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 (which also raises Threads.stop).
231 Threads.stopOnPonderhit = true;
233 while (!Threads.stop && (Threads.ponder || Limits.infinite))
234 {} // Busy wait for a stop or a ponder reset
236 // Stop the threads if not already stopped (also raise the stop if
237 // "ponderhit" just reset Threads.ponder).
240 // Wait until all threads have finished
241 for (Thread* th : Threads)
243 th->wait_for_search_finished();
245 // When playing in 'nodes as time' mode, subtract the searched nodes from
246 // the available ones before exiting.
248 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
250 // Check if there are threads with a better score than main thread
251 Thread* bestThread = this;
252 if ( Options["MultiPV"] == 1
254 && !Skill(Options["Skill Level"]).enabled()
255 && rootMoves[0].pv[0] != MOVE_NONE)
257 std::map<Move, int64_t> votes;
258 Value minScore = this->rootMoves[0].score;
260 // Find out minimum score and reset votes for moves which can be voted
261 for (Thread* th: Threads)
263 minScore = std::min(minScore, th->rootMoves[0].score);
264 votes[th->rootMoves[0].pv[0]] = 0;
267 // Vote according to score and depth
268 auto square = [](int64_t x) { return x * x; };
269 for (Thread* th : Threads)
270 votes[th->rootMoves[0].pv[0]] += 200 + (square(th->rootMoves[0].score - minScore + 1)
271 * int64_t(th->completedDepth));
273 // Select best thread
274 int64_t bestVote = votes[this->rootMoves[0].pv[0]];
275 for (Thread* th : Threads)
277 if (votes[th->rootMoves[0].pv[0]] > bestVote)
279 bestVote = votes[th->rootMoves[0].pv[0]];
285 previousScore = bestThread->rootMoves[0].score;
287 // Send again PV info if we have a new best thread
288 if (bestThread != this)
289 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
291 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
293 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
294 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
296 std::cout << sync_endl;
300 /// Thread::search() is the main iterative deepening loop. It calls search()
301 /// repeatedly with increasing depth until the allocated thinking time has been
302 /// consumed, the user stops the search, or the maximum search depth is reached.
304 void Thread::search() {
306 // To allow access to (ss-5) up to (ss+2), the stack must be oversized.
307 // The former is needed to allow update_continuation_histories(ss-1, ...),
308 // which accesses its argument at ss-4, also near the root.
309 // The latter is needed for statScores and killer initialization.
310 Stack stack[MAX_PLY+8], *ss = stack+5;
312 Value bestValue, alpha, beta, delta;
313 Move lastBestMove = MOVE_NONE;
314 Depth lastBestMoveDepth = DEPTH_ZERO;
315 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
316 double timeReduction = 1.0;
317 Color us = rootPos.side_to_move();
320 std::memset(ss-5, 0, 8 * sizeof(Stack));
321 for (int i = 5; i > 0; i--)
322 (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
325 bestValue = delta = alpha = -VALUE_INFINITE;
326 beta = VALUE_INFINITE;
329 mainThread->bestMoveChanges = 0, failedLow = false;
331 size_t multiPV = Options["MultiPV"];
332 Skill skill(Options["Skill Level"]);
334 // When playing with strength handicap enable MultiPV search that we will
335 // use behind the scenes to retrieve a set of possible moves.
337 multiPV = std::max(multiPV, (size_t)4);
339 multiPV = std::min(multiPV, rootMoves.size());
341 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
343 // In analysis mode, adjust contempt in accordance with user preference
344 if (Limits.infinite || Options["UCI_AnalyseMode"])
345 ct = Options["Analysis Contempt"] == "Off" ? 0
346 : Options["Analysis Contempt"] == "Both" ? ct
347 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
348 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
351 // In evaluate.cpp the evaluation is from the white point of view
352 contempt = (us == WHITE ? make_score(ct, ct / 2)
353 : -make_score(ct, ct / 2));
355 // Iterative deepening loop until requested to stop or the target depth is reached
356 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
358 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
360 // Distribute search depths across the helper threads
363 int i = (idx - 1) % 20;
364 if (((rootDepth / ONE_PLY + SkipPhase[i]) / SkipSize[i]) % 2)
365 continue; // Retry with an incremented rootDepth
368 // Age out PV variability metric
370 mainThread->bestMoveChanges *= 0.517, failedLow = false;
372 // Save the last iteration's scores before first PV line is searched and
373 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
374 for (RootMove& rm : rootMoves)
375 rm.previousScore = rm.score;
380 // MultiPV loop. We perform a full root search for each PV line
381 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
386 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
387 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
391 // Reset UCI info selDepth for each depth and each PV line
394 // Reset aspiration window starting size
395 if (rootDepth >= 5 * ONE_PLY)
397 Value previousScore = rootMoves[pvIdx].previousScore;
399 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
400 beta = std::min(previousScore + delta, VALUE_INFINITE);
402 // Adjust contempt based on root move's previousScore (dynamic contempt)
403 int dct = ct + 88 * previousScore / (abs(previousScore) + 200);
405 contempt = (us == WHITE ? make_score(dct, dct / 2)
406 : -make_score(dct, dct / 2));
409 // Start with a small aspiration window and, in the case of a fail
410 // high/low, re-search with a bigger window until we don't fail
412 int failedHighCnt = 0;
415 Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
416 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
418 // Bring the best move to the front. It is critical that sorting
419 // is done with a stable algorithm because all the values but the
420 // first and eventually the new best one are set to -VALUE_INFINITE
421 // and we want to keep the same order for all the moves except the
422 // new PV that goes to the front. Note that in case of MultiPV
423 // search the already searched PV lines are preserved.
424 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
426 // If search has been stopped, we break immediately. Sorting is
427 // safe because RootMoves is still valid, although it refers to
428 // the previous iteration.
432 // When failing high/low give some update (without cluttering
433 // the UI) before a re-search.
436 && (bestValue <= alpha || bestValue >= beta)
437 && Time.elapsed() > 3000)
438 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
440 // In case of failing low/high increase aspiration window and
441 // re-search, otherwise exit the loop.
442 if (bestValue <= alpha)
444 beta = (alpha + beta) / 2;
445 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
451 Threads.stopOnPonderhit = false;
454 else if (bestValue >= beta)
456 beta = std::min(bestValue + delta, VALUE_INFINITE);
463 delta += delta / 4 + 5;
465 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
468 // Sort the PV lines searched so far and update the GUI
469 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
472 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
473 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
477 completedDepth = rootDepth;
479 if (rootMoves[0].pv[0] != lastBestMove) {
480 lastBestMove = rootMoves[0].pv[0];
481 lastBestMoveDepth = rootDepth;
484 // Have we found a "mate in x"?
486 && bestValue >= VALUE_MATE_IN_MAX_PLY
487 && VALUE_MATE - bestValue <= 2 * Limits.mate)
493 // If skill level is enabled and time is up, pick a sub-optimal best move
494 if (skill.enabled() && skill.time_to_pick(rootDepth))
495 skill.pick_best(multiPV);
497 // Do we have time for the next iteration? Can we stop searching now?
498 if ( Limits.use_time_management()
500 && !Threads.stopOnPonderhit)
502 double fallingEval = (306 + 119 * failedLow + 6 * (mainThread->previousScore - bestValue)) / 581.0;
503 fallingEval = std::max(0.5, std::min(1.5, fallingEval));
505 // If the bestMove is stable over several iterations, reduce time accordingly
507 for (int i : {3, 4, 5})
508 if (lastBestMoveDepth * i < completedDepth)
509 timeReduction *= 1.25;
511 // Use part of the gained time from a previous stable move for the current move
512 double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
513 bestMoveInstability *= std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
515 // Stop the search if we have only one legal move, or if available time elapsed
516 if ( rootMoves.size() == 1
517 || Time.elapsed() > Time.optimum() * bestMoveInstability * fallingEval)
519 // If we are allowed to ponder do not stop the search now but
520 // keep pondering until the GUI sends "ponderhit" or "stop".
522 Threads.stopOnPonderhit = true;
532 mainThread->previousTimeReduction = timeReduction;
534 // If skill level is enabled, swap best PV line with the sub-optimal one
536 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
537 skill.best ? skill.best : skill.pick_best(multiPV)));
543 // search<>() is the main search function for both PV and non-PV nodes
545 template <NodeType NT>
546 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
548 constexpr bool PvNode = NT == PV;
549 const bool rootNode = PvNode && ss->ply == 0;
551 // Check if we have an upcoming move which draws by repetition, or
552 // if the opponent had an alternative move earlier to this position.
553 if ( pos.rule50_count() >= 3
554 && alpha < VALUE_DRAW
556 && pos.has_game_cycle(ss->ply))
558 alpha = value_draw(depth, pos.this_thread());
563 // Dive into quiescence search when the depth reaches zero
565 return qsearch<NT>(pos, ss, alpha, beta);
567 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
568 assert(PvNode || (alpha == beta - 1));
569 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
570 assert(!(PvNode && cutNode));
571 assert(depth / ONE_PLY * ONE_PLY == depth);
573 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
577 Move ttMove, move, excludedMove, bestMove;
578 Depth extension, newDepth;
579 Value bestValue, value, ttValue, eval, maxValue, pureStaticEval;
580 bool ttHit, pvHit, inCheck, givesCheck, improving;
581 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
583 int moveCount, captureCount, quietCount;
585 // Step 1. Initialize node
586 Thread* thisThread = pos.this_thread();
587 inCheck = pos.checkers();
588 Color us = pos.side_to_move();
589 moveCount = captureCount = quietCount = ss->moveCount = 0;
590 bestValue = -VALUE_INFINITE;
591 maxValue = VALUE_INFINITE;
593 // Check for the available remaining time
594 if (thisThread == Threads.main())
595 static_cast<MainThread*>(thisThread)->check_time();
597 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
598 if (PvNode && thisThread->selDepth < ss->ply + 1)
599 thisThread->selDepth = ss->ply + 1;
603 // Step 2. Check for aborted search and immediate draw
604 if ( Threads.stop.load(std::memory_order_relaxed)
605 || pos.is_draw(ss->ply)
606 || ss->ply >= MAX_PLY)
607 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
608 : value_draw(depth, pos.this_thread());
610 // Step 3. Mate distance pruning. Even if we mate at the next move our score
611 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
612 // a shorter mate was found upward in the tree then there is no need to search
613 // because we will never beat the current alpha. Same logic but with reversed
614 // signs applies also in the opposite condition of being mated instead of giving
615 // mate. In this case return a fail-high score.
616 alpha = std::max(mated_in(ss->ply), alpha);
617 beta = std::min(mate_in(ss->ply+1), beta);
622 assert(0 <= ss->ply && ss->ply < MAX_PLY);
624 (ss+1)->ply = ss->ply + 1;
625 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
626 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
627 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
628 Square prevSq = to_sq((ss-1)->currentMove);
630 // Initialize statScore to zero for the grandchildren of the current position.
631 // So statScore is shared between all grandchildren and only the first grandchild
632 // starts with statScore = 0. Later grandchildren start with the last calculated
633 // statScore of the previous grandchild. This influences the reduction rules in
634 // LMR which are based on the statScore of parent position.
635 (ss+2)->statScore = 0;
637 // Step 4. Transposition table lookup. We don't want the score of a partial
638 // search to overwrite a previous full search TT value, so we use a different
639 // position key in case of an excluded move.
640 excludedMove = ss->excludedMove;
641 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
642 tte = TT.probe(posKey, ttHit);
643 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
644 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
645 : ttHit ? tte->move() : MOVE_NONE;
646 pvHit = ttHit ? tte->pv_hit() : false;
648 // At non-PV nodes we check for an early TT cutoff
651 && tte->depth() >= depth
652 && ttValue != VALUE_NONE // Possible in case of TT access race
653 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
654 : (tte->bound() & BOUND_UPPER)))
656 // If ttMove is quiet, update move sorting heuristics on TT hit
661 if (!pos.capture_or_promotion(ttMove))
662 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
664 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
665 if ( ((ss-1)->moveCount == 1 || (ss-1)->currentMove == (ss-1)->killers[0])
666 && !pos.captured_piece())
667 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
669 // Penalty for a quiet ttMove that fails low
670 else if (!pos.capture_or_promotion(ttMove))
672 int penalty = -stat_bonus(depth);
673 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
674 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
680 if ( depth > 6 * ONE_PLY
685 // Step 5. Tablebases probe
686 if (!rootNode && TB::Cardinality)
688 int piecesCount = pos.count<ALL_PIECES>();
690 if ( piecesCount <= TB::Cardinality
691 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
692 && pos.rule50_count() == 0
693 && !pos.can_castle(ANY_CASTLING))
696 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
698 // Force check of time on the next occasion
699 if (thisThread == Threads.main())
700 static_cast<MainThread*>(thisThread)->callsCnt = 0;
702 if (err != TB::ProbeState::FAIL)
704 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
706 int drawScore = TB::UseRule50 ? 1 : 0;
708 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
709 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
710 : VALUE_DRAW + 2 * wdl * drawScore;
712 Bound b = wdl < -drawScore ? BOUND_UPPER
713 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
715 if ( b == BOUND_EXACT
716 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
718 tte->save(posKey, value_to_tt(value, ss->ply), pvHit, b,
719 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
720 MOVE_NONE, VALUE_NONE);
727 if (b == BOUND_LOWER)
728 bestValue = value, alpha = std::max(alpha, bestValue);
736 // Step 6. Static evaluation of the position
739 ss->staticEval = eval = pureStaticEval = VALUE_NONE;
741 goto moves_loop; // Skip early pruning when in check
745 // Never assume anything on values stored in TT
746 ss->staticEval = eval = pureStaticEval = tte->eval();
747 if (eval == VALUE_NONE)
748 ss->staticEval = eval = pureStaticEval = evaluate(pos);
750 // Can ttValue be used as a better position evaluation?
751 if ( ttValue != VALUE_NONE
752 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
757 if ((ss-1)->currentMove != MOVE_NULL)
759 int p = (ss-1)->statScore;
760 int bonus = p > 0 ? (-p - 2500) / 512 :
761 p < 0 ? (-p + 2500) / 512 : 0;
763 pureStaticEval = evaluate(pos);
764 ss->staticEval = eval = pureStaticEval + bonus;
767 ss->staticEval = eval = pureStaticEval = -(ss-1)->staticEval + 2 * Eval::Tempo;
769 tte->save(posKey, VALUE_NONE, pvHit, BOUND_NONE, DEPTH_NONE, MOVE_NONE, pureStaticEval);
772 // Step 7. Razoring (~2 Elo)
773 if ( !rootNode // The required rootNode PV handling is not available in qsearch
774 && depth < 2 * ONE_PLY
775 && eval <= alpha - RazorMargin)
776 return qsearch<NT>(pos, ss, alpha, beta);
778 improving = ss->staticEval >= (ss-2)->staticEval
779 || (ss-2)->staticEval == VALUE_NONE;
781 // Step 8. Futility pruning: child node (~30 Elo)
783 && depth < 7 * ONE_PLY
784 && eval - futility_margin(depth, improving) >= beta
785 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
788 // Step 9. Null move search with verification search (~40 Elo)
790 && (ss-1)->currentMove != MOVE_NULL
791 && (ss-1)->statScore < 23200
793 && pureStaticEval >= beta - 36 * depth / ONE_PLY + 225
795 && pos.non_pawn_material(us)
796 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
798 assert(eval - beta >= 0);
800 // Null move dynamic reduction based on depth and value
801 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 200, 3)) * ONE_PLY;
803 ss->currentMove = MOVE_NULL;
804 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
806 pos.do_null_move(st);
808 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
810 pos.undo_null_move();
812 if (nullValue >= beta)
814 // Do not return unproven mate scores
815 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
818 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 12 * ONE_PLY))
821 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
823 // Do verification search at high depths, with null move pruning disabled
824 // for us, until ply exceeds nmpMinPly.
825 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
826 thisThread->nmpColor = us;
828 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
830 thisThread->nmpMinPly = 0;
837 // Step 10. ProbCut (~10 Elo)
838 // If we have a good enough capture and a reduced search returns a value
839 // much above beta, we can (almost) safely prune the previous move.
841 && depth >= 5 * ONE_PLY
842 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
844 Value raisedBeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
845 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
846 int probCutCount = 0;
848 while ( (move = mp.next_move()) != MOVE_NONE
850 if (move != excludedMove && pos.legal(move))
854 ss->currentMove = move;
855 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
857 assert(depth >= 5 * ONE_PLY);
859 pos.do_move(move, st);
861 // Perform a preliminary qsearch to verify that the move holds
862 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
864 // If the qsearch held perform the regular search
865 if (value >= raisedBeta)
866 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
870 if (value >= raisedBeta)
875 // Step 11. Internal iterative deepening (~2 Elo)
876 if ( depth >= 8 * ONE_PLY
879 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
881 tte = TT.probe(posKey, ttHit);
882 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
883 ttMove = ttHit ? tte->move() : MOVE_NONE;
884 pvHit = ttHit ? tte->pv_hit() : false;
887 moves_loop: // When in check, search starts from here
889 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, nullptr, (ss-4)->continuationHistory };
890 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
892 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
893 &thisThread->captureHistory,
897 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
900 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
901 pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
903 // Step 12. Loop through all pseudo-legal moves until no moves remain
904 // or a beta cutoff occurs.
905 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
909 if (move == excludedMove)
912 // At root obey the "searchmoves" option and skip moves not listed in Root
913 // Move List. As a consequence any illegal move is also skipped. In MultiPV
914 // mode we also skip PV moves which have been already searched and those
915 // of lower "TB rank" if we are in a TB root position.
916 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
917 thisThread->rootMoves.begin() + thisThread->pvLast, move))
920 ss->moveCount = ++moveCount;
922 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
923 sync_cout << "info depth " << depth / ONE_PLY
924 << " currmove " << UCI::move(move, pos.is_chess960())
925 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
927 (ss+1)->pv = nullptr;
929 extension = DEPTH_ZERO;
930 captureOrPromotion = pos.capture_or_promotion(move);
931 movedPiece = pos.moved_piece(move);
932 givesCheck = gives_check(pos, move);
934 moveCountPruning = depth < 16 * ONE_PLY
935 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
937 // Step 13. Extensions (~70 Elo)
939 // Singular extension search (~60 Elo). If all moves but one fail low on a
940 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
941 // then that move is singular and should be extended. To verify this we do
942 // a reduced search on all the other moves but the ttMove and if the
943 // result is lower than ttValue minus a margin then we will extend the ttMove.
944 if ( depth >= 8 * ONE_PLY
947 && !excludedMove // Avoid recursive singular search
948 && ttValue != VALUE_NONE
949 && (tte->bound() & BOUND_LOWER)
950 && tte->depth() >= depth - 3 * ONE_PLY
953 Value singularBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
954 ss->excludedMove = move;
955 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, depth / 2, cutNode);
956 ss->excludedMove = MOVE_NONE;
958 if (value < singularBeta)
962 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
963 // search without the ttMove. So we assume this expected Cut-node is not singular,
964 // that is multiple moves fail high, and we can prune the whole subtree by returning
965 // the hard beta bound.
966 else if (cutNode && singularBeta > beta)
969 else if ( givesCheck // Check extension (~2 Elo)
973 // Extension if castling
974 else if (type_of(move) == CASTLING)
977 // Calculate new depth for this move
978 newDepth = depth - ONE_PLY + extension;
980 // Step 14. Pruning at shallow depth (~170 Elo)
982 && pos.non_pawn_material(us)
983 && bestValue > VALUE_MATED_IN_MAX_PLY)
985 if ( !captureOrPromotion
987 && !pos.advanced_pawn_push(move))
989 // Move count based pruning (~30 Elo)
990 if (moveCountPruning)
996 // Reduced depth of the next LMR search
997 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
999 // Countermoves based pruning (~20 Elo)
1000 if ( lmrDepth < 3 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1001 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1002 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1005 // Futility pruning: parent node (~2 Elo)
1008 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
1011 // Prune moves with negative SEE (~10 Elo)
1012 if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
1015 else if ( !extension // (~20 Elo)
1016 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
1020 // Speculative prefetch as early as possible
1021 prefetch(TT.first_entry(pos.key_after(move)));
1023 // Check for legality just before making the move
1024 if (!rootNode && !pos.legal(move))
1026 ss->moveCount = --moveCount;
1030 // Update the current move (this must be done after singular extension search)
1031 ss->currentMove = move;
1032 ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
1034 // Step 15. Make the move
1035 pos.do_move(move, st, givesCheck);
1037 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1038 // re-searched at full depth.
1039 if ( depth >= 3 * ONE_PLY
1041 && (!captureOrPromotion || moveCountPruning))
1043 Depth r = reduction<PvNode>(improving, depth, moveCount);
1045 // Decrease reduction if position is or has been on the PV
1046 if (pvHit && !PvNode)
1049 // Decrease reduction if opponent's move count is high (~10 Elo)
1050 if ((ss-1)->moveCount > 15)
1053 if (!captureOrPromotion)
1055 // Decrease reduction for exact PV nodes (~0 Elo)
1059 // Increase reduction if ttMove is a capture (~0 Elo)
1063 // Increase reduction for cut nodes (~5 Elo)
1067 // Decrease reduction for moves that escape a capture. Filter out
1068 // castling moves, because they are coded as "king captures rook" and
1069 // hence break make_move(). (~5 Elo)
1070 else if ( type_of(move) == NORMAL
1071 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1074 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1075 + (*contHist[0])[movedPiece][to_sq(move)]
1076 + (*contHist[1])[movedPiece][to_sq(move)]
1077 + (*contHist[3])[movedPiece][to_sq(move)]
1080 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1081 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1084 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1087 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1088 r -= ss->statScore / 20000 * ONE_PLY;
1091 Depth d = std::max(newDepth - std::max(r, DEPTH_ZERO), ONE_PLY);
1093 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1095 doFullDepthSearch = (value > alpha && d != newDepth);
1098 doFullDepthSearch = !PvNode || moveCount > 1;
1100 // Step 17. Full depth search when LMR is skipped or fails high
1101 if (doFullDepthSearch)
1102 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1104 // For PV nodes only, do a full PV search on the first move or after a fail
1105 // high (in the latter case search only if value < beta), otherwise let the
1106 // parent node fail low with value <= alpha and try another move.
1107 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1110 (ss+1)->pv[0] = MOVE_NONE;
1112 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1115 // Step 18. Undo move
1116 pos.undo_move(move);
1118 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1120 // Step 19. Check for a new best move
1121 // Finished searching the move. If a stop occurred, the return value of
1122 // the search cannot be trusted, and we return immediately without
1123 // updating best move, PV and TT.
1124 if (Threads.stop.load(std::memory_order_relaxed))
1129 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1130 thisThread->rootMoves.end(), move);
1132 // PV move or new best move?
1133 if (moveCount == 1 || value > alpha)
1136 rm.selDepth = thisThread->selDepth;
1141 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1142 rm.pv.push_back(*m);
1144 // We record how often the best move has been changed in each
1145 // iteration. This information is used for time management: When
1146 // the best move changes frequently, we allocate some more time.
1147 if (moveCount > 1 && thisThread == Threads.main())
1148 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1151 // All other moves but the PV are set to the lowest value: this
1152 // is not a problem when sorting because the sort is stable and the
1153 // move position in the list is preserved - just the PV is pushed up.
1154 rm.score = -VALUE_INFINITE;
1157 if (value > bestValue)
1165 if (PvNode && !rootNode) // Update pv even in fail-high case
1166 update_pv(ss->pv, move, (ss+1)->pv);
1168 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1172 assert(value >= beta); // Fail high
1179 if (move != bestMove)
1181 if (captureOrPromotion && captureCount < 32)
1182 capturesSearched[captureCount++] = move;
1184 else if (!captureOrPromotion && quietCount < 64)
1185 quietsSearched[quietCount++] = move;
1189 // The following condition would detect a stop only after move loop has been
1190 // completed. But in this case bestValue is valid because we have fully
1191 // searched our subtree, and we can anyhow save the result in TT.
1197 // Step 20. Check for mate and stalemate
1198 // All legal moves have been searched and if there are no legal moves, it
1199 // must be a mate or a stalemate. If we are in a singular extension search then
1200 // return a fail low score.
1202 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1205 bestValue = excludedMove ? alpha
1206 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1209 // Quiet best move: update move sorting heuristics
1210 if (!pos.capture_or_promotion(bestMove))
1211 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1212 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1214 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1216 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1217 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1218 && !pos.captured_piece())
1219 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1222 // Bonus for prior countermove that caused the fail low
1223 else if ( (depth >= 3 * ONE_PLY || PvNode)
1224 && !pos.captured_piece())
1225 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1228 bestValue = std::min(bestValue, maxValue);
1231 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1232 bestValue >= beta ? BOUND_LOWER :
1233 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1234 depth, bestMove, pureStaticEval);
1236 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1242 // qsearch() is the quiescence search function, which is called by the main
1243 // search function with depth zero, or recursively with depth less than ONE_PLY.
1244 template <NodeType NT>
1245 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1247 constexpr bool PvNode = NT == PV;
1249 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1250 assert(PvNode || (alpha == beta - 1));
1251 assert(depth <= DEPTH_ZERO);
1252 assert(depth / ONE_PLY * ONE_PLY == depth);
1258 Move ttMove, move, bestMove;
1260 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1261 bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
1266 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1268 ss->pv[0] = MOVE_NONE;
1271 Thread* thisThread = pos.this_thread();
1272 (ss+1)->ply = ss->ply + 1;
1273 ss->currentMove = bestMove = MOVE_NONE;
1274 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
1275 inCheck = pos.checkers();
1278 // Check for an immediate draw or maximum ply reached
1279 if ( pos.is_draw(ss->ply)
1280 || ss->ply >= MAX_PLY)
1281 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1283 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1285 // Decide whether or not to include checks: this fixes also the type of
1286 // TT entry depth that we are going to use. Note that in qsearch we use
1287 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1288 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1289 : DEPTH_QS_NO_CHECKS;
1290 // Transposition table lookup
1292 tte = TT.probe(posKey, ttHit);
1293 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1294 ttMove = ttHit ? tte->move() : MOVE_NONE;
1295 pvHit = ttHit ? tte->pv_hit() : false;
1299 && tte->depth() >= ttDepth
1300 && ttValue != VALUE_NONE // Only in case of TT access race
1301 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1302 : (tte->bound() & BOUND_UPPER)))
1305 // Evaluate the position statically
1308 ss->staticEval = VALUE_NONE;
1309 bestValue = futilityBase = -VALUE_INFINITE;
1315 // Never assume anything on values stored in TT
1316 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1317 ss->staticEval = bestValue = evaluate(pos);
1319 // Can ttValue be used as a better position evaluation?
1320 if ( ttValue != VALUE_NONE
1321 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1322 bestValue = ttValue;
1325 ss->staticEval = bestValue =
1326 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1327 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1329 // Stand pat. Return immediately if static value is at least beta
1330 if (bestValue >= beta)
1333 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1334 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1339 if (PvNode && bestValue > alpha)
1342 futilityBase = bestValue + 128;
1345 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, nullptr, (ss-4)->continuationHistory };
1347 // Initialize a MovePicker object for the current position, and prepare
1348 // to search the moves. Because the depth is <= 0 here, only captures,
1349 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1351 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1352 &thisThread->captureHistory,
1354 to_sq((ss-1)->currentMove));
1356 // Loop through the moves until no moves remain or a beta cutoff occurs
1357 while ((move = mp.next_move()) != MOVE_NONE)
1359 assert(is_ok(move));
1361 givesCheck = gives_check(pos, move);
1368 && futilityBase > -VALUE_KNOWN_WIN
1369 && !pos.advanced_pawn_push(move))
1371 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1373 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1375 if (futilityValue <= alpha)
1377 bestValue = std::max(bestValue, futilityValue);
1381 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1383 bestValue = std::max(bestValue, futilityBase);
1388 // Detect non-capture evasions that are candidates to be pruned
1389 evasionPrunable = inCheck
1390 && (depth != DEPTH_ZERO || moveCount > 2)
1391 && bestValue > VALUE_MATED_IN_MAX_PLY
1392 && !pos.capture(move);
1394 // Don't search moves with negative SEE values
1395 if ( (!inCheck || evasionPrunable)
1396 && !pos.see_ge(move))
1399 // Speculative prefetch as early as possible
1400 prefetch(TT.first_entry(pos.key_after(move)));
1402 // Check for legality just before making the move
1403 if (!pos.legal(move))
1409 ss->currentMove = move;
1410 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
1412 // Make and search the move
1413 pos.do_move(move, st, givesCheck);
1414 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1415 pos.undo_move(move);
1417 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1419 // Check for a new best move
1420 if (value > bestValue)
1428 if (PvNode) // Update pv even in fail-high case
1429 update_pv(ss->pv, move, (ss+1)->pv);
1431 if (PvNode && value < beta) // Update alpha here!
1439 // All legal moves have been searched. A special case: If we're in check
1440 // and no legal moves were found, it is checkmate.
1441 if (inCheck && bestValue == -VALUE_INFINITE)
1442 return mated_in(ss->ply); // Plies to mate from the root
1444 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1445 bestValue >= beta ? BOUND_LOWER :
1446 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1447 ttDepth, bestMove, ss->staticEval);
1449 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1455 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1456 // "plies to mate from the current position". Non-mate scores are unchanged.
1457 // The function is called before storing a value in the transposition table.
1459 Value value_to_tt(Value v, int ply) {
1461 assert(v != VALUE_NONE);
1463 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1464 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1468 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1469 // from the transposition table (which refers to the plies to mate/be mated
1470 // from current position) to "plies to mate/be mated from the root".
1472 Value value_from_tt(Value v, int ply) {
1474 return v == VALUE_NONE ? VALUE_NONE
1475 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1476 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1480 // update_pv() adds current move and appends child pv[]
1482 void update_pv(Move* pv, Move move, Move* childPv) {
1484 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1490 // update_continuation_histories() updates histories of the move pairs formed
1491 // by moves at ply -1, -2, and -4 with current move.
1493 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1495 for (int i : {1, 2, 4})
1496 if (is_ok((ss-i)->currentMove))
1497 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1501 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1503 void update_capture_stats(const Position& pos, Move move,
1504 Move* captures, int captureCount, int bonus) {
1506 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1507 Piece moved_piece = pos.moved_piece(move);
1508 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1510 if (pos.capture_or_promotion(move))
1511 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1513 // Decrease all the other played capture moves
1514 for (int i = 0; i < captureCount; ++i)
1516 moved_piece = pos.moved_piece(captures[i]);
1517 captured = type_of(pos.piece_on(to_sq(captures[i])));
1518 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1523 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1525 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1526 Move* quiets, int quietCount, int bonus) {
1528 if (ss->killers[0] != move)
1530 ss->killers[1] = ss->killers[0];
1531 ss->killers[0] = move;
1534 Color us = pos.side_to_move();
1535 Thread* thisThread = pos.this_thread();
1536 thisThread->mainHistory[us][from_to(move)] << bonus;
1537 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1539 if (is_ok((ss-1)->currentMove))
1541 Square prevSq = to_sq((ss-1)->currentMove);
1542 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1545 // Decrease all the other played quiet moves
1546 for (int i = 0; i < quietCount; ++i)
1548 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1549 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1553 // When playing with strength handicap, choose best move among a set of RootMoves
1554 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1556 Move Skill::pick_best(size_t multiPV) {
1558 const RootMoves& rootMoves = Threads.main()->rootMoves;
1559 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1561 // RootMoves are already sorted by score in descending order
1562 Value topScore = rootMoves[0].score;
1563 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1564 int weakness = 120 - 2 * level;
1565 int maxScore = -VALUE_INFINITE;
1567 // Choose best move. For each move score we add two terms, both dependent on
1568 // weakness. One is deterministic and bigger for weaker levels, and one is
1569 // random. Then we choose the move with the resulting highest score.
1570 for (size_t i = 0; i < multiPV; ++i)
1572 // This is our magic formula
1573 int push = ( weakness * int(topScore - rootMoves[i].score)
1574 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1576 if (rootMoves[i].score + push >= maxScore)
1578 maxScore = rootMoves[i].score + push;
1579 best = rootMoves[i].pv[0];
1588 /// MainThread::check_time() is used to print debug info and, more importantly,
1589 /// to detect when we are out of available time and thus stop the search.
1591 void MainThread::check_time() {
1596 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1597 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1599 static TimePoint lastInfoTime = now();
1601 TimePoint elapsed = Time.elapsed();
1602 TimePoint tick = Limits.startTime + elapsed;
1604 if (tick - lastInfoTime >= 1000)
1606 lastInfoTime = tick;
1610 // We should not stop pondering until told so by the GUI
1614 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1615 || (Limits.movetime && elapsed >= Limits.movetime)
1616 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1617 Threads.stop = true;
1621 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1622 /// that all (if any) unsearched PV lines are sent using a previous search score.
1624 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1626 std::stringstream ss;
1627 TimePoint elapsed = Time.elapsed() + 1;
1628 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1629 size_t pvIdx = pos.this_thread()->pvIdx;
1630 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1631 uint64_t nodesSearched = Threads.nodes_searched();
1632 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1634 for (size_t i = 0; i < multiPV; ++i)
1636 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1638 if (depth == ONE_PLY && !updated)
1641 Depth d = updated ? depth : depth - ONE_PLY;
1642 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1644 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1645 v = tb ? rootMoves[i].tbScore : v;
1647 if (ss.rdbuf()->in_avail()) // Not at first line
1651 << " depth " << d / ONE_PLY
1652 << " seldepth " << rootMoves[i].selDepth
1653 << " multipv " << i + 1
1654 << " score " << UCI::value(v);
1656 if (!tb && i == pvIdx)
1657 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1659 ss << " nodes " << nodesSearched
1660 << " nps " << nodesSearched * 1000 / elapsed;
1662 if (elapsed > 1000) // Earlier makes little sense
1663 ss << " hashfull " << TT.hashfull();
1665 ss << " tbhits " << tbHits
1666 << " time " << elapsed
1669 for (Move m : rootMoves[i].pv)
1670 ss << " " << UCI::move(m, pos.is_chess960());
1677 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1678 /// before exiting the search, for instance, in case we stop the search during a
1679 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1680 /// otherwise in case of 'ponder on' we have nothing to think on.
1682 bool RootMove::extract_ponder_from_tt(Position& pos) {
1687 assert(pv.size() == 1);
1689 if (pv[0] == MOVE_NONE)
1692 pos.do_move(pv[0], st);
1693 TTEntry* tte = TT.probe(pos.key(), ttHit);
1697 Move m = tte->move(); // Local copy to be SMP safe
1698 if (MoveList<LEGAL>(pos).contains(m))
1702 pos.undo_move(pv[0]);
1703 return pv.size() > 1;
1706 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1709 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1710 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1711 Cardinality = int(Options["SyzygyProbeLimit"]);
1712 bool dtz_available = true;
1714 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1715 // ProbeDepth == DEPTH_ZERO
1716 if (Cardinality > MaxCardinality)
1718 Cardinality = MaxCardinality;
1719 ProbeDepth = DEPTH_ZERO;
1722 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1724 // Rank moves using DTZ tables
1725 RootInTB = root_probe(pos, rootMoves);
1729 // DTZ tables are missing; try to rank moves using WDL tables
1730 dtz_available = false;
1731 RootInTB = root_probe_wdl(pos, rootMoves);
1737 // Sort moves according to TB rank
1738 std::sort(rootMoves.begin(), rootMoves.end(),
1739 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1741 // Probe during search only if DTZ is not available and we are winning
1742 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1747 // Assign the same rank to all moves
1748 for (auto& m : rootMoves)