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 // Reductions lookup table, initialized at startup
75 int Reductions[64]; // [depth or moveNumber]
77 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
78 int r = Reductions[std::min(d / ONE_PLY, 63)] * Reductions[std::min(mn, 63)] / 1024;
79 return ((r + 512) / 1024 + (!i && r > 1024) - PvNode) * ONE_PLY;
82 constexpr int futility_move_count(bool improving, int depth) {
83 return (5 + depth * depth) * (1 + improving) / 2;
86 // History and stats update bonus, based on depth
87 int stat_bonus(Depth depth) {
88 int d = depth / ONE_PLY;
89 return d > 17 ? 0 : 29 * d * d + 138 * d - 134;
92 // Add a small random component to draw evaluations to keep search dynamic
93 // and to avoid 3fold-blindness.
94 Value value_draw(Depth depth, Thread* thisThread) {
95 return depth < 4 ? VALUE_DRAW
96 : VALUE_DRAW + Value(2 * (thisThread->nodes.load(std::memory_order_relaxed) % 2) - 1);
99 // Skill structure is used to implement strength limit
101 explicit Skill(int l) : level(l) {}
102 bool enabled() const { return level < 20; }
103 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
104 Move pick_best(size_t multiPV);
107 Move best = MOVE_NONE;
110 template <NodeType NT>
111 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
113 template <NodeType NT>
114 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
116 Value value_to_tt(Value v, int ply);
117 Value value_from_tt(Value v, int ply);
118 void update_pv(Move* pv, Move move, Move* childPv);
119 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
120 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietCount, int bonus);
121 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCount, int bonus);
123 inline bool gives_check(const Position& pos, Move move) {
124 Color us = pos.side_to_move();
125 return type_of(move) == NORMAL && !(pos.blockers_for_king(~us) & pos.pieces(us))
126 ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
127 : pos.gives_check(move);
130 // perft() is our utility to verify move generation. All the leaf nodes up
131 // to the given depth are generated and counted, and the sum is returned.
133 uint64_t perft(Position& pos, Depth depth) {
136 uint64_t cnt, nodes = 0;
137 const bool leaf = (depth == 2 * ONE_PLY);
139 for (const auto& m : MoveList<LEGAL>(pos))
141 if (Root && depth <= ONE_PLY)
146 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
151 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
159 /// Search::init() is called at startup to initialize various lookup tables
161 void Search::init() {
163 for (int i = 1; i < 64; ++i)
164 Reductions[i] = int(1024 * std::log(i) / std::sqrt(1.95));
168 /// Search::clear() resets search state to its initial value
170 void Search::clear() {
172 Threads.main()->wait_for_search_finished();
174 Time.availableNodes = 0;
177 Tablebases::init(Options["SyzygyPath"]); // Free up mapped files
181 /// MainThread::search() is called by the main thread when the program receives
182 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
184 void MainThread::search() {
188 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
189 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
193 Color us = rootPos.side_to_move();
194 Time.init(Limits, us, rootPos.game_ply());
197 if (rootMoves.empty())
199 rootMoves.emplace_back(MOVE_NONE);
200 sync_cout << "info depth 0 score "
201 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
206 for (Thread* th : Threads)
208 th->start_searching();
210 Thread::search(); // Let's start searching!
213 // When we reach the maximum depth, we can arrive here without a raise of
214 // Threads.stop. However, if we are pondering or in an infinite search,
215 // the UCI protocol states that we shouldn't print the best move before the
216 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
217 // until the GUI sends one of those commands.
219 while (!Threads.stop && (ponder || Limits.infinite))
220 {} // Busy wait for a stop or a ponder reset
222 // Stop the threads if not already stopped (also raise the stop if
223 // "ponderhit" just reset Threads.ponder).
226 // Wait until all threads have finished
227 for (Thread* th : Threads)
229 th->wait_for_search_finished();
231 // When playing in 'nodes as time' mode, subtract the searched nodes from
232 // the available ones before exiting.
234 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
236 // Check if there are threads with a better score than main thread
237 Thread* bestThread = this;
238 if ( Options["MultiPV"] == 1
240 && !Skill(Options["Skill Level"]).enabled()
241 && rootMoves[0].pv[0] != MOVE_NONE)
243 std::map<Move, int64_t> votes;
244 Value minScore = this->rootMoves[0].score;
246 // Find out minimum score and reset votes for moves which can be voted
247 for (Thread* th: Threads)
248 minScore = std::min(minScore, th->rootMoves[0].score);
250 // Vote according to score and depth
251 for (Thread* th : Threads)
253 int64_t s = th->rootMoves[0].score - minScore + 1;
254 votes[th->rootMoves[0].pv[0]] += 200 + s * s * int(th->completedDepth);
257 // Select best thread
258 auto bestVote = votes[this->rootMoves[0].pv[0]];
259 for (Thread* th : Threads)
260 if (votes[th->rootMoves[0].pv[0]] > bestVote)
262 bestVote = votes[th->rootMoves[0].pv[0]];
267 previousScore = bestThread->rootMoves[0].score;
269 // Send again PV info if we have a new best thread
270 if (bestThread != this)
271 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
273 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
275 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
276 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
278 std::cout << sync_endl;
282 /// Thread::search() is the main iterative deepening loop. It calls search()
283 /// repeatedly with increasing depth until the allocated thinking time has been
284 /// consumed, the user stops the search, or the maximum search depth is reached.
286 void Thread::search() {
288 // To allow access to (ss-5) up to (ss+2), the stack must be oversized.
289 // The former is needed to allow update_continuation_histories(ss-1, ...),
290 // which accesses its argument at ss-4, also near the root.
291 // The latter is needed for statScores and killer initialization.
292 Stack stack[MAX_PLY+10], *ss = stack+7;
294 Value bestValue, alpha, beta, delta;
295 Move lastBestMove = MOVE_NONE;
296 Depth lastBestMoveDepth = DEPTH_ZERO;
297 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
298 double timeReduction = 1.0;
299 Color us = rootPos.side_to_move();
301 std::memset(ss-7, 0, 10 * sizeof(Stack));
302 for (int i = 7; i > 0; i--)
303 (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
306 bestValue = delta = alpha = -VALUE_INFINITE;
307 beta = VALUE_INFINITE;
310 mainThread->bestMoveChanges = 0;
312 size_t multiPV = Options["MultiPV"];
313 Skill skill(Options["Skill Level"]);
315 // When playing with strength handicap enable MultiPV search that we will
316 // use behind the scenes to retrieve a set of possible moves.
318 multiPV = std::max(multiPV, (size_t)4);
320 multiPV = std::min(multiPV, rootMoves.size());
322 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
324 // In analysis mode, adjust contempt in accordance with user preference
325 if (Limits.infinite || Options["UCI_AnalyseMode"])
326 ct = Options["Analysis Contempt"] == "Off" ? 0
327 : Options["Analysis Contempt"] == "Both" ? ct
328 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
329 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
332 // In evaluate.cpp the evaluation is from the white point of view
333 contempt = (us == WHITE ? make_score(ct, ct / 2)
334 : -make_score(ct, ct / 2));
336 // Iterative deepening loop until requested to stop or the target depth is reached
337 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
339 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
341 // Distribute search depths across the helper threads
344 int i = (idx - 1) % 20;
345 if (((rootDepth / ONE_PLY + SkipPhase[i]) / SkipSize[i]) % 2)
346 continue; // Retry with an incremented rootDepth
349 // Age out PV variability metric
351 mainThread->bestMoveChanges *= 0.517;
353 // Save the last iteration's scores before first PV line is searched and
354 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
355 for (RootMove& rm : rootMoves)
356 rm.previousScore = rm.score;
361 // MultiPV loop. We perform a full root search for each PV line
362 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
367 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
368 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
372 // Reset UCI info selDepth for each depth and each PV line
375 // Reset aspiration window starting size
376 if (rootDepth >= 5 * ONE_PLY)
378 Value previousScore = rootMoves[pvIdx].previousScore;
380 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
381 beta = std::min(previousScore + delta, VALUE_INFINITE);
383 // Adjust contempt based on root move's previousScore (dynamic contempt)
384 int dct = ct + 88 * previousScore / (abs(previousScore) + 200);
386 contempt = (us == WHITE ? make_score(dct, dct / 2)
387 : -make_score(dct, dct / 2));
390 // Start with a small aspiration window and, in the case of a fail
391 // high/low, re-search with a bigger window until we don't fail
393 int failedHighCnt = 0;
396 Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
397 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
399 // Bring the best move to the front. It is critical that sorting
400 // is done with a stable algorithm because all the values but the
401 // first and eventually the new best one are set to -VALUE_INFINITE
402 // and we want to keep the same order for all the moves except the
403 // new PV that goes to the front. Note that in case of MultiPV
404 // search the already searched PV lines are preserved.
405 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
407 // If search has been stopped, we break immediately. Sorting is
408 // safe because RootMoves is still valid, although it refers to
409 // the previous iteration.
413 // When failing high/low give some update (without cluttering
414 // the UI) before a re-search.
417 && (bestValue <= alpha || bestValue >= beta)
418 && Time.elapsed() > 3000)
419 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
421 // In case of failing low/high increase aspiration window and
422 // re-search, otherwise exit the loop.
423 if (bestValue <= alpha)
425 beta = (alpha + beta) / 2;
426 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
431 mainThread->stopOnPonderhit = false;
434 else if (bestValue >= beta)
436 beta = std::min(bestValue + delta, VALUE_INFINITE);
443 delta += delta / 4 + 5;
445 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
448 // Sort the PV lines searched so far and update the GUI
449 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
452 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
453 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
457 completedDepth = rootDepth;
459 if (rootMoves[0].pv[0] != lastBestMove) {
460 lastBestMove = rootMoves[0].pv[0];
461 lastBestMoveDepth = rootDepth;
464 // Have we found a "mate in x"?
466 && bestValue >= VALUE_MATE_IN_MAX_PLY
467 && VALUE_MATE - bestValue <= 2 * Limits.mate)
473 // If skill level is enabled and time is up, pick a sub-optimal best move
474 if (skill.enabled() && skill.time_to_pick(rootDepth))
475 skill.pick_best(multiPV);
477 // Do we have time for the next iteration? Can we stop searching now?
478 if ( Limits.use_time_management()
480 && !mainThread->stopOnPonderhit)
482 double fallingEval = (306 + 9 * (mainThread->previousScore - bestValue)) / 581.0;
483 fallingEval = std::max(0.5, std::min(1.5, fallingEval));
485 // If the bestMove is stable over several iterations, reduce time accordingly
486 timeReduction = lastBestMoveDepth + 10 * ONE_PLY < completedDepth ? 1.95 : 1.0;
487 double reduction = std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
489 // Use part of the gained time from a previous stable move for the current move
490 double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
492 // Stop the search if we have only one legal move, or if available time elapsed
493 if ( rootMoves.size() == 1
494 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
496 // If we are allowed to ponder do not stop the search now but
497 // keep pondering until the GUI sends "ponderhit" or "stop".
498 if (mainThread->ponder)
499 mainThread->stopOnPonderhit = true;
509 mainThread->previousTimeReduction = timeReduction;
511 // If skill level is enabled, swap best PV line with the sub-optimal one
513 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
514 skill.best ? skill.best : skill.pick_best(multiPV)));
520 // search<>() is the main search function for both PV and non-PV nodes
522 template <NodeType NT>
523 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
525 constexpr bool PvNode = NT == PV;
526 const bool rootNode = PvNode && ss->ply == 0;
528 // Check if we have an upcoming move which draws by repetition, or
529 // if the opponent had an alternative move earlier to this position.
530 if ( pos.rule50_count() >= 3
531 && alpha < VALUE_DRAW
533 && pos.has_game_cycle(ss->ply))
535 alpha = value_draw(depth, pos.this_thread());
540 // Dive into quiescence search when the depth reaches zero
542 return qsearch<NT>(pos, ss, alpha, beta);
544 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
545 assert(PvNode || (alpha == beta - 1));
546 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
547 assert(!(PvNode && cutNode));
548 assert(depth / ONE_PLY * ONE_PLY == depth);
550 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
554 Move ttMove, move, excludedMove, bestMove;
555 Depth extension, newDepth;
556 Value bestValue, value, ttValue, eval, maxValue, pureStaticEval;
557 bool ttHit, ttPv, inCheck, givesCheck, improving;
558 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
560 int moveCount, captureCount, quietCount;
562 // Step 1. Initialize node
563 Thread* thisThread = pos.this_thread();
564 inCheck = pos.checkers();
565 Color us = pos.side_to_move();
566 moveCount = captureCount = quietCount = ss->moveCount = 0;
567 bestValue = -VALUE_INFINITE;
568 maxValue = VALUE_INFINITE;
570 // Check for the available remaining time
571 if (thisThread == Threads.main())
572 static_cast<MainThread*>(thisThread)->check_time();
574 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
575 if (PvNode && thisThread->selDepth < ss->ply + 1)
576 thisThread->selDepth = ss->ply + 1;
580 // Step 2. Check for aborted search and immediate draw
581 if ( Threads.stop.load(std::memory_order_relaxed)
582 || pos.is_draw(ss->ply)
583 || ss->ply >= MAX_PLY)
584 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
585 : value_draw(depth, pos.this_thread());
587 // Step 3. Mate distance pruning. Even if we mate at the next move our score
588 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
589 // a shorter mate was found upward in the tree then there is no need to search
590 // because we will never beat the current alpha. Same logic but with reversed
591 // signs applies also in the opposite condition of being mated instead of giving
592 // mate. In this case return a fail-high score.
593 alpha = std::max(mated_in(ss->ply), alpha);
594 beta = std::min(mate_in(ss->ply+1), beta);
599 assert(0 <= ss->ply && ss->ply < MAX_PLY);
601 (ss+1)->ply = ss->ply + 1;
602 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
603 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
604 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
605 Square prevSq = to_sq((ss-1)->currentMove);
607 // Initialize statScore to zero for the grandchildren of the current position.
608 // So statScore is shared between all grandchildren and only the first grandchild
609 // starts with statScore = 0. Later grandchildren start with the last calculated
610 // statScore of the previous grandchild. This influences the reduction rules in
611 // LMR which are based on the statScore of parent position.
612 (ss+2)->statScore = 0;
614 // Step 4. Transposition table lookup. We don't want the score of a partial
615 // search to overwrite a previous full search TT value, so we use a different
616 // position key in case of an excluded move.
617 excludedMove = ss->excludedMove;
618 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
619 tte = TT.probe(posKey, ttHit);
620 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
621 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
622 : ttHit ? tte->move() : MOVE_NONE;
623 ttPv = (ttHit && tte->is_pv()) || (PvNode && depth > 4 * ONE_PLY);
625 // At non-PV nodes we check for an early TT cutoff
628 && tte->depth() >= depth
629 && ttValue != VALUE_NONE // Possible in case of TT access race
630 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
631 : (tte->bound() & BOUND_UPPER)))
633 // If ttMove is quiet, update move sorting heuristics on TT hit
638 if (!pos.capture_or_promotion(ttMove))
639 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
641 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
642 if ( ((ss-1)->moveCount == 1 || (ss-1)->currentMove == (ss-1)->killers[0])
643 && !pos.captured_piece())
644 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
646 // Penalty for a quiet ttMove that fails low
647 else if (!pos.capture_or_promotion(ttMove))
649 int penalty = -stat_bonus(depth);
650 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
651 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
657 // Step 5. Tablebases probe
658 if (!rootNode && TB::Cardinality)
660 int piecesCount = pos.count<ALL_PIECES>();
662 if ( piecesCount <= TB::Cardinality
663 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
664 && pos.rule50_count() == 0
665 && !pos.can_castle(ANY_CASTLING))
668 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
670 // Force check of time on the next occasion
671 if (thisThread == Threads.main())
672 static_cast<MainThread*>(thisThread)->callsCnt = 0;
674 if (err != TB::ProbeState::FAIL)
676 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
678 int drawScore = TB::UseRule50 ? 1 : 0;
680 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
681 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
682 : VALUE_DRAW + 2 * wdl * drawScore;
684 Bound b = wdl < -drawScore ? BOUND_UPPER
685 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
687 if ( b == BOUND_EXACT
688 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
690 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
691 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
692 MOVE_NONE, VALUE_NONE);
699 if (b == BOUND_LOWER)
700 bestValue = value, alpha = std::max(alpha, bestValue);
708 // Step 6. Static evaluation of the position
711 ss->staticEval = eval = pureStaticEval = VALUE_NONE;
713 goto moves_loop; // Skip early pruning when in check
717 // Never assume anything on values stored in TT
718 ss->staticEval = eval = pureStaticEval = tte->eval();
719 if (eval == VALUE_NONE)
720 ss->staticEval = eval = pureStaticEval = evaluate(pos);
722 // Can ttValue be used as a better position evaluation?
723 if ( ttValue != VALUE_NONE
724 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
729 if ((ss-1)->currentMove != MOVE_NULL)
731 int bonus = -(ss-1)->statScore / 512;
733 pureStaticEval = evaluate(pos);
734 ss->staticEval = eval = pureStaticEval + bonus;
737 ss->staticEval = eval = pureStaticEval = -(ss-1)->staticEval + 2 * Eval::Tempo;
739 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, pureStaticEval);
742 // Step 7. Razoring (~2 Elo)
743 if ( !rootNode // The required rootNode PV handling is not available in qsearch
744 && depth < 2 * ONE_PLY
745 && eval <= alpha - RazorMargin)
746 return qsearch<NT>(pos, ss, alpha, beta);
748 improving = ss->staticEval >= (ss-2)->staticEval
749 || (ss-2)->staticEval == VALUE_NONE;
751 // Step 8. Futility pruning: child node (~30 Elo)
753 && depth < 7 * ONE_PLY
754 && eval - futility_margin(depth, improving) >= beta
755 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
758 // Step 9. Null move search with verification search (~40 Elo)
760 && (ss-1)->currentMove != MOVE_NULL
761 && (ss-1)->statScore < 23200
763 && pureStaticEval >= beta - 36 * depth / ONE_PLY + 225
765 && pos.non_pawn_material(us)
766 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
768 assert(eval - beta >= 0);
770 // Null move dynamic reduction based on depth and value
771 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 200, 3)) * ONE_PLY;
773 ss->currentMove = MOVE_NULL;
774 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
776 pos.do_null_move(st);
778 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
780 pos.undo_null_move();
782 if (nullValue >= beta)
784 // Do not return unproven mate scores
785 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
788 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 12 * ONE_PLY))
791 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
793 // Do verification search at high depths, with null move pruning disabled
794 // for us, until ply exceeds nmpMinPly.
795 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
796 thisThread->nmpColor = us;
798 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
800 thisThread->nmpMinPly = 0;
807 // Step 10. ProbCut (~10 Elo)
808 // If we have a good enough capture and a reduced search returns a value
809 // much above beta, we can (almost) safely prune the previous move.
811 && depth >= 5 * ONE_PLY
812 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
814 Value raisedBeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
815 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
816 int probCutCount = 0;
818 while ( (move = mp.next_move()) != MOVE_NONE
819 && probCutCount < 2 + 2 * cutNode)
820 if (move != excludedMove && pos.legal(move))
824 ss->currentMove = move;
825 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
827 assert(depth >= 5 * ONE_PLY);
829 pos.do_move(move, st);
831 // Perform a preliminary qsearch to verify that the move holds
832 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
834 // If the qsearch held, perform the regular search
835 if (value >= raisedBeta)
836 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
840 if (value >= raisedBeta)
845 // Step 11. Internal iterative deepening (~2 Elo)
846 if ( depth >= 8 * ONE_PLY
849 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
851 tte = TT.probe(posKey, ttHit);
852 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
853 ttMove = ttHit ? tte->move() : MOVE_NONE;
856 moves_loop: // When in check, search starts from here
858 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
859 nullptr, (ss-4)->continuationHistory,
860 nullptr, (ss-6)->continuationHistory };
861 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
863 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
864 &thisThread->captureHistory,
868 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
870 moveCountPruning = false;
871 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
873 // Step 12. Loop through all pseudo-legal moves until no moves remain
874 // or a beta cutoff occurs.
875 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
879 if (move == excludedMove)
882 // At root obey the "searchmoves" option and skip moves not listed in Root
883 // Move List. As a consequence any illegal move is also skipped. In MultiPV
884 // mode we also skip PV moves which have been already searched and those
885 // of lower "TB rank" if we are in a TB root position.
886 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
887 thisThread->rootMoves.begin() + thisThread->pvLast, move))
890 ss->moveCount = ++moveCount;
892 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
893 sync_cout << "info depth " << depth / ONE_PLY
894 << " currmove " << UCI::move(move, pos.is_chess960())
895 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
897 (ss+1)->pv = nullptr;
899 extension = DEPTH_ZERO;
900 captureOrPromotion = pos.capture_or_promotion(move);
901 movedPiece = pos.moved_piece(move);
902 givesCheck = gives_check(pos, move);
904 // Step 13. Extensions (~70 Elo)
906 // Singular extension search (~60 Elo). If all moves but one fail low on a
907 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
908 // then that move is singular and should be extended. To verify this we do
909 // a reduced search on all the other moves but the ttMove and if the
910 // result is lower than ttValue minus a margin then we will extend the ttMove.
911 if ( depth >= 8 * ONE_PLY
914 && !excludedMove // Avoid recursive singular search
915 /* && ttValue != VALUE_NONE Already implicit in the next condition */
916 && abs(ttValue) < VALUE_KNOWN_WIN
917 && (tte->bound() & BOUND_LOWER)
918 && tte->depth() >= depth - 3 * ONE_PLY
921 Value singularBeta = ttValue - 2 * depth / ONE_PLY;
922 ss->excludedMove = move;
923 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, depth / 2, cutNode);
924 ss->excludedMove = MOVE_NONE;
926 if (value < singularBeta)
930 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
931 // search without the ttMove. So we assume this expected Cut-node is not singular,
932 // that is multiple moves fail high, and we can prune the whole subtree by returning
933 // the hard beta bound.
934 else if (cutNode && singularBeta > beta)
938 // Check extension (~2 Elo)
940 && (pos.blockers_for_king(~us) & from_sq(move) || pos.see_ge(move)))
943 // Castling extension
944 else if (type_of(move) == CASTLING)
947 // Calculate new depth for this move
948 newDepth = depth - ONE_PLY + extension;
950 // Step 14. Pruning at shallow depth (~170 Elo)
952 && pos.non_pawn_material(us)
953 && bestValue > VALUE_MATED_IN_MAX_PLY)
955 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
956 moveCountPruning = moveCount >= futility_move_count(improving,depth / ONE_PLY);
958 if ( !captureOrPromotion
960 && !pos.advanced_pawn_push(move))
962 // Move count based pruning (~30 Elo)
963 if (moveCountPruning)
966 // Reduced depth of the next LMR search
967 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
969 // Countermoves based pruning (~20 Elo)
970 if ( lmrDepth < 3 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
971 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
972 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
975 // Futility pruning: parent node (~2 Elo)
978 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
981 // Prune moves with negative SEE (~10 Elo)
982 if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
985 else if ( !extension // (~20 Elo)
986 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
990 // Speculative prefetch as early as possible
991 prefetch(TT.first_entry(pos.key_after(move)));
993 // Check for legality just before making the move
994 if (!rootNode && !pos.legal(move))
996 ss->moveCount = --moveCount;
1000 // Update the current move (this must be done after singular extension search)
1001 ss->currentMove = move;
1002 ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
1004 // Step 15. Make the move
1005 pos.do_move(move, st, givesCheck);
1007 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1008 // re-searched at full depth.
1009 if ( depth >= 3 * ONE_PLY
1011 && (!captureOrPromotion || moveCountPruning))
1013 Depth r = reduction<PvNode>(improving, depth, moveCount);
1015 // Decrease reduction if position is or has been on the PV
1019 // Decrease reduction if opponent's move count is high (~10 Elo)
1020 if ((ss-1)->moveCount > 15)
1023 if (!captureOrPromotion)
1025 // Increase reduction if ttMove is a capture (~0 Elo)
1029 // Increase reduction for cut nodes (~5 Elo)
1033 // Decrease reduction for moves that escape a capture. Filter out
1034 // castling moves, because they are coded as "king captures rook" and
1035 // hence break make_move(). (~5 Elo)
1036 else if ( type_of(move) == NORMAL
1037 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1040 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1041 + (*contHist[0])[movedPiece][to_sq(move)]
1042 + (*contHist[1])[movedPiece][to_sq(move)]
1043 + (*contHist[3])[movedPiece][to_sq(move)]
1046 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1047 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1050 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1053 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1054 r -= ss->statScore / 20000 * ONE_PLY;
1057 Depth d = std::max(newDepth - std::max(r, DEPTH_ZERO), ONE_PLY);
1059 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1061 doFullDepthSearch = (value > alpha && d != newDepth);
1064 doFullDepthSearch = !PvNode || moveCount > 1;
1066 // Step 17. Full depth search when LMR is skipped or fails high
1067 if (doFullDepthSearch)
1068 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1070 // For PV nodes only, do a full PV search on the first move or after a fail
1071 // high (in the latter case search only if value < beta), otherwise let the
1072 // parent node fail low with value <= alpha and try another move.
1073 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1076 (ss+1)->pv[0] = MOVE_NONE;
1078 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1081 // Step 18. Undo move
1082 pos.undo_move(move);
1084 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1086 // Step 19. Check for a new best move
1087 // Finished searching the move. If a stop occurred, the return value of
1088 // the search cannot be trusted, and we return immediately without
1089 // updating best move, PV and TT.
1090 if (Threads.stop.load(std::memory_order_relaxed))
1095 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1096 thisThread->rootMoves.end(), move);
1098 // PV move or new best move?
1099 if (moveCount == 1 || value > alpha)
1102 rm.selDepth = thisThread->selDepth;
1107 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1108 rm.pv.push_back(*m);
1110 // We record how often the best move has been changed in each
1111 // iteration. This information is used for time management: When
1112 // the best move changes frequently, we allocate some more time.
1113 if (moveCount > 1 && thisThread == Threads.main())
1114 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1117 // All other moves but the PV are set to the lowest value: this
1118 // is not a problem when sorting because the sort is stable and the
1119 // move position in the list is preserved - just the PV is pushed up.
1120 rm.score = -VALUE_INFINITE;
1123 if (value > bestValue)
1131 if (PvNode && !rootNode) // Update pv even in fail-high case
1132 update_pv(ss->pv, move, (ss+1)->pv);
1134 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1138 assert(value >= beta); // Fail high
1145 if (move != bestMove)
1147 if (captureOrPromotion && captureCount < 32)
1148 capturesSearched[captureCount++] = move;
1150 else if (!captureOrPromotion && quietCount < 64)
1151 quietsSearched[quietCount++] = move;
1155 // The following condition would detect a stop only after move loop has been
1156 // completed. But in this case bestValue is valid because we have fully
1157 // searched our subtree, and we can anyhow save the result in TT.
1163 // Step 20. Check for mate and stalemate
1164 // All legal moves have been searched and if there are no legal moves, it
1165 // must be a mate or a stalemate. If we are in a singular extension search then
1166 // return a fail low score.
1168 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1171 bestValue = excludedMove ? alpha
1172 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1175 // Quiet best move: update move sorting heuristics
1176 if (!pos.capture_or_promotion(bestMove))
1177 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1178 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1180 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1182 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1183 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1184 && !pos.captured_piece())
1185 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1188 // Bonus for prior countermove that caused the fail low
1189 else if ( (depth >= 3 * ONE_PLY || PvNode)
1190 && !pos.captured_piece())
1191 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1194 bestValue = std::min(bestValue, maxValue);
1197 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1198 bestValue >= beta ? BOUND_LOWER :
1199 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1200 depth, bestMove, pureStaticEval);
1202 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1208 // qsearch() is the quiescence search function, which is called by the main
1209 // search function with depth zero, or recursively with depth less than ONE_PLY.
1210 template <NodeType NT>
1211 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1213 constexpr bool PvNode = NT == PV;
1215 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1216 assert(PvNode || (alpha == beta - 1));
1217 assert(depth <= DEPTH_ZERO);
1218 assert(depth / ONE_PLY * ONE_PLY == depth);
1224 Move ttMove, move, bestMove;
1226 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1227 bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
1232 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1234 ss->pv[0] = MOVE_NONE;
1237 Thread* thisThread = pos.this_thread();
1238 (ss+1)->ply = ss->ply + 1;
1239 ss->currentMove = bestMove = MOVE_NONE;
1240 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
1241 inCheck = pos.checkers();
1244 // Check for an immediate draw or maximum ply reached
1245 if ( pos.is_draw(ss->ply)
1246 || ss->ply >= MAX_PLY)
1247 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1249 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1251 // Decide whether or not to include checks: this fixes also the type of
1252 // TT entry depth that we are going to use. Note that in qsearch we use
1253 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1254 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1255 : DEPTH_QS_NO_CHECKS;
1256 // Transposition table lookup
1258 tte = TT.probe(posKey, ttHit);
1259 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1260 ttMove = ttHit ? tte->move() : MOVE_NONE;
1261 pvHit = ttHit && tte->is_pv();
1265 && tte->depth() >= ttDepth
1266 && ttValue != VALUE_NONE // Only in case of TT access race
1267 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1268 : (tte->bound() & BOUND_UPPER)))
1271 // Evaluate the position statically
1274 ss->staticEval = VALUE_NONE;
1275 bestValue = futilityBase = -VALUE_INFINITE;
1281 // Never assume anything on values stored in TT
1282 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1283 ss->staticEval = bestValue = evaluate(pos);
1285 // Can ttValue be used as a better position evaluation?
1286 if ( ttValue != VALUE_NONE
1287 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1288 bestValue = ttValue;
1291 ss->staticEval = bestValue =
1292 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1293 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1295 // Stand pat. Return immediately if static value is at least beta
1296 if (bestValue >= beta)
1299 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1300 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1305 if (PvNode && bestValue > alpha)
1308 futilityBase = bestValue + 128;
1311 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1312 nullptr, (ss-4)->continuationHistory,
1313 nullptr, (ss-6)->continuationHistory };
1315 // Initialize a MovePicker object for the current position, and prepare
1316 // to search the moves. Because the depth is <= 0 here, only captures,
1317 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1319 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1320 &thisThread->captureHistory,
1322 to_sq((ss-1)->currentMove));
1324 // Loop through the moves until no moves remain or a beta cutoff occurs
1325 while ((move = mp.next_move()) != MOVE_NONE)
1327 assert(is_ok(move));
1329 givesCheck = gives_check(pos, move);
1336 && futilityBase > -VALUE_KNOWN_WIN
1337 && !pos.advanced_pawn_push(move))
1339 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1341 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1343 if (futilityValue <= alpha)
1345 bestValue = std::max(bestValue, futilityValue);
1349 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1351 bestValue = std::max(bestValue, futilityBase);
1356 // Detect non-capture evasions that are candidates to be pruned
1357 evasionPrunable = inCheck
1358 && (depth != DEPTH_ZERO || moveCount > 2)
1359 && bestValue > VALUE_MATED_IN_MAX_PLY
1360 && !pos.capture(move);
1362 // Don't search moves with negative SEE values
1363 if ( (!inCheck || evasionPrunable)
1364 && !pos.see_ge(move))
1367 // Speculative prefetch as early as possible
1368 prefetch(TT.first_entry(pos.key_after(move)));
1370 // Check for legality just before making the move
1371 if (!pos.legal(move))
1377 ss->currentMove = move;
1378 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
1380 // Make and search the move
1381 pos.do_move(move, st, givesCheck);
1382 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1383 pos.undo_move(move);
1385 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1387 // Check for a new best move
1388 if (value > bestValue)
1396 if (PvNode) // Update pv even in fail-high case
1397 update_pv(ss->pv, move, (ss+1)->pv);
1399 if (PvNode && value < beta) // Update alpha here!
1407 // All legal moves have been searched. A special case: If we're in check
1408 // and no legal moves were found, it is checkmate.
1409 if (inCheck && bestValue == -VALUE_INFINITE)
1410 return mated_in(ss->ply); // Plies to mate from the root
1412 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1413 bestValue >= beta ? BOUND_LOWER :
1414 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1415 ttDepth, bestMove, ss->staticEval);
1417 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1423 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1424 // "plies to mate from the current position". Non-mate scores are unchanged.
1425 // The function is called before storing a value in the transposition table.
1427 Value value_to_tt(Value v, int ply) {
1429 assert(v != VALUE_NONE);
1431 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1432 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1436 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1437 // from the transposition table (which refers to the plies to mate/be mated
1438 // from current position) to "plies to mate/be mated from the root".
1440 Value value_from_tt(Value v, int ply) {
1442 return v == VALUE_NONE ? VALUE_NONE
1443 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1444 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1448 // update_pv() adds current move and appends child pv[]
1450 void update_pv(Move* pv, Move move, Move* childPv) {
1452 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1458 // update_continuation_histories() updates histories of the move pairs formed
1459 // by moves at ply -1, -2, and -4 with current move.
1461 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1463 for (int i : {1, 2, 4, 6})
1464 if (is_ok((ss-i)->currentMove))
1465 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1469 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1471 void update_capture_stats(const Position& pos, Move move,
1472 Move* captures, int captureCount, int bonus) {
1474 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1475 Piece moved_piece = pos.moved_piece(move);
1476 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1478 if (pos.capture_or_promotion(move))
1479 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1481 // Decrease all the other played capture moves
1482 for (int i = 0; i < captureCount; ++i)
1484 moved_piece = pos.moved_piece(captures[i]);
1485 captured = type_of(pos.piece_on(to_sq(captures[i])));
1486 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1491 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1493 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1494 Move* quiets, int quietCount, int bonus) {
1496 if (ss->killers[0] != move)
1498 ss->killers[1] = ss->killers[0];
1499 ss->killers[0] = move;
1502 Color us = pos.side_to_move();
1503 Thread* thisThread = pos.this_thread();
1504 thisThread->mainHistory[us][from_to(move)] << bonus;
1505 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1507 if (is_ok((ss-1)->currentMove))
1509 Square prevSq = to_sq((ss-1)->currentMove);
1510 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1513 // Decrease all the other played quiet moves
1514 for (int i = 0; i < quietCount; ++i)
1516 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1517 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1521 // When playing with strength handicap, choose best move among a set of RootMoves
1522 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1524 Move Skill::pick_best(size_t multiPV) {
1526 const RootMoves& rootMoves = Threads.main()->rootMoves;
1527 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1529 // RootMoves are already sorted by score in descending order
1530 Value topScore = rootMoves[0].score;
1531 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1532 int weakness = 120 - 2 * level;
1533 int maxScore = -VALUE_INFINITE;
1535 // Choose best move. For each move score we add two terms, both dependent on
1536 // weakness. One is deterministic and bigger for weaker levels, and one is
1537 // random. Then we choose the move with the resulting highest score.
1538 for (size_t i = 0; i < multiPV; ++i)
1540 // This is our magic formula
1541 int push = ( weakness * int(topScore - rootMoves[i].score)
1542 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1544 if (rootMoves[i].score + push >= maxScore)
1546 maxScore = rootMoves[i].score + push;
1547 best = rootMoves[i].pv[0];
1556 /// MainThread::check_time() is used to print debug info and, more importantly,
1557 /// to detect when we are out of available time and thus stop the search.
1559 void MainThread::check_time() {
1564 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1565 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1567 static TimePoint lastInfoTime = now();
1569 TimePoint elapsed = Time.elapsed();
1570 TimePoint tick = Limits.startTime + elapsed;
1572 if (tick - lastInfoTime >= 1000)
1574 lastInfoTime = tick;
1578 // We should not stop pondering until told so by the GUI
1582 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1583 || (Limits.movetime && elapsed >= Limits.movetime)
1584 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1585 Threads.stop = true;
1589 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1590 /// that all (if any) unsearched PV lines are sent using a previous search score.
1592 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1594 std::stringstream ss;
1595 TimePoint elapsed = Time.elapsed() + 1;
1596 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1597 size_t pvIdx = pos.this_thread()->pvIdx;
1598 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1599 uint64_t nodesSearched = Threads.nodes_searched();
1600 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1602 for (size_t i = 0; i < multiPV; ++i)
1604 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1606 if (depth == ONE_PLY && !updated)
1609 Depth d = updated ? depth : depth - ONE_PLY;
1610 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1612 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1613 v = tb ? rootMoves[i].tbScore : v;
1615 if (ss.rdbuf()->in_avail()) // Not at first line
1619 << " depth " << d / ONE_PLY
1620 << " seldepth " << rootMoves[i].selDepth
1621 << " multipv " << i + 1
1622 << " score " << UCI::value(v);
1624 if (!tb && i == pvIdx)
1625 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1627 ss << " nodes " << nodesSearched
1628 << " nps " << nodesSearched * 1000 / elapsed;
1630 if (elapsed > 1000) // Earlier makes little sense
1631 ss << " hashfull " << TT.hashfull();
1633 ss << " tbhits " << tbHits
1634 << " time " << elapsed
1637 for (Move m : rootMoves[i].pv)
1638 ss << " " << UCI::move(m, pos.is_chess960());
1645 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1646 /// before exiting the search, for instance, in case we stop the search during a
1647 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1648 /// otherwise in case of 'ponder on' we have nothing to think on.
1650 bool RootMove::extract_ponder_from_tt(Position& pos) {
1655 assert(pv.size() == 1);
1657 if (pv[0] == MOVE_NONE)
1660 pos.do_move(pv[0], st);
1661 TTEntry* tte = TT.probe(pos.key(), ttHit);
1665 Move m = tte->move(); // Local copy to be SMP safe
1666 if (MoveList<LEGAL>(pos).contains(m))
1670 pos.undo_move(pv[0]);
1671 return pv.size() > 1;
1674 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1677 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1678 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1679 Cardinality = int(Options["SyzygyProbeLimit"]);
1680 bool dtz_available = true;
1682 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1683 // ProbeDepth == DEPTH_ZERO
1684 if (Cardinality > MaxCardinality)
1686 Cardinality = MaxCardinality;
1687 ProbeDepth = DEPTH_ZERO;
1690 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1692 // Rank moves using DTZ tables
1693 RootInTB = root_probe(pos, rootMoves);
1697 // DTZ tables are missing; try to rank moves using WDL tables
1698 dtz_available = false;
1699 RootInTB = root_probe_wdl(pos, rootMoves);
1705 // Sort moves according to TB rank
1706 std::sort(rootMoves.begin(), rootMoves.end(),
1707 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1709 // Probe during search only if DTZ is not available and we are winning
1710 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1715 // Assign the same rank to all moves
1716 for (auto& m : rootMoves)