = false;
367 // Save the last iteration's scores before first PV line is searched and
368 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
369 for (RootMove& rm : rootMoves)
370 rm.previousScore = rm.score;
375 // MultiPV loop. We perform a full root search for each PV line
376 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
381 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
382 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
386 // Reset UCI info selDepth for each depth and each PV line
389 // Reset aspiration window starting size
390 if (rootDepth >= 5 * ONE_PLY)
392 Value previousScore = rootMoves[pvIdx].previousScore;
394 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
395 beta = std::min(previousScore + delta, VALUE_INFINITE);
397 // Adjust contempt based on root move's previousScore (dynamic contempt)
398 int dct = ct + 88 * previousScore / (abs(previousScore) + 200);
400 contempt = (us == WHITE ? make_score(dct, dct / 2)
401 : -make_score(dct, dct / 2));
404 // Start with a small aspiration window and, in the case of a fail
405 // high/low, re-search with a bigger window until we don't fail
407 int failedHighCnt = 0;
410 Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
411 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
413 // Bring the best move to the front. It is critical that sorting
414 // is done with a stable algorithm because all the values but the
415 // first and eventually the new best one are set to -VALUE_INFINITE
416 // and we want to keep the same order for all the moves except the
417 // new PV that goes to the front. Note that in case of MultiPV
418 // search the already searched PV lines are preserved.
419 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
421 // If search has been stopped, we break immediately. Sorting is
422 // safe because RootMoves is still valid, although it refers to
423 // the previous iteration.
427 // When failing high/low give some update (without cluttering
428 // the UI) before a re-search.
431 && (bestValue <= alpha || bestValue >= beta)
432 && Time.elapsed() > 3000)
433 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
435 // In case of failing low/high increase aspiration window and
436 // re-search, otherwise exit the loop.
437 if (bestValue <= alpha)
439 beta = (alpha + beta) / 2;
440 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
446 mainThread->stopOnPonderhit = false;
449 else if (bestValue >= beta)
451 beta = std::min(bestValue + delta, VALUE_INFINITE);
458 delta += delta / 4 + 5;
460 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
463 // Sort the PV lines searched so far and update the GUI
464 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
467 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
468 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
472 completedDepth = rootDepth;
474 if (rootMoves[0].pv[0] != lastBestMove) {
475 lastBestMove = rootMoves[0].pv[0];
476 lastBestMoveDepth = rootDepth;
479 // Have we found a "mate in x"?
481 && bestValue >= VALUE_MATE_IN_MAX_PLY
482 && VALUE_MATE - bestValue <= 2 * Limits.mate)
488 // If skill level is enabled and time is up, pick a sub-optimal best move
489 if (skill.enabled() && skill.time_to_pick(rootDepth))
490 skill.pick_best(multiPV);
492 // Do we have time for the next iteration? Can we stop searching now?
493 if ( Limits.use_time_management()
495 && !mainThread->stopOnPonderhit)
497 double fallingEval = (306 + 119 * failedLow + 6 * (mainThread->previousScore - bestValue)) / 581.0;
498 fallingEval = std::max(0.5, std::min(1.5, fallingEval));
500 // If the bestMove is stable over several iterations, reduce time accordingly
501 timeReduction = lastBestMoveDepth + 10 * ONE_PLY < completedDepth ? 1.95 : 1.0;
503 // Use part of the gained time from a previous stable move for the current move
504 double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
505 bestMoveInstability *= std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
507 // Stop the search if we have only one legal move, or if available time elapsed
508 if ( rootMoves.size() == 1
509 || Time.elapsed() > Time.optimum() * bestMoveInstability * fallingEval)
511 // If we are allowed to ponder do not stop the search now but
512 // keep pondering until the GUI sends "ponderhit" or "stop".
513 if (mainThread->ponder)
514 mainThread->stopOnPonderhit = true;
524 mainThread->previousTimeReduction = timeReduction;
526 // If skill level is enabled, swap best PV line with the sub-optimal one
528 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
529 skill.best ? skill.best : skill.pick_best(multiPV)));
535 // search<>() is the main search function for both PV and non-PV nodes
537 template <NodeType NT>
538 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
540 constexpr bool PvNode = NT == PV;
541 const bool rootNode = PvNode && ss->ply == 0;
543 // Check if we have an upcoming move which draws by repetition, or
544 // if the opponent had an alternative move earlier to this position.
545 if ( pos.rule50_count() >= 3
546 && alpha < VALUE_DRAW
548 && pos.has_game_cycle(ss->ply))
550 alpha = value_draw(depth, pos.this_thread());
555 // Dive into quiescence search when the depth reaches zero
557 return qsearch<NT>(pos, ss, alpha, beta);
559 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
560 assert(PvNode || (alpha == beta - 1));
561 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
562 assert(!(PvNode && cutNode));
563 assert(depth / ONE_PLY * ONE_PLY == depth);
565 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
569 Move ttMove, move, excludedMove, bestMove;
570 Depth extension, newDepth;
571 Value bestValue, value, ttValue, eval, maxValue, pureStaticEval;
572 bool ttHit, ttPv, inCheck, givesCheck, improving;
573 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture;
575 int moveCount, captureCount, quietCount;
577 // Step 1. Initialize node
578 Thread* thisThread = pos.this_thread();
579 inCheck = pos.checkers();
580 Color us = pos.side_to_move();
581 moveCount = captureCount = quietCount = ss->moveCount = 0;
582 bestValue = -VALUE_INFINITE;
583 maxValue = VALUE_INFINITE;
585 // Check for the available remaining time
586 if (thisThread == Threads.main())
587 static_cast<MainThread*>(thisThread)->check_time();
589 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
590 if (PvNode && thisThread->selDepth < ss->ply + 1)
591 thisThread->selDepth = ss->ply + 1;
595 // Step 2. Check for aborted search and immediate draw
596 if ( Threads.stop.load(std::memory_order_relaxed)
597 || pos.is_draw(ss->ply)
598 || ss->ply >= MAX_PLY)
599 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
600 : value_draw(depth, pos.this_thread());
602 // Step 3. Mate distance pruning. Even if we mate at the next move our score
603 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
604 // a shorter mate was found upward in the tree then there is no need to search
605 // because we will never beat the current alpha. Same logic but with reversed
606 // signs applies also in the opposite condition of being mated instead of giving
607 // mate. In this case return a fail-high score.
608 alpha = std::max(mated_in(ss->ply), alpha);
609 beta = std::min(mate_in(ss->ply+1), beta);
614 assert(0 <= ss->ply && ss->ply < MAX_PLY);
616 (ss+1)->ply = ss->ply + 1;
617 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
618 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
619 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
620 Square prevSq = to_sq((ss-1)->currentMove);
622 // Initialize statScore to zero for the grandchildren of the current position.
623 // So statScore is shared between all grandchildren and only the first grandchild
624 // starts with statScore = 0. Later grandchildren start with the last calculated
625 // statScore of the previous grandchild. This influences the reduction rules in
626 // LMR which are based on the statScore of parent position.
627 (ss+2)->statScore = 0;
629 // Step 4. Transposition table lookup. We don't want the score of a partial
630 // search to overwrite a previous full search TT value, so we use a different
631 // position key in case of an excluded move.
632 excludedMove = ss->excludedMove;
633 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
634 tte = TT.probe(posKey, ttHit);
635 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
636 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
637 : ttHit ? tte->move() : MOVE_NONE;
638 ttPv = (ttHit && tte->is_pv()) || (PvNode && depth > 4 * ONE_PLY);
640 // At non-PV nodes we check for an early TT cutoff
643 && tte->depth() >= depth
644 && ttValue != VALUE_NONE // Possible in case of TT access race
645 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
646 : (tte->bound() & BOUND_UPPER)))
648 // If ttMove is quiet, update move sorting heuristics on TT hit
653 if (!pos.capture_or_promotion(ttMove))
654 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
656 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
657 if ( ((ss-1)->moveCount == 1 || (ss-1)->currentMove == (ss-1)->killers[0])
658 && !pos.captured_piece())
659 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
661 // Penalty for a quiet ttMove that fails low
662 else if (!pos.capture_or_promotion(ttMove))
664 int penalty = -stat_bonus(depth);
665 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
666 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
672 // Step 5. Tablebases probe
673 if (!rootNode && TB::Cardinality)
675 int piecesCount = pos.count<ALL_PIECES>();
677 if ( piecesCount <= TB::Cardinality
678 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
679 && pos.rule50_count() == 0
680 && !pos.can_castle(ANY_CASTLING))
683 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
685 // Force check of time on the next occasion
686 if (thisThread == Threads.main())
687 static_cast<MainThread*>(thisThread)->callsCnt = 0;
689 if (err != TB::ProbeState::FAIL)
691 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
693 int drawScore = TB::UseRule50 ? 1 : 0;
695 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
696 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
697 : VALUE_DRAW + 2 * wdl * drawScore;
699 Bound b = wdl < -drawScore ? BOUND_UPPER
700 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
702 if ( b == BOUND_EXACT
703 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
705 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
706 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
707 MOVE_NONE, VALUE_NONE);
714 if (b == BOUND_LOWER)
715 bestValue = value, alpha = std::max(alpha, bestValue);
723 // Step 6. Static evaluation of the position
726 ss->staticEval = eval = pureStaticEval = VALUE_NONE;
728 goto moves_loop; // Skip early pruning when in check
732 // Never assume anything on values stored in TT
733 ss->staticEval = eval = pureStaticEval = tte->eval();
734 if (eval == VALUE_NONE)
735 ss->staticEval = eval = pureStaticEval = evaluate(pos);
737 // Can ttValue be used as a better position evaluation?
738 if ( ttValue != VALUE_NONE
739 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
744 if ((ss-1)->currentMove != MOVE_NULL)
746 int bonus = -(ss-1)->statScore / 512;
748 pureStaticEval = evaluate(pos);
749 ss->staticEval = eval = pureStaticEval + bonus;
752 ss->staticEval = eval = pureStaticEval = -(ss-1)->staticEval + 2 * Eval::Tempo;
754 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, pureStaticEval);
757 // Step 7. Razoring (~2 Elo)
758 if ( !rootNode // The required rootNode PV handling is not available in qsearch
759 && depth < 2 * ONE_PLY
760 && eval <= alpha - RazorMargin)
761 return qsearch<NT>(pos, ss, alpha, beta);
763 improving = ss->staticEval >= (ss-2)->staticEval
764 || (ss-2)->staticEval == VALUE_NONE;
766 // Step 8. Futility pruning: child node (~30 Elo)
768 && depth < 7 * ONE_PLY
769 && eval - futility_margin(depth, improving) >= beta
770 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
773 // Step 9. Null move search with verification search (~40 Elo)
775 && (ss-1)->currentMove != MOVE_NULL
776 && (ss-1)->statScore < 23200
778 && pureStaticEval >= beta - 36 * depth / ONE_PLY + 225
780 && pos.non_pawn_material(us)
781 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
783 assert(eval - beta >= 0);
785 // Null move dynamic reduction based on depth and value
786 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 200, 3)) * ONE_PLY;
788 ss->currentMove = MOVE_NULL;
789 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
791 pos.do_null_move(st);
793 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
795 pos.undo_null_move();
797 if (nullValue >= beta)
799 // Do not return unproven mate scores
800 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
803 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 12 * ONE_PLY))
806 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
808 // Do verification search at high depths, with null move pruning disabled
809 // for us, until ply exceeds nmpMinPly.
810 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
811 thisThread->nmpColor = us;
813 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
815 thisThread->nmpMinPly = 0;
822 // Step 10. ProbCut (~10 Elo)
823 // If we have a good enough capture and a reduced search returns a value
824 // much above beta, we can (almost) safely prune the previous move.
826 && depth >= 5 * ONE_PLY
827 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
829 Value raisedBeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
830 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
831 int probCutCount = 0;
833 while ( (move = mp.next_move()) != MOVE_NONE
835 if (move != excludedMove && pos.legal(move))
839 ss->currentMove = move;
840 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
842 assert(depth >= 5 * ONE_PLY);
844 pos.do_move(move, st);
846 // Perform a preliminary qsearch to verify that the move holds
847 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
849 // If the qsearch held perform the regular search
850 if (value >= raisedBeta)
851 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
855 if (value >= raisedBeta)
860 // Step 11. Internal iterative deepening (~2 Elo)
861 if ( depth >= 8 * ONE_PLY
864 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
866 tte = TT.probe(posKey, ttHit);
867 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
868 ttMove = ttHit ? tte->move() : MOVE_NONE;
871 moves_loop: // When in check, search starts from here
873 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, nullptr, (ss-4)->continuationHistory };
874 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
876 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
877 &thisThread->captureHistory,
881 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
884 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
886 // Step 12. Loop through all pseudo-legal moves until no moves remain
887 // or a beta cutoff occurs.
888 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
892 if (move == excludedMove)
895 // At root obey the "searchmoves" option and skip moves not listed in Root
896 // Move List. As a consequence any illegal move is also skipped. In MultiPV
897 // mode we also skip PV moves which have been already searched and those
898 // of lower "TB rank" if we are in a TB root position.
899 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
900 thisThread->rootMoves.begin() + thisThread->pvLast, move))
903 ss->moveCount = ++moveCount;
905 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
906 sync_cout << "info depth " << depth / ONE_PLY
907 << " currmove " << UCI::move(move, pos.is_chess960())
908 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
910 (ss+1)->pv = nullptr;
912 extension = DEPTH_ZERO;
913 captureOrPromotion = pos.capture_or_promotion(move);
914 movedPiece = pos.moved_piece(move);
915 givesCheck = gives_check(pos, move);
917 moveCountPruning = depth < 16 * ONE_PLY
918 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
920 // Step 13. Extensions (~70 Elo)
922 // Singular extension search (~60 Elo). If all moves but one fail low on a
923 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
924 // then that move is singular and should be extended. To verify this we do
925 // a reduced search on all the other moves but the ttMove and if the
926 // result is lower than ttValue minus a margin then we will extend the ttMove.
927 if ( depth >= 8 * ONE_PLY
930 && !excludedMove // Avoid recursive singular search
931 && ttValue != VALUE_NONE
932 && (tte->bound() & BOUND_LOWER)
933 && tte->depth() >= depth - 3 * ONE_PLY
936 Value singularBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
937 ss->excludedMove = move;
938 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, depth / 2, cutNode);
939 ss->excludedMove = MOVE_NONE;
941 if (value < singularBeta)
945 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
946 // search without the ttMove. So we assume this expected Cut-node is not singular,
947 // that is multiple moves fail high, and we can prune the whole subtree by returning
948 // the hard beta bound.
949 else if (cutNode && singularBeta > beta)
953 // Check extension (~2 Elo)
955 && (pos.blockers_for_king(~us) & from_sq(move) || pos.see_ge(move)))
958 // Castling extension
959 else if (type_of(move) == CASTLING)
962 // Calculate new depth for this move
963 newDepth = depth - ONE_PLY + extension;
965 // Step 14. Pruning at shallow depth (~170 Elo)
967 && pos.non_pawn_material(us)
968 && bestValue > VALUE_MATED_IN_MAX_PLY)
970 if ( !captureOrPromotion
972 && !pos.advanced_pawn_push(move))
974 // Move count based pruning (~30 Elo)
975 if (moveCountPruning)
981 // Reduced depth of the next LMR search
982 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
984 // Countermoves based pruning (~20 Elo)
985 if ( lmrDepth < 3 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
986 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
987 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
990 // Futility pruning: parent node (~2 Elo)
993 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
996 // Prune moves with negative SEE (~10 Elo)
997 if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
1000 else if ( !extension // (~20 Elo)
1001 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
1005 // Speculative prefetch as early as possible
1006 prefetch(TT.first_entry(pos.key_after(move)));
1008 // Check for legality just before making the move
1009 if (!rootNode && !pos.legal(move))
1011 ss->moveCount = --moveCount;
1015 // Update the current move (this must be done after singular extension search)
1016 ss->currentMove = move;
1017 ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
1019 // Step 15. Make the move
1020 pos.do_move(move, st, givesCheck);
1022 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1023 // re-searched at full depth.
1024 if ( depth >= 3 * ONE_PLY
1026 && (!captureOrPromotion || moveCountPruning))
1028 Depth r = reduction<PvNode>(improving, depth, moveCount);
1030 // Decrease reduction if position is or has been on the PV
1034 // Decrease reduction if opponent's move count is high (~10 Elo)
1035 if ((ss-1)->moveCount > 15)
1038 if (!captureOrPromotion)
1040 // Increase reduction if ttMove is a capture (~0 Elo)
1044 // Increase reduction for cut nodes (~5 Elo)
1048 // Decrease reduction for moves that escape a capture. Filter out
1049 // castling moves, because they are coded as "king captures rook" and
1050 // hence break make_move(). (~5 Elo)
1051 else if ( type_of(move) == NORMAL
1052 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1055 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1056 + (*contHist[0])[movedPiece][to_sq(move)]
1057 + (*contHist[1])[movedPiece][to_sq(move)]
1058 + (*contHist[3])[movedPiece][to_sq(move)]
1061 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1062 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1065 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1068 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1069 r -= ss->statScore / 20000 * ONE_PLY;
1072 Depth d = std::max(newDepth - std::max(r, DEPTH_ZERO), ONE_PLY);
1074 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1076 doFullDepthSearch = (value > alpha && d != newDepth);
1079 doFullDepthSearch = !PvNode || moveCount > 1;
1081 // Step 17. Full depth search when LMR is skipped or fails high
1082 if (doFullDepthSearch)
1083 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1085 // For PV nodes only, do a full PV search on the first move or after a fail
1086 // high (in the latter case search only if value < beta), otherwise let the
1087 // parent node fail low with value <= alpha and try another move.
1088 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1091 (ss+1)->pv[0] = MOVE_NONE;
1093 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1096 // Step 18. Undo move
1097 pos.undo_move(move);
1099 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1101 // Step 19. Check for a new best move
1102 // Finished searching the move. If a stop occurred, the return value of
1103 // the search cannot be trusted, and we return immediately without
1104 // updating best move, PV and TT.
1105 if (Threads.stop.load(std::memory_order_relaxed))
1110 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1111 thisThread->rootMoves.end(), move);
1113 // PV move or new best move?
1114 if (moveCount == 1 || value > alpha)
1117 rm.selDepth = thisThread->selDepth;
1122 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1123 rm.pv.push_back(*m);
1125 // We record how often the best move has been changed in each
1126 // iteration. This information is used for time management: When
1127 // the best move changes frequently, we allocate some more time.
1128 if (moveCount > 1 && thisThread == Threads.main())
1129 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1132 // All other moves but the PV are set to the lowest value: this
1133 // is not a problem when sorting because the sort is stable and the
1134 // move position in the list is preserved - just the PV is pushed up.
1135 rm.score = -VALUE_INFINITE;
1138 if (value > bestValue)
1146 if (PvNode && !rootNode) // Update pv even in fail-high case
1147 update_pv(ss->pv, move, (ss+1)->pv);
1149 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1153 assert(value >= beta); // Fail high
1160 if (move != bestMove)
1162 if (captureOrPromotion && captureCount < 32)
1163 capturesSearched[captureCount++] = move;
1165 else if (!captureOrPromotion && quietCount < 64)
1166 quietsSearched[quietCount++] = move;
1170 // The following condition would detect a stop only after move loop has been
1171 // completed. But in this case bestValue is valid because we have fully
1172 // searched our subtree, and we can anyhow save the result in TT.
1178 // Step 20. Check for mate and stalemate
1179 // All legal moves have been searched and if there are no legal moves, it
1180 // must be a mate or a stalemate. If we are in a singular extension search then
1181 // return a fail low score.
1183 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1186 bestValue = excludedMove ? alpha
1187 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1190 // Quiet best move: update move sorting heuristics
1191 if (!pos.capture_or_promotion(bestMove))
1192 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1193 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1195 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1197 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1198 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1199 && !pos.captured_piece())
1200 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1203 // Bonus for prior countermove that caused the fail low
1204 else if ( (depth >= 3 * ONE_PLY || PvNode)
1205 && !pos.captured_piece())
1206 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1209 bestValue = std::min(bestValue, maxValue);
1212 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1213 bestValue >= beta ? BOUND_LOWER :
1214 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1215 depth, bestMove, pureStaticEval);
1217 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1223 // qsearch() is the quiescence search function, which is called by the main
1224 // search function with depth zero, or recursively with depth less than ONE_PLY.
1225 template <NodeType NT>
1226 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1228 constexpr bool PvNode = NT == PV;
1230 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1231 assert(PvNode || (alpha == beta - 1));
1232 assert(depth <= DEPTH_ZERO);
1233 assert(depth / ONE_PLY * ONE_PLY == depth);
1239 Move ttMove, move, bestMove;
1241 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1242 bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
1247 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1249 ss->pv[0] = MOVE_NONE;
1252 Thread* thisThread = pos.this_thread();
1253 (ss+1)->ply = ss->ply + 1;
1254 ss->currentMove = bestMove = MOVE_NONE;
1255 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
1256 inCheck = pos.checkers();
1259 // Check for an immediate draw or maximum ply reached
1260 if ( pos.is_draw(ss->ply)
1261 || ss->ply >= MAX_PLY)
1262 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1264 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1266 // Decide whether or not to include checks: this fixes also the type of
1267 // TT entry depth that we are going to use. Note that in qsearch we use
1268 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1269 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1270 : DEPTH_QS_NO_CHECKS;
1271 // Transposition table lookup
1273 tte = TT.probe(posKey, ttHit);
1274 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1275 ttMove = ttHit ? tte->move() : MOVE_NONE;
1276 pvHit = ttHit && tte->is_pv();
1280 && tte->depth() >= ttDepth
1281 && ttValue != VALUE_NONE // Only in case of TT access race
1282 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1283 : (tte->bound() & BOUND_UPPER)))
1286 // Evaluate the position statically
1289 ss->staticEval = VALUE_NONE;
1290 bestValue = futilityBase = -VALUE_INFINITE;
1296 // Never assume anything on values stored in TT
1297 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1298 ss->staticEval = bestValue = evaluate(pos);
1300 // Can ttValue be used as a better position evaluation?
1301 if ( ttValue != VALUE_NONE
1302 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1303 bestValue = ttValue;
1306 ss->staticEval = bestValue =
1307 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1308 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1310 // Stand pat. Return immediately if static value is at least beta
1311 if (bestValue >= beta)
1314 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1315 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1320 if (PvNode && bestValue > alpha)
1323 futilityBase = bestValue + 128;
1326 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, nullptr, (ss-4)->continuationHistory };
1328 // Initialize a MovePicker object for the current position, and prepare
1329 // to search the moves. Because the depth is <= 0 here, only captures,
1330 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1332 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1333 &thisThread->captureHistory,
1335 to_sq((ss-1)->currentMove));
1337 // Loop through the moves until no moves remain or a beta cutoff occurs
1338 while ((move = mp.next_move()) != MOVE_NONE)
1340 assert(is_ok(move));
1342 givesCheck = gives_check(pos, move);
1349 && futilityBase > -VALUE_KNOWN_WIN
1350 && !pos.advanced_pawn_push(move))
1352 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1354 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1356 if (futilityValue <= alpha)
1358 bestValue = std::max(bestValue, futilityValue);
1362 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1364 bestValue = std::max(bestValue, futilityBase);
1369 // Detect non-capture evasions that are candidates to be pruned
1370 evasionPrunable = inCheck
1371 && (depth != DEPTH_ZERO || moveCount > 2)
1372 && bestValue > VALUE_MATED_IN_MAX_PLY
1373 && !pos.capture(move);
1375 // Don't search moves with negative SEE values
1376 if ( (!inCheck || evasionPrunable)
1377 && !pos.see_ge(move))
1380 // Speculative prefetch as early as possible
1381 prefetch(TT.first_entry(pos.key_after(move)));
1383 // Check for legality just before making the move
1384 if (!pos.legal(move))
1390 ss->currentMove = move;
1391 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
1393 // Make and search the move
1394 pos.do_move(move, st, givesCheck);
1395 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1396 pos.undo_move(move);
1398 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1400 // Check for a new best move
1401 if (value > bestValue)
1409 if (PvNode) // Update pv even in fail-high case
1410 update_pv(ss->pv, move, (ss+1)->pv);
1412 if (PvNode && value < beta) // Update alpha here!
1420 // All legal moves have been searched. A special case: If we're in check
1421 // and no legal moves were found, it is checkmate.
1422 if (inCheck && bestValue == -VALUE_INFINITE)
1423 return mated_in(ss->ply); // Plies to mate from the root
1425 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1426 bestValue >= beta ? BOUND_LOWER :
1427 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1428 ttDepth, bestMove, ss->staticEval);
1430 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1436 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1437 // "plies to mate from the current position". Non-mate scores are unchanged.
1438 // The function is called before storing a value in the transposition table.
1440 Value value_to_tt(Value v, int ply) {
1442 assert(v != VALUE_NONE);
1444 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1445 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1449 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1450 // from the transposition table (which refers to the plies to mate/be mated
1451 // from current position) to "plies to mate/be mated from the root".
1453 Value value_from_tt(Value v, int ply) {
1455 return v == VALUE_NONE ? VALUE_NONE
1456 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1457 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1461 // update_pv() adds current move and appends child pv[]
1463 void update_pv(Move* pv, Move move, Move* childPv) {
1465 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1471 // update_continuation_histories() updates histories of the move pairs formed
1472 // by moves at ply -1, -2, and -4 with current move.
1474 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1476 for (int i : {1, 2, 4})
1477 if (is_ok((ss-i)->currentMove))
1478 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1482 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1484 void update_capture_stats(const Position& pos, Move move,
1485 Move* captures, int captureCount, int bonus) {
1487 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1488 Piece moved_piece = pos.moved_piece(move);
1489 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1491 if (pos.capture_or_promotion(move))
1492 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1494 // Decrease all the other played capture moves
1495 for (int i = 0; i < captureCount; ++i)
1497 moved_piece = pos.moved_piece(captures[i]);
1498 captured = type_of(pos.piece_on(to_sq(captures[i])));
1499 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1504 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1506 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1507 Move* quiets, int quietCount, int bonus) {
1509 if (ss->killers[0] != move)
1511 ss->killers[1] = ss->killers[0];
1512 ss->killers[0] = move;
1515 Color us = pos.side_to_move();
1516 Thread* thisThread = pos.this_thread();
1517 thisThread->mainHistory[us][from_to(move)] << bonus;
1518 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1520 if (is_ok((ss-1)->currentMove))
1522 Square prevSq = to_sq((ss-1)->currentMove);
1523 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1526 // Decrease all the other played quiet moves
1527 for (int i = 0; i < quietCount; ++i)
1529 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1530 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1534 // When playing with strength handicap, choose best move among a set of RootMoves
1535 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1537 Move Skill::pick_best(size_t multiPV) {
1539 const RootMoves& rootMoves = Threads.main()->rootMoves;
1540 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1542 // RootMoves are already sorted by score in descending order
1543 Value topScore = rootMoves[0].score;
1544 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1545 int weakness = 120 - 2 * level;
1546 int maxScore = -VALUE_INFINITE;
1548 // Choose best move. For each move score we add two terms, both dependent on
1549 // weakness. One is deterministic and bigger for weaker levels, and one is
1550 // random. Then we choose the move with the resulting highest score.
1551 for (size_t i = 0; i < multiPV; ++i)
1553 // This is our magic formula
1554 int push = ( weakness * int(topScore - rootMoves[i].score)
1555 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1557 if (rootMoves[i].score + push >= maxScore)
1559 maxScore = rootMoves[i].score + push;
1560 best = rootMoves[i].pv[0];
1569 /// MainThread::check_time() is used to print debug info and, more importantly,
1570 /// to detect when we are out of available time and thus stop the search.
1572 void MainThread::check_time() {
1577 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1578 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1580 static TimePoint lastInfoTime = now();
1582 TimePoint elapsed = Time.elapsed();
1583 TimePoint tick = Limits.startTime + elapsed;
1585 if (tick - lastInfoTime >= 1000)
1587 lastInfoTime = tick;
1591 // We should not stop pondering until told so by the GUI
1595 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1596 || (Limits.movetime && elapsed >= Limits.movetime)
1597 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1598 Threads.stop = true;
1602 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1603 /// that all (if any) unsearched PV lines are sent using a previous search score.
1605 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1607 std::stringstream ss;
1608 TimePoint elapsed = Time.elapsed() + 1;
1609 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1610 size_t pvIdx = pos.this_thread()->pvIdx;
1611 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1612 uint64_t nodesSearched = Threads.nodes_searched();
1613 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1615 for (size_t i = 0; i < multiPV; ++i)
1617 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1619 if (depth == ONE_PLY && !updated)
1622 Depth d = updated ? depth : depth - ONE_PLY;
1623 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1625 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1626 v = tb ? rootMoves[i].tbScore : v;
1628 if (ss.rdbuf()->in_avail()) // Not at first line
1632 << " depth " << d / ONE_PLY
1633 << " seldepth " << rootMoves[i].selDepth
1634 << " multipv " << i + 1
1635 << " score " << UCI::value(v);
1637 if (!tb && i == pvIdx)
1638 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1640 ss << " nodes " << nodesSearched
1641 << " nps " << nodesSearched * 1000 / elapsed;
1643 if (elapsed > 1000) // Earlier makes little sense
1644 ss << " hashfull " << TT.hashfull();
1646 ss << " tbhits " << tbHits
1647 << " time " << elapsed
1650 for (Move m : rootMoves[i].pv)
1651 ss << " " << UCI::move(m, pos.is_chess960());
1658 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1659 /// before exiting the search, for instance, in case we stop the search during a
1660 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1661 /// otherwise in case of 'ponder on' we have nothing to think on.
1663 bool RootMove::extract_ponder_from_tt(Position& pos) {
1668 assert(pv.size() == 1);
1670 if (pv[0] == MOVE_NONE)
1673 pos.do_move(pv[0], st);
1674 TTEntry* tte = TT.probe(pos.key(), ttHit);
1678 Move m = tte->move(); // Local copy to be SMP safe
1679 if (MoveList<LEGAL>(pos).contains(m))
1683 pos.undo_move(pv[0]);
1684 return pv.size() > 1;
1687 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1690 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1691 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1692 Cardinality = int(Options["SyzygyProbeLimit"]);
1693 bool dtz_available = true;
1695 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1696 // ProbeDepth == DEPTH_ZERO
1697 if (Cardinality > MaxCardinality)
1699 Cardinality = MaxCardinality;
1700 ProbeDepth = DEPTH_ZERO;
1703 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1705 // Rank moves using DTZ tables
1706 RootInTB = root_probe(pos, rootMoves);
1710 // DTZ tables are missing; try to rank moves using WDL tables
1711 dtz_available = false;
1712 RootInTB = root_probe_wdl(pos, rootMoves);
1718 // Sort moves according to TB rank
1719 std::sort(rootMoves.begin(), rootMoves.end(),
1720 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1722 // Probe during search only if DTZ is not available and we are winning
1723 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1728 // Assign the same rank to all moves
1729 for (auto& m : rootMoves)