2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2018 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
38 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
54 namespace TB = Tablebases;
58 using namespace Search;
62 // Different node types, used as a template parameter
63 enum NodeType { NonPV, PV };
65 // Sizes and phases of the skip-blocks, used for distributing search depths across the threads
66 const int SkipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
67 const int SkipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
69 // Razoring and futility margins
70 const int RazorMargin = 590;
71 Value futility_margin(Depth d) { return Value(150 * d / ONE_PLY); }
73 // Futility and reductions lookup tables, initialized at startup
74 int FutilityMoveCounts[2][16]; // [improving][depth]
75 int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
77 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
78 return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
81 // History and stats update bonus, based on depth
82 int stat_bonus(Depth depth) {
83 int d = depth / ONE_PLY;
84 return d > 17 ? 0 : d * d + 2 * d - 2;
87 // Skill structure is used to implement strength limit
89 explicit Skill(int l) : level(l) {}
90 bool enabled() const { return level < 20; }
91 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
92 Move pick_best(size_t multiPV);
95 Move best = MOVE_NONE;
98 template <NodeType NT>
99 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning);
101 template <NodeType NT, bool InCheck>
102 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
104 Value value_to_tt(Value v, int ply);
105 Value value_from_tt(Value v, int ply);
106 void update_pv(Move* pv, Move move, Move* childPv);
107 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
108 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
109 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCnt, int bonus);
111 inline bool gives_check(const Position& pos, Move move) {
112 Color us = pos.side_to_move();
113 return type_of(move) == NORMAL && !(pos.blockers_for_king(~us) & pos.pieces(us))
114 ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
115 : pos.gives_check(move);
118 // perft() is our utility to verify move generation. All the leaf nodes up
119 // to the given depth are generated and counted, and the sum is returned.
121 uint64_t perft(Position& pos, Depth depth) {
124 uint64_t cnt, nodes = 0;
125 const bool leaf = (depth == 2 * ONE_PLY);
127 for (const auto& m : MoveList<LEGAL>(pos))
129 if (Root && depth <= ONE_PLY)
134 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
139 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
147 /// Search::init() is called at startup to initialize various lookup tables
149 void Search::init() {
151 for (int imp = 0; imp <= 1; ++imp)
152 for (int d = 1; d < 64; ++d)
153 for (int mc = 1; mc < 64; ++mc)
155 double r = log(d) * log(mc) / 1.95;
157 Reductions[NonPV][imp][d][mc] = int(std::round(r));
158 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
160 // Increase reduction for non-PV nodes when eval is not improving
161 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
162 Reductions[NonPV][imp][d][mc]++;
165 for (int d = 0; d < 16; ++d)
167 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
168 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
173 /// Search::clear() resets search state to its initial value
175 void Search::clear() {
177 Threads.main()->wait_for_search_finished();
179 Time.availableNodes = 0;
185 /// MainThread::search() is called by the main thread when the program receives
186 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
188 void MainThread::search() {
192 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
193 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
197 Color us = rootPos.side_to_move();
198 Time.init(Limits, us, rootPos.game_ply());
201 if (rootMoves.empty())
203 rootMoves.emplace_back(MOVE_NONE);
204 sync_cout << "info depth 0 score "
205 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
210 for (Thread* th : Threads)
212 th->start_searching();
214 Thread::search(); // Let's start searching!
217 // When we reach the maximum depth, we can arrive here without a raise of
218 // Threads.stop. However, if we are pondering or in an infinite search,
219 // the UCI protocol states that we shouldn't print the best move before the
220 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
221 // until the GUI sends one of those commands (which also raises Threads.stop).
222 Threads.stopOnPonderhit = true;
224 while (!Threads.stop && (Threads.ponder || Limits.infinite))
225 {} // Busy wait for a stop or a ponder reset
227 // Stop the threads if not already stopped (also raise the stop if
228 // "ponderhit" just reset Threads.ponder).
231 // Wait until all threads have finished
232 for (Thread* th : Threads)
234 th->wait_for_search_finished();
236 // When playing in 'nodes as time' mode, subtract the searched nodes from
237 // the available ones before exiting.
239 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
241 // Check if there are threads with a better score than main thread
242 Thread* bestThread = this;
243 if ( Options["MultiPV"] == 1
245 && !Skill(Options["Skill Level"]).enabled()
246 && rootMoves[0].pv[0] != MOVE_NONE)
248 for (Thread* th : Threads)
250 Depth depthDiff = th->completedDepth - bestThread->completedDepth;
251 Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
253 // Select the thread with the best score, always if it is a mate
255 && (depthDiff >= 0 || th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY))
260 previousScore = bestThread->rootMoves[0].score;
262 // Send again PV info if we have a new best thread
263 if (bestThread != this)
264 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
266 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
268 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
269 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
271 std::cout << sync_endl;
275 /// Thread::search() is the main iterative deepening loop. It calls search()
276 /// repeatedly with increasing depth until the allocated thinking time has been
277 /// consumed, the user stops the search, or the maximum search depth is reached.
279 void Thread::search() {
281 Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
282 Value bestValue, alpha, beta, delta;
283 Move lastBestMove = MOVE_NONE;
284 Depth lastBestMoveDepth = DEPTH_ZERO;
285 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
286 double timeReduction = 1.0;
287 Color us = rootPos.side_to_move();
289 std::memset(ss-4, 0, 7 * sizeof(Stack));
290 for (int i = 4; i > 0; i--)
291 (ss-i)->contHistory = &this->contHistory[NO_PIECE][0]; // Use as sentinel
293 bestValue = delta = alpha = -VALUE_INFINITE;
294 beta = VALUE_INFINITE;
297 mainThread->bestMoveChanges = 0, mainThread->failedLow = false;
299 size_t multiPV = Options["MultiPV"];
300 Skill skill(Options["Skill Level"]);
302 // When playing with strength handicap enable MultiPV search that we will
303 // use behind the scenes to retrieve a set of possible moves.
305 multiPV = std::max(multiPV, (size_t)4);
307 multiPV = std::min(multiPV, rootMoves.size());
309 int ct = Options["Contempt"] * PawnValueEg / 100; // From centipawns
310 Eval::Contempt = (us == WHITE ? make_score(ct, ct / 2)
311 : -make_score(ct, ct / 2));
313 // Iterative deepening loop until requested to stop or the target depth is reached
314 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
316 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
318 // Distribute search depths across the helper threads
321 int i = (idx - 1) % 20;
322 if (((rootDepth / ONE_PLY + rootPos.game_ply() + SkipPhase[i]) / SkipSize[i]) % 2)
323 continue; // Retry with an incremented rootDepth
326 // Age out PV variability metric
328 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
330 // Save the last iteration's scores before first PV line is searched and
331 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
332 for (RootMove& rm : rootMoves)
333 rm.previousScore = rm.score;
335 // MultiPV loop. We perform a full root search for each PV line
336 for (PVIdx = 0; PVIdx < multiPV && !Threads.stop; ++PVIdx)
338 // Reset UCI info selDepth for each depth and each PV line
341 // Reset aspiration window starting size
342 if (rootDepth >= 5 * ONE_PLY)
345 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
346 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
348 // Adjust contempt based on current bestValue
349 ct = Options["Contempt"] * PawnValueEg / 100 // From centipawns
350 + (bestValue > 500 ? 50: // Dynamic contempt
351 bestValue < -500 ? -50:
354 Eval::Contempt = (us == WHITE ? make_score(ct, ct / 2)
355 : -make_score(ct, ct / 2));
358 // Start with a small aspiration window and, in the case of a fail
359 // high/low, re-search with a bigger window until we don't fail
363 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false, false);
365 // Bring the best move to the front. It is critical that sorting
366 // is done with a stable algorithm because all the values but the
367 // first and eventually the new best one are set to -VALUE_INFINITE
368 // and we want to keep the same order for all the moves except the
369 // new PV that goes to the front. Note that in case of MultiPV
370 // search the already searched PV lines are preserved.
371 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
373 // If search has been stopped, we break immediately. Sorting is
374 // safe because RootMoves is still valid, although it refers to
375 // the previous iteration.
379 // When failing high/low give some update (without cluttering
380 // the UI) before a re-search.
383 && (bestValue <= alpha || bestValue >= beta)
384 && Time.elapsed() > 3000)
385 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
387 // In case of failing low/high increase aspiration window and
388 // re-search, otherwise exit the loop.
389 if (bestValue <= alpha)
391 beta = (alpha + beta) / 2;
392 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
396 mainThread->failedLow = true;
397 Threads.stopOnPonderhit = false;
400 else if (bestValue >= beta)
401 beta = std::min(bestValue + delta, VALUE_INFINITE);
405 delta += delta / 4 + 5;
407 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
410 // Sort the PV lines searched so far and update the GUI
411 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
414 && (Threads.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000))
415 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
419 completedDepth = rootDepth;
421 if (rootMoves[0].pv[0] != lastBestMove) {
422 lastBestMove = rootMoves[0].pv[0];
423 lastBestMoveDepth = rootDepth;
426 // Have we found a "mate in x"?
428 && bestValue >= VALUE_MATE_IN_MAX_PLY
429 && VALUE_MATE - bestValue <= 2 * Limits.mate)
435 // If skill level is enabled and time is up, pick a sub-optimal best move
436 if (skill.enabled() && skill.time_to_pick(rootDepth))
437 skill.pick_best(multiPV);
439 // Do we have time for the next iteration? Can we stop searching now?
440 if ( Limits.use_time_management()
442 && !Threads.stopOnPonderhit)
444 const int F[] = { mainThread->failedLow,
445 bestValue - mainThread->previousScore };
447 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
449 // If the bestMove is stable over several iterations, reduce time accordingly
451 for (int i : {3, 4, 5})
452 if (lastBestMoveDepth * i < completedDepth)
453 timeReduction *= 1.3;
455 // Use part of the gained time from a previous stable move for the current move
456 double unstablePvFactor = 1.0 + mainThread->bestMoveChanges;
457 unstablePvFactor *= std::pow(mainThread->previousTimeReduction, 0.51) / timeReduction;
459 // Stop the search if we have only one legal move, or if available time elapsed
460 if ( rootMoves.size() == 1
461 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 605)
463 // If we are allowed to ponder do not stop the search now but
464 // keep pondering until the GUI sends "ponderhit" or "stop".
466 Threads.stopOnPonderhit = true;
476 mainThread->previousTimeReduction = timeReduction;
478 // If skill level is enabled, swap best PV line with the sub-optimal one
480 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
481 skill.best ? skill.best : skill.pick_best(multiPV)));
487 // search<>() is the main search function for both PV and non-PV nodes
489 template <NodeType NT>
490 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode, bool skipEarlyPruning) {
492 const bool PvNode = NT == PV;
493 const bool rootNode = PvNode && ss->ply == 0;
495 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
496 assert(PvNode || (alpha == beta - 1));
497 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
498 assert(!(PvNode && cutNode));
499 assert(depth / ONE_PLY * ONE_PLY == depth);
501 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
505 Move ttMove, move, excludedMove, bestMove;
506 Depth extension, newDepth;
507 Value bestValue, value, ttValue, eval, maxValue;
508 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
509 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
511 int moveCount, captureCount, quietCount;
513 // Step 1. Initialize node
514 Thread* thisThread = pos.this_thread();
515 inCheck = pos.checkers();
516 moveCount = captureCount = quietCount = ss->moveCount = 0;
517 bestValue = -VALUE_INFINITE;
518 maxValue = VALUE_INFINITE;
520 // Check for the available remaining time
521 if (thisThread == Threads.main())
522 static_cast<MainThread*>(thisThread)->check_time();
524 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
525 if (PvNode && thisThread->selDepth < ss->ply + 1)
526 thisThread->selDepth = ss->ply + 1;
530 // Step 2. Check for aborted search and immediate draw
531 if ( Threads.stop.load(std::memory_order_relaxed)
532 || pos.is_draw(ss->ply)
533 || ss->ply >= MAX_PLY)
534 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
536 // Step 3. Mate distance pruning. Even if we mate at the next move our score
537 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
538 // a shorter mate was found upward in the tree then there is no need to search
539 // because we will never beat the current alpha. Same logic but with reversed
540 // signs applies also in the opposite condition of being mated instead of giving
541 // mate. In this case return a fail-high score.
542 alpha = std::max(mated_in(ss->ply), alpha);
543 beta = std::min(mate_in(ss->ply+1), beta);
548 assert(0 <= ss->ply && ss->ply < MAX_PLY);
550 (ss+1)->ply = ss->ply + 1;
551 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
552 ss->contHistory = &thisThread->contHistory[NO_PIECE][0];
553 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
554 Square prevSq = to_sq((ss-1)->currentMove);
556 // Initialize statScore to zero for the grandchildren of the current position.
557 // So statScore is shared between all grandchildren and only the first grandchild
558 // starts with statScore = 0. Later grandchildren start with the last calculated
559 // statScore of the previous grandchild. This influences the reduction rules in
560 // LMR which are based on the statScore of parent position.
561 (ss+2)->statScore = 0;
563 // Step 4. Transposition table lookup. We don't want the score of a partial
564 // search to overwrite a previous full search TT value, so we use a different
565 // position key in case of an excluded move.
566 excludedMove = ss->excludedMove;
567 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
568 tte = TT.probe(posKey, ttHit);
569 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
570 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
571 : ttHit ? tte->move() : MOVE_NONE;
573 // At non-PV nodes we check for an early TT cutoff
576 && tte->depth() >= depth
577 && ttValue != VALUE_NONE // Possible in case of TT access race
578 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
579 : (tte->bound() & BOUND_UPPER)))
581 // If ttMove is quiet, update move sorting heuristics on TT hit
586 if (!pos.capture_or_promotion(ttMove))
587 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
589 // Extra penalty for a quiet TT move in previous ply when it gets refuted
590 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
591 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
593 // Penalty for a quiet ttMove that fails low
594 else if (!pos.capture_or_promotion(ttMove))
596 int penalty = -stat_bonus(depth);
597 thisThread->mainHistory.update(pos.side_to_move(), ttMove, penalty);
598 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
604 // Step 5. Tablebases probe
605 if (!rootNode && TB::Cardinality)
607 int piecesCount = pos.count<ALL_PIECES>();
609 if ( piecesCount <= TB::Cardinality
610 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
611 && pos.rule50_count() == 0
612 && !pos.can_castle(ANY_CASTLING))
615 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
617 if (err != TB::ProbeState::FAIL)
619 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
621 int drawScore = TB::UseRule50 ? 1 : 0;
623 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
624 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
625 : VALUE_DRAW + 2 * wdl * drawScore;
627 Bound b = wdl < -drawScore ? BOUND_UPPER
628 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
630 if ( b == BOUND_EXACT
631 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
633 tte->save(posKey, value_to_tt(value, ss->ply), b,
634 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
635 MOVE_NONE, VALUE_NONE, TT.generation());
642 if (b == BOUND_LOWER)
643 bestValue = value, alpha = std::max(alpha, bestValue);
651 // Step 6. Evaluate the position statically
654 ss->staticEval = eval = VALUE_NONE;
659 // Never assume anything on values stored in TT
660 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
661 eval = ss->staticEval = evaluate(pos);
663 // Can ttValue be used as a better position evaluation?
664 if ( ttValue != VALUE_NONE
665 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
670 ss->staticEval = eval =
671 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
672 : -(ss-1)->staticEval + 2 * Eval::Tempo;
674 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
675 ss->staticEval, TT.generation());
678 if (skipEarlyPruning || !pos.non_pawn_material(pos.side_to_move()))
681 // Step 7. Razoring (skipped when in check)
684 && eval + RazorMargin <= alpha)
685 return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
687 // Step 8. Futility pruning: child node (skipped when in check)
689 && depth < 7 * ONE_PLY
690 && eval - futility_margin(depth) >= beta
691 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
694 // Step 9. Null move search with verification search
697 && ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
698 && (ss->ply >= thisThread->nmp_ply || ss->ply % 2 != thisThread->nmp_odd))
700 assert(eval - beta >= 0);
702 // Null move dynamic reduction based on depth and value
703 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
705 ss->currentMove = MOVE_NULL;
706 ss->contHistory = &thisThread->contHistory[NO_PIECE][0];
708 pos.do_null_move(st);
709 Value nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1)
710 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode, true);
711 pos.undo_null_move();
713 if (nullValue >= beta)
715 // Do not return unproven mate scores
716 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
719 if (abs(beta) < VALUE_KNOWN_WIN && (depth < 12 * ONE_PLY || thisThread->nmp_ply))
722 // Do verification search at high depths. Disable null move pruning
723 // for side to move for the first part of the remaining search tree.
724 thisThread->nmp_ply = ss->ply + 3 * (depth-R) / 4;
725 thisThread->nmp_odd = ss->ply % 2;
727 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
728 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
730 thisThread->nmp_odd = thisThread->nmp_ply = 0;
737 // Step 10. ProbCut (skipped when in check)
738 // If we have a good enough capture and a reduced search returns a value
739 // much above beta, we can (almost) safely prune the previous move.
741 && depth >= 5 * ONE_PLY
742 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
744 assert(is_ok((ss-1)->currentMove));
746 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
747 MovePicker mp(pos, ttMove, rbeta - ss->staticEval, &thisThread->captureHistory);
749 while ((move = mp.next_move()) != MOVE_NONE)
752 ss->currentMove = move;
753 ss->contHistory = &thisThread->contHistory[pos.moved_piece(move)][to_sq(move)];
755 assert(depth >= 5 * ONE_PLY);
757 pos.do_move(move, st);
759 // Perform a preliminary search at depth 1 to verify that the move holds.
760 // We will only do this search if the depth is not 5, thus avoiding two
761 // searches at depth 1 in a row.
762 if (depth != 5 * ONE_PLY)
763 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, ONE_PLY, !cutNode, true);
765 // If the first search was skipped or was performed and held, perform
766 // the regular search.
767 if (depth == 5 * ONE_PLY || value >= rbeta)
768 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode, false);
776 // Step 11. Internal iterative deepening (skipped when in check)
777 if ( depth >= 6 * ONE_PLY
779 && (PvNode || ss->staticEval + 256 >= beta))
781 Depth d = 3 * depth / 4 - 2 * ONE_PLY;
782 search<NT>(pos, ss, alpha, beta, d, cutNode, true);
784 tte = TT.probe(posKey, ttHit);
785 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
786 ttMove = ttHit ? tte->move() : MOVE_NONE;
789 moves_loop: // When in check, search starts from here
791 const PieceToHistory* contHist[] = { (ss-1)->contHistory, (ss-2)->contHistory, nullptr, (ss-4)->contHistory };
792 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
794 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory, contHist, countermove, ss->killers);
795 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
796 improving = ss->staticEval >= (ss-2)->staticEval
797 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
798 ||(ss-2)->staticEval == VALUE_NONE;
800 singularExtensionNode = !rootNode
801 && depth >= 8 * ONE_PLY
802 && ttMove != MOVE_NONE
803 && ttValue != VALUE_NONE
804 && !excludedMove // Recursive singular search is not allowed
805 && (tte->bound() & BOUND_LOWER)
806 && tte->depth() >= depth - 3 * ONE_PLY;
809 pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
811 // Step 12. Loop through all pseudo-legal moves until no moves remain
812 // or a beta cutoff occurs.
813 while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
817 if (move == excludedMove)
820 // At root obey the "searchmoves" option and skip moves not listed in Root
821 // Move List. As a consequence any illegal move is also skipped. In MultiPV
822 // mode we also skip PV moves which have been already searched.
823 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
824 thisThread->rootMoves.end(), move))
827 ss->moveCount = ++moveCount;
829 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
830 sync_cout << "info depth " << depth / ONE_PLY
831 << " currmove " << UCI::move(move, pos.is_chess960())
832 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
834 (ss+1)->pv = nullptr;
836 extension = DEPTH_ZERO;
837 captureOrPromotion = pos.capture_or_promotion(move);
838 movedPiece = pos.moved_piece(move);
839 givesCheck = gives_check(pos, move);
841 moveCountPruning = depth < 16 * ONE_PLY
842 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
844 // Step 13. Extensions
846 // Singular extension search. If all moves but one fail low on a search
847 // of (alpha-s, beta-s), and just one fails high on (alpha, beta), then
848 // that move is singular and should be extended. To verify this we do a
849 // reduced search on on all the other moves but the ttMove and if the
850 // result is lower than ttValue minus a margin then we will extend the ttMove.
851 if ( singularExtensionNode
855 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
856 ss->excludedMove = move;
857 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode, true);
858 ss->excludedMove = MOVE_NONE;
863 else if ( givesCheck // Check extension
868 // Calculate new depth for this move
869 newDepth = depth - ONE_PLY + extension;
871 // Step 14. Pruning at shallow depth
873 && pos.non_pawn_material(pos.side_to_move())
874 && bestValue > VALUE_MATED_IN_MAX_PLY)
876 if ( !captureOrPromotion
878 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
880 // Move count based pruning
881 if (moveCountPruning)
887 // Reduced depth of the next LMR search
888 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
890 // Countermoves based pruning
892 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
893 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
896 // Futility pruning: parent node
899 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
902 // Prune moves with negative SEE
904 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
907 else if ( depth < 7 * ONE_PLY
909 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
913 // Speculative prefetch as early as possible
914 prefetch(TT.first_entry(pos.key_after(move)));
916 // Check for legality just before making the move
917 if (!rootNode && !pos.legal(move))
919 ss->moveCount = --moveCount;
923 if (move == ttMove && captureOrPromotion)
926 // Update the current move (this must be done after singular extension search)
927 ss->currentMove = move;
928 ss->contHistory = &thisThread->contHistory[movedPiece][to_sq(move)];
930 // Step 15. Make the move
931 pos.do_move(move, st, givesCheck);
933 // Step 16. Reduced depth search (LMR). If the move fails high it will be
934 // re-searched at full depth.
935 if ( depth >= 3 * ONE_PLY
937 && (!captureOrPromotion || moveCountPruning))
939 Depth r = reduction<PvNode>(improving, depth, moveCount);
941 if (captureOrPromotion)
942 r -= r ? ONE_PLY : DEPTH_ZERO;
945 // Decrease reduction if opponent's move count is high
946 if ((ss-1)->moveCount > 15)
949 // Decrease reduction for exact PV nodes
953 // Increase reduction if ttMove is a capture
957 // Increase reduction for cut nodes
961 // Decrease reduction for moves that escape a capture. Filter out
962 // castling moves, because they are coded as "king captures rook" and
963 // hence break make_move().
964 else if ( type_of(move) == NORMAL
965 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
968 ss->statScore = thisThread->mainHistory[~pos.side_to_move()][from_to(move)]
969 + (*contHist[0])[movedPiece][to_sq(move)]
970 + (*contHist[1])[movedPiece][to_sq(move)]
971 + (*contHist[3])[movedPiece][to_sq(move)]
974 // Decrease/increase reduction by comparing opponent's stat score
975 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
978 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
981 // Decrease/increase reduction for moves with a good/bad history
982 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->statScore / 20000) * ONE_PLY);
985 Depth d = std::max(newDepth - r, ONE_PLY);
987 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true, false);
989 doFullDepthSearch = (value > alpha && d != newDepth);
992 doFullDepthSearch = !PvNode || moveCount > 1;
994 // Step 17. Full depth search when LMR is skipped or fails high
995 if (doFullDepthSearch)
996 value = newDepth < ONE_PLY ?
997 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha)
998 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha)
999 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode, false);
1001 // For PV nodes only, do a full PV search on the first move or after a fail
1002 // high (in the latter case search only if value < beta), otherwise let the
1003 // parent node fail low with value <= alpha and try another move.
1004 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1007 (ss+1)->pv[0] = MOVE_NONE;
1009 value = newDepth < ONE_PLY ?
1010 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha)
1011 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha)
1012 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false, false);
1015 // Step 18. Undo move
1016 pos.undo_move(move);
1018 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1020 // Step 19. Check for a new best move
1021 // Finished searching the move. If a stop occurred, the return value of
1022 // the search cannot be trusted, and we return immediately without
1023 // updating best move, PV and TT.
1024 if (Threads.stop.load(std::memory_order_relaxed))
1029 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1030 thisThread->rootMoves.end(), move);
1032 // PV move or new best move?
1033 if (moveCount == 1 || value > alpha)
1036 rm.selDepth = thisThread->selDepth;
1041 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1042 rm.pv.push_back(*m);
1044 // We record how often the best move has been changed in each
1045 // iteration. This information is used for time management: When
1046 // the best move changes frequently, we allocate some more time.
1047 if (moveCount > 1 && thisThread == Threads.main())
1048 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1051 // All other moves but the PV are set to the lowest value: this
1052 // is not a problem when sorting because the sort is stable and the
1053 // move position in the list is preserved - just the PV is pushed up.
1054 rm.score = -VALUE_INFINITE;
1057 if (value > bestValue)
1065 if (PvNode && !rootNode) // Update pv even in fail-high case
1066 update_pv(ss->pv, move, (ss+1)->pv);
1068 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1072 assert(value >= beta); // Fail high
1078 if (move != bestMove)
1080 if (captureOrPromotion && captureCount < 32)
1081 capturesSearched[captureCount++] = move;
1083 else if (!captureOrPromotion && quietCount < 64)
1084 quietsSearched[quietCount++] = move;
1088 // The following condition would detect a stop only after move loop has been
1089 // completed. But in this case bestValue is valid because we have fully
1090 // searched our subtree, and we can anyhow save the result in TT.
1096 // Step 20. Check for mate and stalemate
1097 // All legal moves have been searched and if there are no legal moves, it
1098 // must be a mate or a stalemate. If we are in a singular extension search then
1099 // return a fail low score.
1101 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1104 bestValue = excludedMove ? alpha
1105 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1108 // Quiet best move: update move sorting heuristics
1109 if (!pos.capture_or_promotion(bestMove))
1110 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
1112 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth));
1114 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1115 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1116 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1118 // Bonus for prior countermove that caused the fail low
1119 else if ( depth >= 3 * ONE_PLY
1120 && !pos.captured_piece()
1121 && is_ok((ss-1)->currentMove))
1122 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1125 bestValue = std::min(bestValue, maxValue);
1128 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1129 bestValue >= beta ? BOUND_LOWER :
1130 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1131 depth, bestMove, ss->staticEval, TT.generation());
1133 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1139 // qsearch() is the quiescence search function, which is called by the main
1140 // search function with depth zero, or recursively with depth less than ONE_PLY.
1142 template <NodeType NT, bool InCheck>
1143 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1145 const bool PvNode = NT == PV;
1147 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1148 assert(PvNode || (alpha == beta - 1));
1149 assert(depth <= DEPTH_ZERO);
1150 assert(depth / ONE_PLY * ONE_PLY == depth);
1151 assert(InCheck == bool(pos.checkers()));
1157 Move ttMove, move, bestMove;
1159 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1160 bool ttHit, givesCheck, evasionPrunable;
1165 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1167 ss->pv[0] = MOVE_NONE;
1170 (ss+1)->ply = ss->ply + 1;
1171 ss->currentMove = bestMove = MOVE_NONE;
1174 // Check for an immediate draw or maximum ply reached
1175 if ( pos.is_draw(ss->ply)
1176 || ss->ply >= MAX_PLY)
1177 return (ss->ply >= MAX_PLY && !InCheck) ? evaluate(pos) : VALUE_DRAW;
1179 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1181 // Decide whether or not to include checks: this fixes also the type of
1182 // TT entry depth that we are going to use. Note that in qsearch we use
1183 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1184 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1185 : DEPTH_QS_NO_CHECKS;
1186 // Transposition table lookup
1188 tte = TT.probe(posKey, ttHit);
1189 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1190 ttMove = ttHit ? tte->move() : MOVE_NONE;
1194 && tte->depth() >= ttDepth
1195 && ttValue != VALUE_NONE // Only in case of TT access race
1196 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1197 : (tte->bound() & BOUND_UPPER)))
1200 // Evaluate the position statically
1203 ss->staticEval = VALUE_NONE;
1204 bestValue = futilityBase = -VALUE_INFINITE;
1210 // Never assume anything on values stored in TT
1211 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1212 ss->staticEval = bestValue = evaluate(pos);
1214 // Can ttValue be used as a better position evaluation?
1215 if ( ttValue != VALUE_NONE
1216 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1217 bestValue = ttValue;
1220 ss->staticEval = bestValue =
1221 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1222 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1224 // Stand pat. Return immediately if static value is at least beta
1225 if (bestValue >= beta)
1228 tte->save(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1229 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1234 if (PvNode && bestValue > alpha)
1237 futilityBase = bestValue + 128;
1240 // Initialize a MovePicker object for the current position, and prepare
1241 // to search the moves. Because the depth is <= 0 here, only captures,
1242 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1244 MovePicker mp(pos, ttMove, depth, &pos.this_thread()->mainHistory, &pos.this_thread()->captureHistory, to_sq((ss-1)->currentMove));
1246 // Loop through the moves until no moves remain or a beta cutoff occurs
1247 while ((move = mp.next_move()) != MOVE_NONE)
1249 assert(is_ok(move));
1251 givesCheck = gives_check(pos, move);
1258 && futilityBase > -VALUE_KNOWN_WIN
1259 && !pos.advanced_pawn_push(move))
1261 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1263 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1265 if (futilityValue <= alpha)
1267 bestValue = std::max(bestValue, futilityValue);
1271 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1273 bestValue = std::max(bestValue, futilityBase);
1278 // Detect non-capture evasions that are candidates to be pruned
1279 evasionPrunable = InCheck
1280 && (depth != DEPTH_ZERO || moveCount > 2)
1281 && bestValue > VALUE_MATED_IN_MAX_PLY
1282 && !pos.capture(move);
1284 // Don't search moves with negative SEE values
1285 if ( (!InCheck || evasionPrunable)
1286 && !pos.see_ge(move))
1289 // Speculative prefetch as early as possible
1290 prefetch(TT.first_entry(pos.key_after(move)));
1292 // Check for legality just before making the move
1293 if (!pos.legal(move))
1299 ss->currentMove = move;
1301 // Make and search the move
1302 pos.do_move(move, st, givesCheck);
1303 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1304 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1305 pos.undo_move(move);
1307 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1309 // Check for a new best move
1310 if (value > bestValue)
1316 if (PvNode) // Update pv even in fail-high case
1317 update_pv(ss->pv, move, (ss+1)->pv);
1319 if (PvNode && value < beta) // Update alpha here!
1326 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1327 ttDepth, move, ss->staticEval, TT.generation());
1335 // All legal moves have been searched. A special case: If we're in check
1336 // and no legal moves were found, it is checkmate.
1337 if (InCheck && bestValue == -VALUE_INFINITE)
1338 return mated_in(ss->ply); // Plies to mate from the root
1340 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1341 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1342 ttDepth, bestMove, ss->staticEval, TT.generation());
1344 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1350 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1351 // "plies to mate from the current position". Non-mate scores are unchanged.
1352 // The function is called before storing a value in the transposition table.
1354 Value value_to_tt(Value v, int ply) {
1356 assert(v != VALUE_NONE);
1358 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1359 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1363 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1364 // from the transposition table (which refers to the plies to mate/be mated
1365 // from current position) to "plies to mate/be mated from the root".
1367 Value value_from_tt(Value v, int ply) {
1369 return v == VALUE_NONE ? VALUE_NONE
1370 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1371 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1375 // update_pv() adds current move and appends child pv[]
1377 void update_pv(Move* pv, Move move, Move* childPv) {
1379 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1385 // update_continuation_histories() updates histories of the move pairs formed
1386 // by moves at ply -1, -2, and -4 with current move.
1388 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1390 for (int i : {1, 2, 4})
1391 if (is_ok((ss-i)->currentMove))
1392 (ss-i)->contHistory->update(pc, to, bonus);
1396 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1398 void update_capture_stats(const Position& pos, Move move,
1399 Move* captures, int captureCnt, int bonus) {
1401 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1402 Piece moved_piece = pos.moved_piece(move);
1403 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1404 captureHistory.update(moved_piece, to_sq(move), captured, bonus);
1406 // Decrease all the other played capture moves
1407 for (int i = 0; i < captureCnt; ++i)
1409 moved_piece = pos.moved_piece(captures[i]);
1410 captured = type_of(pos.piece_on(to_sq(captures[i])));
1411 captureHistory.update(moved_piece, to_sq(captures[i]), captured, -bonus);
1416 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1418 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1419 Move* quiets, int quietsCnt, int bonus) {
1421 if (ss->killers[0] != move)
1423 ss->killers[1] = ss->killers[0];
1424 ss->killers[0] = move;
1427 Color us = pos.side_to_move();
1428 Thread* thisThread = pos.this_thread();
1429 thisThread->mainHistory.update(us, move, bonus);
1430 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1432 if (is_ok((ss-1)->currentMove))
1434 Square prevSq = to_sq((ss-1)->currentMove);
1435 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1438 // Decrease all the other played quiet moves
1439 for (int i = 0; i < quietsCnt; ++i)
1441 thisThread->mainHistory.update(us, quiets[i], -bonus);
1442 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1446 // When playing with strength handicap, choose best move among a set of RootMoves
1447 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1449 Move Skill::pick_best(size_t multiPV) {
1451 const RootMoves& rootMoves = Threads.main()->rootMoves;
1452 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1454 // RootMoves are already sorted by score in descending order
1455 Value topScore = rootMoves[0].score;
1456 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1457 int weakness = 120 - 2 * level;
1458 int maxScore = -VALUE_INFINITE;
1460 // Choose best move. For each move score we add two terms, both dependent on
1461 // weakness. One is deterministic and bigger for weaker levels, and one is
1462 // random. Then we choose the move with the resulting highest score.
1463 for (size_t i = 0; i < multiPV; ++i)
1465 // This is our magic formula
1466 int push = ( weakness * int(topScore - rootMoves[i].score)
1467 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1469 if (rootMoves[i].score + push >= maxScore)
1471 maxScore = rootMoves[i].score + push;
1472 best = rootMoves[i].pv[0];
1481 /// MainThread::check_time() is used to print debug info and, more importantly,
1482 /// to detect when we are out of available time and thus stop the search.
1484 void MainThread::check_time() {
1489 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1490 callsCnt = Limits.nodes ? std::min(4096, int(Limits.nodes / 1024)) : 4096;
1492 static TimePoint lastInfoTime = now();
1494 int elapsed = Time.elapsed();
1495 TimePoint tick = Limits.startTime + elapsed;
1497 if (tick - lastInfoTime >= 1000)
1499 lastInfoTime = tick;
1503 // We should not stop pondering until told so by the GUI
1507 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1508 || (Limits.movetime && elapsed >= Limits.movetime)
1509 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1510 Threads.stop = true;
1514 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1515 /// that all (if any) unsearched PV lines are sent using a previous search score.
1517 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1519 std::stringstream ss;
1520 int elapsed = Time.elapsed() + 1;
1521 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1522 size_t PVIdx = pos.this_thread()->PVIdx;
1523 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1524 uint64_t nodesSearched = Threads.nodes_searched();
1525 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1527 for (size_t i = 0; i < multiPV; ++i)
1529 bool updated = (i <= PVIdx && rootMoves[i].score != -VALUE_INFINITE);
1531 if (depth == ONE_PLY && !updated)
1534 Depth d = updated ? depth : depth - ONE_PLY;
1535 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1537 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1538 v = tb ? TB::Score : v;
1540 if (ss.rdbuf()->in_avail()) // Not at first line
1544 << " depth " << d / ONE_PLY
1545 << " seldepth " << rootMoves[i].selDepth
1546 << " multipv " << i + 1
1547 << " score " << UCI::value(v);
1549 if (!tb && i == PVIdx)
1550 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1552 ss << " nodes " << nodesSearched
1553 << " nps " << nodesSearched * 1000 / elapsed;
1555 if (elapsed > 1000) // Earlier makes little sense
1556 ss << " hashfull " << TT.hashfull();
1558 ss << " tbhits " << tbHits
1559 << " time " << elapsed
1562 for (Move m : rootMoves[i].pv)
1563 ss << " " << UCI::move(m, pos.is_chess960());
1570 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1571 /// before exiting the search, for instance, in case we stop the search during a
1572 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1573 /// otherwise in case of 'ponder on' we have nothing to think on.
1575 bool RootMove::extract_ponder_from_tt(Position& pos) {
1580 assert(pv.size() == 1);
1585 pos.do_move(pv[0], st);
1586 TTEntry* tte = TT.probe(pos.key(), ttHit);
1590 Move m = tte->move(); // Local copy to be SMP safe
1591 if (MoveList<LEGAL>(pos).contains(m))
1595 pos.undo_move(pv[0]);
1596 return pv.size() > 1;
1600 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1603 UseRule50 = Options["Syzygy50MoveRule"];
1604 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1605 Cardinality = Options["SyzygyProbeLimit"];
1607 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1608 if (Cardinality > MaxCardinality)
1610 Cardinality = MaxCardinality;
1611 ProbeDepth = DEPTH_ZERO;
1614 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1617 // Don't filter any moves if the user requested analysis on multiple
1618 if (Options["MultiPV"] != 1)
1621 // If the current root position is in the tablebases, then RootMoves
1622 // contains only moves that preserve the draw or the win.
1623 RootInTB = root_probe(pos, rootMoves, TB::Score);
1626 Cardinality = 0; // Do not probe tablebases during the search
1628 else // If DTZ tables are missing, use WDL tables as a fallback
1630 // Filter out moves that do not preserve the draw or the win.
1631 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1633 // Only probe during search if winning
1634 if (RootInTB && TB::Score <= VALUE_DRAW)
1638 if (RootInTB && !UseRule50)
1639 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1640 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
1643 // Since root_probe() and root_probe_wdl() dirty the root move scores,
1644 // we reset them to -VALUE_INFINITE
1645 for (RootMove& rm : rootMoves)
1646 rm.score = -VALUE_INFINITE;