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
6 Stockfish is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation, either version 3 of the License, or
9 (at your option) any later version.
11 Stockfish is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>.
23 #include <cstring> // For std::memset
36 #include "syzygy/tbprobe.h"
40 volatile SignalsType Signals;
42 RootMoveVector RootMoves;
44 StateStackPtr SetupStates;
47 namespace Tablebases {
57 namespace TB = Tablebases;
61 using namespace Search;
65 // Different node types, used as template parameter
66 enum NodeType { Root, PV, NonPV };
68 // Razoring and futility margin based on depth
69 Value razor_margin(Depth d) { return Value(512 + 32 * d); }
70 Value futility_margin(Depth d) { return Value(200 * d); }
72 // Futility and reductions lookup tables, initialized at startup
73 int FutilityMoveCounts[2][16]; // [improving][depth]
74 Depth Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
76 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
77 return Reductions[PvNode][i][std::min(d, 63 * ONE_PLY)][std::min(mn, 63)];
80 // Skill struct is used to implement strength limiting
82 Skill(int l) : level(l) {}
83 bool enabled() const { return level < 20; }
84 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
85 Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
86 Move pick_best(size_t multiPV);
89 Move best = MOVE_NONE;
92 // EasyMoveManager struct is used to detect a so called 'easy move'; when PV is
93 // stable across multiple search iterations we can fast return the best move.
94 struct EasyMoveManager {
99 pv[0] = pv[1] = pv[2] = MOVE_NONE;
102 Move get(Key key) const {
103 return expectedPosKey == key ? pv[2] : MOVE_NONE;
106 void update(Position& pos, const std::vector<Move>& newPv) {
108 assert(newPv.size() >= 3);
110 // Keep track of how many times in a row 3rd ply remains stable
111 stableCnt = (newPv[2] == pv[2]) ? stableCnt + 1 : 0;
113 if (!std::equal(newPv.begin(), newPv.begin() + 3, pv))
115 std::copy(newPv.begin(), newPv.begin() + 3, pv);
118 pos.do_move(newPv[0], st[0], pos.gives_check(newPv[0], CheckInfo(pos)));
119 pos.do_move(newPv[1], st[1], pos.gives_check(newPv[1], CheckInfo(pos)));
120 expectedPosKey = pos.key();
121 pos.undo_move(newPv[1]);
122 pos.undo_move(newPv[0]);
132 EasyMoveManager EasyMove;
133 double BestMoveChanges;
134 Value DrawValue[COLOR_NB];
135 HistoryStats History;
136 CounterMovesHistoryStats CounterMovesHistory;
138 MovesStats Countermoves;
140 template <NodeType NT, bool SpNode>
141 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
143 template <NodeType NT, bool InCheck>
144 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
146 void id_loop(Position& pos);
147 Value value_to_tt(Value v, int ply);
148 Value value_from_tt(Value v, int ply);
149 void update_pv(Move* pv, Move move, Move* childPv);
150 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
155 /// Search::init() is called during startup to initialize various lookup tables
157 void Search::init() {
159 const double K[][2] = {{ 0.83, 2.25 }, { 0.50, 3.00 }};
161 for (int pv = 0; pv <= 1; ++pv)
162 for (int imp = 0; imp <= 1; ++imp)
163 for (int d = 1; d < 64; ++d)
164 for (int mc = 1; mc < 64; ++mc)
166 double r = K[pv][0] + log(d) * log(mc) / K[pv][1];
169 Reductions[pv][imp][d][mc] = int(r) * ONE_PLY;
171 // Increase reduction when eval is not improving
172 if (!pv && !imp && Reductions[pv][imp][d][mc] >= 2 * ONE_PLY)
173 Reductions[pv][imp][d][mc] += ONE_PLY;
176 for (int d = 0; d < 16; ++d)
178 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
179 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
184 /// Search::reset() clears all search memory, to restore a deterministic state
186 void Search::reset () {
190 CounterMovesHistory.clear();
192 Countermoves.clear();
196 /// Search::perft() is our utility to verify move generation. All the leaf nodes
197 /// up to the given depth are generated and counted and the sum returned.
199 uint64_t Search::perft(Position& pos, Depth depth) {
202 uint64_t cnt, nodes = 0;
204 const bool leaf = (depth == 2 * ONE_PLY);
206 for (const auto& m : MoveList<LEGAL>(pos))
208 if (Root && depth <= ONE_PLY)
212 pos.do_move(m, st, pos.gives_check(m, ci));
213 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
218 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
223 template uint64_t Search::perft<true>(Position& pos, Depth depth);
226 /// Search::think() is the external interface to Stockfish's search, and is
227 /// called by the main thread when the program receives the UCI 'go' command. It
228 /// searches from RootPos and at the end prints the "bestmove" to output.
230 void Search::think() {
232 Color us = RootPos.side_to_move();
233 Time.init(Limits, us, RootPos.game_ply(), now());
235 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
236 DrawValue[ us] = VALUE_DRAW - Value(contempt);
237 DrawValue[~us] = VALUE_DRAW + Value(contempt);
240 TB::RootInTB = false;
241 TB::UseRule50 = Options["Syzygy50MoveRule"];
242 TB::ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
243 TB::Cardinality = Options["SyzygyProbeLimit"];
245 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
246 if (TB::Cardinality > TB::MaxCardinality)
248 TB::Cardinality = TB::MaxCardinality;
249 TB::ProbeDepth = DEPTH_ZERO;
252 if (RootMoves.empty())
254 RootMoves.push_back(RootMove(MOVE_NONE));
255 sync_cout << "info depth 0 score "
256 << UCI::value(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
261 if (TB::Cardinality >= RootPos.count<ALL_PIECES>(WHITE)
262 + RootPos.count<ALL_PIECES>(BLACK))
264 // If the current root position is in the tablebases then RootMoves
265 // contains only moves that preserve the draw or win.
266 TB::RootInTB = Tablebases::root_probe(RootPos, RootMoves, TB::Score);
269 TB::Cardinality = 0; // Do not probe tablebases during the search
271 else // If DTZ tables are missing, use WDL tables as a fallback
273 // Filter out moves that do not preserve a draw or win
274 TB::RootInTB = Tablebases::root_probe_wdl(RootPos, RootMoves, TB::Score);
276 // Only probe during search if winning
277 if (TB::Score <= VALUE_DRAW)
283 TB::Hits = RootMoves.size();
286 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
287 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
292 for (Thread* th : Threads)
295 th->notify_one(); // Wake up all the threads
298 Threads.timer->run = true;
299 Threads.timer->notify_one(); // Start the recurring timer
301 id_loop(RootPos); // Let's start searching !
303 Threads.timer->run = false;
306 // When playing in 'nodes as time' mode, subtract the searched nodes from
307 // the available ones before to exit.
309 Time.availableNodes += Limits.inc[us] - RootPos.nodes_searched();
311 // When we reach the maximum depth, we can arrive here without a raise of
312 // Signals.stop. However, if we are pondering or in an infinite search,
313 // the UCI protocol states that we shouldn't print the best move before the
314 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
315 // until the GUI sends one of those commands (which also raises Signals.stop).
316 if (!Signals.stop && (Limits.ponder || Limits.infinite))
318 Signals.stopOnPonderhit = true;
319 RootPos.this_thread()->wait_for(Signals.stop);
322 sync_cout << "bestmove " << UCI::move(RootMoves[0].pv[0], RootPos.is_chess960());
324 if (RootMoves[0].pv.size() > 1 || RootMoves[0].extract_ponder_from_tt(RootPos))
325 std::cout << " ponder " << UCI::move(RootMoves[0].pv[1], RootPos.is_chess960());
327 std::cout << sync_endl;
333 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
334 // with increasing depth until the allocated thinking time has been consumed,
335 // user stops the search, or the maximum search depth is reached.
337 void id_loop(Position& pos) {
339 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
341 Value bestValue, alpha, beta, delta;
343 Move easyMove = EasyMove.get(pos.key());
346 std::memset(ss-2, 0, 5 * sizeof(Stack));
350 bestValue = delta = alpha = -VALUE_INFINITE;
351 beta = VALUE_INFINITE;
355 size_t multiPV = Options["MultiPV"];
356 Skill skill(Options["Skill Level"]);
358 // When playing with strength handicap enable MultiPV search that we will
359 // use behind the scenes to retrieve a set of possible moves.
361 multiPV = std::max(multiPV, (size_t)4);
363 multiPV = std::min(multiPV, RootMoves.size());
365 // Iterative deepening loop until requested to stop or target depth reached
366 while (++depth < DEPTH_MAX && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
368 // Age out PV variability metric
369 BestMoveChanges *= 0.5;
371 // Save the last iteration's scores before first PV line is searched and
372 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
373 for (RootMove& rm : RootMoves)
374 rm.previousScore = rm.score;
376 // MultiPV loop. We perform a full root search for each PV line
377 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
379 // Reset aspiration window starting size
380 if (depth >= 5 * ONE_PLY)
383 alpha = std::max(RootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
384 beta = std::min(RootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
387 // Start with a small aspiration window and, in the case of a fail
388 // high/low, re-search with a bigger window until we're not failing
392 bestValue = search<Root, false>(pos, ss, alpha, beta, depth, false);
394 // Bring the best move to the front. It is critical that sorting
395 // is done with a stable algorithm because all the values but the
396 // first and eventually the new best one are set to -VALUE_INFINITE
397 // and we want to keep the same order for all the moves except the
398 // new PV that goes to the front. Note that in case of MultiPV
399 // search the already searched PV lines are preserved.
400 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
402 // Write PV back to transposition table in case the relevant
403 // entries have been overwritten during the search.
404 for (size_t i = 0; i <= PVIdx; ++i)
405 RootMoves[i].insert_pv_in_tt(pos);
407 // If search has been stopped break immediately. Sorting and
408 // writing PV back to TT is safe because RootMoves is still
409 // valid, although it refers to previous iteration.
413 // When failing high/low give some update (without cluttering
414 // the UI) before a re-search.
416 && (bestValue <= alpha || bestValue >= beta)
417 && Time.elapsed() > 3000)
418 sync_cout << UCI::pv(pos, depth, alpha, beta) << sync_endl;
420 // In case of failing low/high increase aspiration window and
421 // re-search, otherwise exit the loop.
422 if (bestValue <= alpha)
424 beta = (alpha + beta) / 2;
425 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
427 Signals.failedLowAtRoot = true;
428 Signals.stopOnPonderhit = false;
430 else if (bestValue >= beta)
432 alpha = (alpha + beta) / 2;
433 beta = std::min(bestValue + delta, VALUE_INFINITE);
440 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
443 // Sort the PV lines searched so far and update the GUI
444 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
447 sync_cout << "info nodes " << RootPos.nodes_searched()
448 << " time " << Time.elapsed() << sync_endl;
450 else if (PVIdx + 1 == multiPV || Time.elapsed() > 3000)
451 sync_cout << UCI::pv(pos, depth, alpha, beta) << sync_endl;
454 // If skill level is enabled and time is up, pick a sub-optimal best move
455 if (skill.enabled() && skill.time_to_pick(depth))
456 skill.pick_best(multiPV);
458 // Have we found a "mate in x"?
460 && bestValue >= VALUE_MATE_IN_MAX_PLY
461 && VALUE_MATE - bestValue <= 2 * Limits.mate)
464 // Do we have time for the next iteration? Can we stop searching now?
465 if (Limits.use_time_management())
467 if (!Signals.stop && !Signals.stopOnPonderhit)
469 // Take some extra time if the best move has changed
470 if (depth > 4 * ONE_PLY && multiPV == 1)
471 Time.pv_instability(BestMoveChanges);
473 // Stop the search if only one legal move is available or all
474 // of the available time has been used or we matched an easyMove
475 // from the previous search and just did a fast verification.
476 if ( RootMoves.size() == 1
477 || Time.elapsed() > Time.available()
478 || ( RootMoves[0].pv[0] == easyMove
479 && BestMoveChanges < 0.03
480 && Time.elapsed() > Time.available() / 10))
482 // If we are allowed to ponder do not stop the search now but
483 // keep pondering until the GUI sends "ponderhit" or "stop".
485 Signals.stopOnPonderhit = true;
491 if (RootMoves[0].pv.size() >= 3)
492 EasyMove.update(pos, RootMoves[0].pv);
498 // Clear any candidate easy move that wasn't stable for the last search
499 // iterations; the second condition prevents consecutive fast moves.
500 if (EasyMove.stableCnt < 6 || Time.elapsed() < Time.available())
503 // If skill level is enabled, swap best PV line with the sub-optimal one
505 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
506 RootMoves.end(), skill.best_move(multiPV)));
510 // search<>() is the main search function for both PV and non-PV nodes and for
511 // normal and SplitPoint nodes. When called just after a split point the search
512 // is simpler because we have already probed the hash table, done a null move
513 // search, and searched the first move before splitting, so we don't have to
514 // repeat all this work again. We also don't need to store anything to the hash
515 // table here: This is taken care of after we return from the split point.
517 template <NodeType NT, bool SpNode>
518 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
520 const bool RootNode = NT == Root;
521 const bool PvNode = NT == PV || NT == Root;
523 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
524 assert(PvNode || (alpha == beta - 1));
525 assert(depth > DEPTH_ZERO);
527 Move pv[MAX_PLY+1], quietsSearched[64];
530 SplitPoint* splitPoint;
532 Move ttMove, move, excludedMove, bestMove;
533 Depth extension, newDepth, predictedDepth;
534 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
535 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
536 bool captureOrPromotion, dangerous, doFullDepthSearch;
537 int moveCount, quietCount;
539 // Step 1. Initialize node
540 Thread* thisThread = pos.this_thread();
541 inCheck = pos.checkers();
545 splitPoint = ss->splitPoint;
546 bestMove = splitPoint->bestMove;
547 bestValue = splitPoint->bestValue;
550 ttMove = excludedMove = MOVE_NONE;
551 ttValue = VALUE_NONE;
553 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
558 moveCount = quietCount = 0;
559 bestValue = -VALUE_INFINITE;
560 ss->ply = (ss-1)->ply + 1;
562 // Used to send selDepth info to GUI
563 if (PvNode && thisThread->maxPly < ss->ply)
564 thisThread->maxPly = ss->ply;
568 // Step 2. Check for aborted search and immediate draw
569 if (Signals.stop || pos.is_draw() || ss->ply >= MAX_PLY)
570 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
572 // Step 3. Mate distance pruning. Even if we mate at the next move our score
573 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
574 // a shorter mate was found upward in the tree then there is no need to search
575 // because we will never beat the current alpha. Same logic but with reversed
576 // signs applies also in the opposite condition of being mated instead of giving
577 // mate. In this case return a fail-high score.
578 alpha = std::max(mated_in(ss->ply), alpha);
579 beta = std::min(mate_in(ss->ply+1), beta);
584 assert(0 <= ss->ply && ss->ply < MAX_PLY);
586 ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
587 (ss+1)->skipEarlyPruning = false; (ss+1)->reduction = DEPTH_ZERO;
588 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
590 // Step 4. Transposition table lookup
591 // We don't want the score of a partial search to overwrite a previous full search
592 // TT value, so we use a different position key in case of an excluded move.
593 excludedMove = ss->excludedMove;
594 posKey = excludedMove ? pos.exclusion_key() : pos.key();
595 tte = TT.probe(posKey, ttHit);
596 ss->ttMove = ttMove = RootNode ? RootMoves[PVIdx].pv[0] : ttHit ? tte->move() : MOVE_NONE;
597 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
599 // At non-PV nodes we check for a fail high/low. We don't prune at PV nodes
602 && tte->depth() >= depth
603 && ttValue != VALUE_NONE // Only in case of TT access race
604 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
605 : (tte->bound() & BOUND_UPPER)))
607 ss->currentMove = ttMove; // Can be MOVE_NONE
609 // If ttMove is quiet, update killers, history, counter move on TT hit
610 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove))
611 update_stats(pos, ss, ttMove, depth, nullptr, 0);
616 // Step 4a. Tablebase probe
617 if (!RootNode && TB::Cardinality)
619 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
621 if ( piecesCnt <= TB::Cardinality
622 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
623 && pos.rule50_count() == 0)
625 int found, v = Tablebases::probe_wdl(pos, &found);
631 int drawScore = TB::UseRule50 ? 1 : 0;
633 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
634 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
635 : VALUE_DRAW + 2 * v * drawScore;
637 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
638 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
639 MOVE_NONE, VALUE_NONE, TT.generation());
646 // Step 5. Evaluate the position statically and update parent's gain statistics
649 ss->staticEval = eval = VALUE_NONE;
655 // Never assume anything on values stored in TT
656 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
657 eval = ss->staticEval = evaluate(pos);
659 // Can ttValue be used as a better position evaluation?
660 if (ttValue != VALUE_NONE)
661 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
666 eval = ss->staticEval =
667 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
669 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
672 if (ss->skipEarlyPruning)
675 if ( !pos.captured_piece_type()
676 && ss->staticEval != VALUE_NONE
677 && (ss-1)->staticEval != VALUE_NONE
678 && (move = (ss-1)->currentMove) != MOVE_NULL
680 && type_of(move) == NORMAL)
682 Square to = to_sq(move);
683 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
686 // Step 6. Razoring (skipped when in check)
688 && depth < 4 * ONE_PLY
689 && eval + razor_margin(depth) <= alpha
690 && ttMove == MOVE_NONE
691 && !pos.pawn_on_7th(pos.side_to_move()))
693 if ( depth <= ONE_PLY
694 && eval + razor_margin(3 * ONE_PLY) <= alpha)
695 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
697 Value ralpha = alpha - razor_margin(depth);
698 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
703 // Step 7. Futility pruning: child node (skipped when in check)
705 && depth < 7 * ONE_PLY
706 && eval - futility_margin(depth) >= beta
707 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
708 && pos.non_pawn_material(pos.side_to_move()))
709 return eval - futility_margin(depth);
711 // Step 8. Null move search with verification search (is omitted in PV nodes)
713 && depth >= 2 * ONE_PLY
715 && pos.non_pawn_material(pos.side_to_move()))
717 ss->currentMove = MOVE_NULL;
719 assert(eval - beta >= 0);
721 // Null move dynamic reduction based on depth and value
722 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
724 pos.do_null_move(st);
725 (ss+1)->skipEarlyPruning = true;
726 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
727 : - search<NonPV, false>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
728 (ss+1)->skipEarlyPruning = false;
729 pos.undo_null_move();
731 if (nullValue >= beta)
733 // Do not return unproven mate scores
734 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
737 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
740 // Do verification search at high depths
741 ss->skipEarlyPruning = true;
742 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
743 : search<NonPV, false>(pos, ss, beta-1, beta, depth-R, false);
744 ss->skipEarlyPruning = false;
751 // Step 9. ProbCut (skipped when in check)
752 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
753 // and a reduced search returns a value much above beta, we can (almost) safely
754 // prune the previous move.
756 && depth >= 5 * ONE_PLY
757 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
759 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
760 Depth rdepth = depth - 4 * ONE_PLY;
762 assert(rdepth >= ONE_PLY);
763 assert((ss-1)->currentMove != MOVE_NONE);
764 assert((ss-1)->currentMove != MOVE_NULL);
766 MovePicker mp(pos, ttMove, History, CounterMovesHistory, pos.captured_piece_type());
769 while ((move = mp.next_move<false>()) != MOVE_NONE)
770 if (pos.legal(move, ci.pinned))
772 ss->currentMove = move;
773 pos.do_move(move, st, pos.gives_check(move, ci));
774 value = -search<NonPV, false>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
781 // Step 10. Internal iterative deepening (skipped when in check)
782 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
784 && (PvNode || ss->staticEval + 256 >= beta))
786 Depth d = 2 * (depth - 2 * ONE_PLY) - (PvNode ? DEPTH_ZERO : depth / 2);
787 ss->skipEarlyPruning = true;
788 search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
789 ss->skipEarlyPruning = false;
791 tte = TT.probe(posKey, ttHit);
792 ttMove = ttHit ? tte->move() : MOVE_NONE;
795 moves_loop: // When in check and at SpNode search starts from here
797 Square prevMoveSq = to_sq((ss-1)->currentMove);
798 Move countermove = Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq];
800 MovePicker mp(pos, ttMove, depth, History, CounterMovesHistory, countermove, ss);
802 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
803 improving = ss->staticEval >= (ss-2)->staticEval
804 || ss->staticEval == VALUE_NONE
805 ||(ss-2)->staticEval == VALUE_NONE;
807 singularExtensionNode = !RootNode
809 && depth >= 8 * ONE_PLY
810 && ttMove != MOVE_NONE
811 /* && ttValue != VALUE_NONE Already implicit in the next condition */
812 && abs(ttValue) < VALUE_KNOWN_WIN
813 && !excludedMove // Recursive singular search is not allowed
814 && (tte->bound() & BOUND_LOWER)
815 && tte->depth() >= depth - 3 * ONE_PLY;
817 // Step 11. Loop through moves
818 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
819 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
823 if (move == excludedMove)
826 // At root obey the "searchmoves" option and skip moves not listed in Root
827 // Move List. As a consequence any illegal move is also skipped. In MultiPV
828 // mode we also skip PV moves which have been already searched.
829 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
834 // Shared counter cannot be decremented later if the move turns out to be illegal
835 if (!pos.legal(move, ci.pinned))
838 moveCount = ++splitPoint->moveCount;
839 splitPoint->spinlock.release();
846 Signals.firstRootMove = (moveCount == 1);
848 if (thisThread == Threads.main() && Time.elapsed() > 3000)
849 sync_cout << "info depth " << depth / ONE_PLY
850 << " currmove " << UCI::move(move, pos.is_chess960())
851 << " currmovenumber " << moveCount + PVIdx << sync_endl;
855 (ss+1)->pv = nullptr;
857 extension = DEPTH_ZERO;
858 captureOrPromotion = pos.capture_or_promotion(move);
860 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
861 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
862 : pos.gives_check(move, ci);
864 dangerous = givesCheck
865 || type_of(move) != NORMAL
866 || pos.advanced_pawn_push(move);
868 // Step 12. Extend checks
869 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
872 // Singular extension search. If all moves but one fail low on a search of
873 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
874 // is singular and should be extended. To verify this we do a reduced search
875 // on all the other moves but the ttMove and if the result is lower than
876 // ttValue minus a margin then we extend the ttMove.
877 if ( singularExtensionNode
880 && pos.legal(move, ci.pinned))
882 Value rBeta = ttValue - 2 * depth / ONE_PLY;
883 ss->excludedMove = move;
884 ss->skipEarlyPruning = true;
885 value = search<NonPV, false>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
886 ss->skipEarlyPruning = false;
887 ss->excludedMove = MOVE_NONE;
893 // Update the current move (this must be done after singular extension search)
894 newDepth = depth - ONE_PLY + extension;
896 // Step 13. Pruning at shallow depth
898 && !captureOrPromotion
901 && bestValue > VALUE_MATED_IN_MAX_PLY)
903 // Move count based pruning
904 if ( depth < 16 * ONE_PLY
905 && moveCount >= FutilityMoveCounts[improving][depth])
908 splitPoint->spinlock.acquire();
913 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
915 // Futility pruning: parent node
916 if (predictedDepth < 7 * ONE_PLY)
918 futilityValue = ss->staticEval + futility_margin(predictedDepth)
919 + 128 + Gains[pos.moved_piece(move)][to_sq(move)];
921 if (futilityValue <= alpha)
923 bestValue = std::max(bestValue, futilityValue);
927 splitPoint->spinlock.acquire();
928 if (bestValue > splitPoint->bestValue)
929 splitPoint->bestValue = bestValue;
935 // Prune moves with negative SEE at low depths
936 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
939 splitPoint->spinlock.acquire();
945 // Speculative prefetch as early as possible
946 prefetch(TT.first_entry(pos.key_after(move)));
948 // Check for legality just before making the move
949 if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
955 ss->currentMove = move;
957 // Step 14. Make the move
958 pos.do_move(move, st, givesCheck);
960 // Step 15. Reduced depth search (LMR). If the move fails high it will be
961 // re-searched at full depth.
962 if ( depth >= 3 * ONE_PLY
964 && !captureOrPromotion
965 && move != ss->killers[0]
966 && move != ss->killers[1])
968 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
970 if ( (!PvNode && cutNode)
971 || ( History[pos.piece_on(to_sq(move))][to_sq(move)] < VALUE_ZERO
972 && CounterMovesHistory[pos.piece_on(prevMoveSq)][prevMoveSq]
973 [pos.piece_on(to_sq(move))][to_sq(move)] <= VALUE_ZERO))
974 ss->reduction += ONE_PLY;
976 if (move == countermove)
977 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
979 // Decrease reduction for moves that escape a capture
981 && type_of(move) == NORMAL
982 && type_of(pos.piece_on(to_sq(move))) != PAWN
983 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
984 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
986 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
988 alpha = splitPoint->alpha;
990 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d, true);
992 // Re-search at intermediate depth if reduction is very high
993 if (value > alpha && ss->reduction >= 4 * ONE_PLY)
995 Depth d2 = std::max(newDepth - 2 * ONE_PLY, ONE_PLY);
996 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d2, true);
999 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
1000 ss->reduction = DEPTH_ZERO;
1003 doFullDepthSearch = !PvNode || moveCount > 1;
1005 // Step 16. Full depth search, when LMR is skipped or fails high
1006 if (doFullDepthSearch)
1009 alpha = splitPoint->alpha;
1011 value = newDepth < ONE_PLY ?
1012 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1013 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1014 : - search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1017 // For PV nodes only, do a full PV search on the first move or after a fail
1018 // high (in the latter case search only if value < beta), otherwise let the
1019 // parent node fail low with value <= alpha and to try another move.
1020 if (PvNode && (moveCount == 1 || (value > alpha && (RootNode || value < beta))))
1023 (ss+1)->pv[0] = MOVE_NONE;
1025 value = newDepth < ONE_PLY ?
1026 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1027 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1028 : - search<PV, false>(pos, ss+1, -beta, -alpha, newDepth, false);
1031 // Step 17. Undo move
1032 pos.undo_move(move);
1034 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1036 // Step 18. Check for new best move
1039 splitPoint->spinlock.acquire();
1040 bestValue = splitPoint->bestValue;
1041 alpha = splitPoint->alpha;
1044 // Finished searching the move. If a stop or a cutoff occurred, the return
1045 // value of the search cannot be trusted, and we return immediately without
1046 // updating best move, PV and TT.
1047 if (Signals.stop || thisThread->cutoff_occurred())
1052 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
1054 // PV move or new best move ?
1055 if (moveCount == 1 || value > alpha)
1062 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1063 rm.pv.push_back(*m);
1065 // We record how often the best move has been changed in each
1066 // iteration. This information is used for time management: When
1067 // the best move changes frequently, we allocate some more time.
1072 // All other moves but the PV are set to the lowest value: this is
1073 // not a problem when sorting because the sort is stable and the
1074 // move position in the list is preserved - just the PV is pushed up.
1075 rm.score = -VALUE_INFINITE;
1078 if (value > bestValue)
1080 bestValue = SpNode ? splitPoint->bestValue = value : value;
1084 // If there is an easy move for this position, clear it if unstable
1086 && EasyMove.get(pos.key())
1087 && (move != EasyMove.get(pos.key()) || moveCount > 1))
1090 bestMove = SpNode ? splitPoint->bestMove = move : move;
1092 if (PvNode && !RootNode) // Update pv even in fail-high case
1093 update_pv(SpNode ? splitPoint->ss->pv : ss->pv, move, (ss+1)->pv);
1095 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1096 alpha = SpNode ? splitPoint->alpha = value : value;
1099 assert(value >= beta); // Fail high
1102 splitPoint->cutoff = true;
1109 if (!SpNode && !captureOrPromotion && move != bestMove && quietCount < 64)
1110 quietsSearched[quietCount++] = move;
1112 // Step 19. Check for splitting the search
1114 && Threads.size() >= 2
1115 && depth >= Threads.minimumSplitDepth
1116 && ( !thisThread->activeSplitPoint
1117 || !thisThread->activeSplitPoint->allSlavesSearching
1118 || ( Threads.size() > MAX_SLAVES_PER_SPLITPOINT
1119 && thisThread->activeSplitPoint->slavesMask.count() == MAX_SLAVES_PER_SPLITPOINT))
1120 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1122 assert(bestValue > -VALUE_INFINITE && bestValue < beta);
1124 thisThread->split(pos, ss, alpha, beta, &bestValue, &bestMove,
1125 depth, moveCount, &mp, NT, cutNode);
1127 if (Signals.stop || thisThread->cutoff_occurred())
1130 if (bestValue >= beta)
1138 // Following condition would detect a stop or a cutoff set only after move
1139 // loop has been completed. But in this case bestValue is valid because we
1140 // have fully searched our subtree, and we can anyhow save the result in TT.
1142 if (Signals.stop || thisThread->cutoff_occurred())
1146 // Step 20. Check for mate and stalemate
1147 // All legal moves have been searched and if there are no legal moves, it
1148 // must be mate or stalemate. If we are in a singular extension search then
1149 // return a fail low score.
1151 bestValue = excludedMove ? alpha
1152 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1154 // Quiet best move: update killers, history and countermoves
1155 else if (bestMove && !pos.capture_or_promotion(bestMove))
1156 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount);
1158 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1159 bestValue >= beta ? BOUND_LOWER :
1160 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1161 depth, bestMove, ss->staticEval, TT.generation());
1163 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1169 // qsearch() is the quiescence search function, which is called by the main
1170 // search function when the remaining depth is zero (or, to be more precise,
1171 // less than ONE_PLY).
1173 template <NodeType NT, bool InCheck>
1174 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1176 const bool PvNode = NT == PV;
1178 assert(NT == PV || NT == NonPV);
1179 assert(InCheck == !!pos.checkers());
1180 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1181 assert(PvNode || (alpha == beta - 1));
1182 assert(depth <= DEPTH_ZERO);
1188 Move ttMove, move, bestMove;
1189 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1190 bool ttHit, givesCheck, evasionPrunable;
1195 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1197 ss->pv[0] = MOVE_NONE;
1200 ss->currentMove = bestMove = MOVE_NONE;
1201 ss->ply = (ss-1)->ply + 1;
1203 // Check for an instant draw or if the maximum ply has been reached
1204 if (pos.is_draw() || ss->ply >= MAX_PLY)
1205 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
1207 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1209 // Decide whether or not to include checks: this fixes also the type of
1210 // TT entry depth that we are going to use. Note that in qsearch we use
1211 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1212 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1213 : DEPTH_QS_NO_CHECKS;
1215 // Transposition table lookup
1217 tte = TT.probe(posKey, ttHit);
1218 ttMove = ttHit ? tte->move() : MOVE_NONE;
1219 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1223 && tte->depth() >= ttDepth
1224 && ttValue != VALUE_NONE // Only in case of TT access race
1225 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1226 : (tte->bound() & BOUND_UPPER)))
1228 ss->currentMove = ttMove; // Can be MOVE_NONE
1232 // Evaluate the position statically
1235 ss->staticEval = VALUE_NONE;
1236 bestValue = futilityBase = -VALUE_INFINITE;
1242 // Never assume anything on values stored in TT
1243 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1244 ss->staticEval = bestValue = evaluate(pos);
1246 // Can ttValue be used as a better position evaluation?
1247 if (ttValue != VALUE_NONE)
1248 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1249 bestValue = ttValue;
1252 ss->staticEval = bestValue =
1253 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
1255 // Stand pat. Return immediately if static value is at least beta
1256 if (bestValue >= beta)
1259 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1260 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1265 if (PvNode && bestValue > alpha)
1268 futilityBase = bestValue + 128;
1271 // Initialize a MovePicker object for the current position, and prepare
1272 // to search the moves. Because the depth is <= 0 here, only captures,
1273 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1275 MovePicker mp(pos, ttMove, depth, History, CounterMovesHistory, to_sq((ss-1)->currentMove));
1278 // Loop through the moves until no moves remain or a beta cutoff occurs
1279 while ((move = mp.next_move<false>()) != MOVE_NONE)
1281 assert(is_ok(move));
1283 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1284 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1285 : pos.gives_check(move, ci);
1290 && futilityBase > -VALUE_KNOWN_WIN
1291 && !pos.advanced_pawn_push(move))
1293 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1295 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1297 if (futilityValue <= alpha)
1299 bestValue = std::max(bestValue, futilityValue);
1303 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1305 bestValue = std::max(bestValue, futilityBase);
1310 // Detect non-capture evasions that are candidates to be pruned
1311 evasionPrunable = InCheck
1312 && bestValue > VALUE_MATED_IN_MAX_PLY
1313 && !pos.capture(move);
1315 // Don't search moves with negative SEE values
1316 if ( (!InCheck || evasionPrunable)
1317 && type_of(move) != PROMOTION
1318 && pos.see_sign(move) < VALUE_ZERO)
1321 // Speculative prefetch as early as possible
1322 prefetch(TT.first_entry(pos.key_after(move)));
1324 // Check for legality just before making the move
1325 if (!pos.legal(move, ci.pinned))
1328 ss->currentMove = move;
1330 // Make and search the move
1331 pos.do_move(move, st, givesCheck);
1332 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1333 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1334 pos.undo_move(move);
1336 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1338 // Check for new best move
1339 if (value > bestValue)
1345 if (PvNode) // Update pv even in fail-high case
1346 update_pv(ss->pv, move, (ss+1)->pv);
1348 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1355 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1356 ttDepth, move, ss->staticEval, TT.generation());
1364 // All legal moves have been searched. A special case: If we're in check
1365 // and no legal moves were found, it is checkmate.
1366 if (InCheck && bestValue == -VALUE_INFINITE)
1367 return mated_in(ss->ply); // Plies to mate from the root
1369 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1370 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1371 ttDepth, bestMove, ss->staticEval, TT.generation());
1373 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1379 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1380 // "plies to mate from the current position". Non-mate scores are unchanged.
1381 // The function is called before storing a value in the transposition table.
1383 Value value_to_tt(Value v, int ply) {
1385 assert(v != VALUE_NONE);
1387 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1388 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1392 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1393 // from the transposition table (which refers to the plies to mate/be mated
1394 // from current position) to "plies to mate/be mated from the root".
1396 Value value_from_tt(Value v, int ply) {
1398 return v == VALUE_NONE ? VALUE_NONE
1399 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1400 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1404 // update_pv() adds current move and appends child pv[]
1406 void update_pv(Move* pv, Move move, Move* childPv) {
1408 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1414 // update_stats() updates killers, history, countermove history and
1415 // countermoves stats for a quiet best move.
1417 void update_stats(const Position& pos, Stack* ss, Move move,
1418 Depth depth, Move* quiets, int quietsCnt) {
1420 if (ss->killers[0] != move)
1422 ss->killers[1] = ss->killers[0];
1423 ss->killers[0] = move;
1426 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY));
1428 Square prevSq = to_sq((ss-1)->currentMove);
1429 HistoryStats& cmh = CounterMovesHistory[pos.piece_on(prevSq)][prevSq];
1431 History.update(pos.moved_piece(move), to_sq(move), bonus);
1433 if (is_ok((ss-1)->currentMove))
1435 Countermoves.update(pos.piece_on(prevSq), prevSq, move);
1436 cmh.update(pos.moved_piece(move), to_sq(move), bonus);
1439 // Decrease all the other played quiet moves
1440 for (int i = 0; i < quietsCnt; ++i)
1442 History.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1444 if (is_ok((ss-1)->currentMove))
1445 cmh.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1448 // Extra penalty for TT move in previous ply when it gets refuted
1449 if (is_ok((ss-2)->currentMove) && (ss-1)->currentMove == (ss-1)->ttMove)
1451 Square prevPrevSq = to_sq((ss-2)->currentMove);
1452 HistoryStats& ttMoveCmh = CounterMovesHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
1453 ttMoveCmh.update(pos.piece_on(prevSq), prevSq, -bonus - 2 * depth / ONE_PLY - 1);
1458 // When playing with strength handicap, choose best move among a set of RootMoves
1459 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1461 Move Skill::pick_best(size_t multiPV) {
1463 // PRNG sequence should be non-deterministic, so we seed it with the time at init
1464 static PRNG rng(now());
1466 // RootMoves are already sorted by score in descending order
1467 int variance = std::min(RootMoves[0].score - RootMoves[multiPV - 1].score, PawnValueMg);
1468 int weakness = 120 - 2 * level;
1469 int maxScore = -VALUE_INFINITE;
1471 // Choose best move. For each move score we add two terms both dependent on
1472 // weakness. One deterministic and bigger for weaker levels, and one random,
1473 // then we choose the move with the resulting highest score.
1474 for (size_t i = 0; i < multiPV; ++i)
1476 // This is our magic formula
1477 int push = ( weakness * int(RootMoves[0].score - RootMoves[i].score)
1478 + variance * (rng.rand<unsigned>() % weakness)) / 128;
1480 if (RootMoves[i].score + push > maxScore)
1482 maxScore = RootMoves[i].score + push;
1483 best = RootMoves[i].pv[0];
1492 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1493 /// that all (if any) unsearched PV lines are sent using a previous search score.
1495 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1497 std::stringstream ss;
1498 int elapsed = Time.elapsed() + 1;
1499 size_t multiPV = std::min((size_t)Options["MultiPV"], RootMoves.size());
1502 for (Thread* th : Threads)
1503 if (th->maxPly > selDepth)
1504 selDepth = th->maxPly;
1506 for (size_t i = 0; i < multiPV; ++i)
1508 bool updated = (i <= PVIdx);
1510 if (depth == ONE_PLY && !updated)
1513 Depth d = updated ? depth : depth - ONE_PLY;
1514 Value v = updated ? RootMoves[i].score : RootMoves[i].previousScore;
1516 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1517 v = tb ? TB::Score : v;
1519 if (ss.rdbuf()->in_avail()) // Not at first line
1523 << " depth " << d / ONE_PLY
1524 << " seldepth " << selDepth
1525 << " multipv " << i + 1
1526 << " score " << UCI::value(v);
1528 if (!tb && i == PVIdx)
1529 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1531 ss << " nodes " << pos.nodes_searched()
1532 << " nps " << pos.nodes_searched() * 1000 / elapsed;
1534 if (elapsed > 1000) // Earlier makes little sense
1535 ss << " hashfull " << TT.hashfull();
1537 ss << " tbhits " << TB::Hits
1538 << " time " << elapsed
1541 for (Move m : RootMoves[i].pv)
1542 ss << " " << UCI::move(m, pos.is_chess960());
1549 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1550 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1551 /// first, even if the old TT entries have been overwritten.
1553 void RootMove::insert_pv_in_tt(Position& pos) {
1555 StateInfo state[MAX_PLY], *st = state;
1560 assert(MoveList<LEGAL>(pos).contains(m));
1562 TTEntry* tte = TT.probe(pos.key(), ttHit);
1564 if (!ttHit || tte->move() != m) // Don't overwrite correct entries
1565 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, m, VALUE_NONE, TT.generation());
1567 pos.do_move(m, *st++, pos.gives_check(m, CheckInfo(pos)));
1570 for (size_t i = pv.size(); i > 0; )
1571 pos.undo_move(pv[--i]);
1575 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move before
1576 /// exiting the search, for instance in case we stop the search during a fail high at
1577 /// root. We try hard to have a ponder move to return to the GUI, otherwise in case of
1578 /// 'ponder on' we have nothing to think on.
1580 bool RootMove::extract_ponder_from_tt(Position& pos)
1585 assert(pv.size() == 1);
1587 pos.do_move(pv[0], st, pos.gives_check(pv[0], CheckInfo(pos)));
1588 TTEntry* tte = TT.probe(pos.key(), ttHit);
1589 pos.undo_move(pv[0]);
1593 Move m = tte->move(); // Local copy to be SMP safe
1594 if (MoveList<LEGAL>(pos).contains(m))
1595 return pv.push_back(m), true;
1602 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1604 void Thread::idle_loop() {
1606 // Pointer 'this_sp' is not null only if we are called from split(), and not
1607 // at the thread creation. This means we are the split point's master.
1608 SplitPoint* this_sp = activeSplitPoint;
1610 assert(!this_sp || (this_sp->master == this && searching));
1612 while (!exit && !(this_sp && this_sp->slavesMask.none()))
1614 // If this thread has been assigned work, launch a search
1619 assert(activeSplitPoint);
1620 SplitPoint* sp = activeSplitPoint;
1624 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
1625 Position pos(*sp->pos, this);
1627 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1628 ss->splitPoint = sp;
1630 sp->spinlock.acquire();
1632 assert(activePosition == nullptr);
1634 activePosition = &pos;
1636 if (sp->nodeType == NonPV)
1637 search<NonPV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1639 else if (sp->nodeType == PV)
1640 search<PV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1642 else if (sp->nodeType == Root)
1643 search<Root, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1651 activePosition = nullptr;
1652 sp->slavesMask.reset(idx);
1653 sp->allSlavesSearching = false;
1654 sp->nodes += pos.nodes_searched();
1656 // After releasing the lock we can't access any SplitPoint related data
1657 // in a safe way because it could have been released under our feet by
1659 sp->spinlock.release();
1661 // Try to late join to another split point if none of its slaves has
1662 // already finished.
1663 SplitPoint* bestSp = NULL;
1664 int minLevel = INT_MAX;
1666 for (Thread* th : Threads)
1668 const size_t size = th->splitPointsSize; // Local copy
1669 sp = size ? &th->splitPoints[size - 1] : nullptr;
1672 && sp->allSlavesSearching
1673 && sp->slavesMask.count() < MAX_SLAVES_PER_SPLITPOINT
1677 assert(!(this_sp && this_sp->slavesMask.none()));
1678 assert(Threads.size() > 2);
1680 // Prefer to join to SP with few parents to reduce the probability
1681 // that a cut-off occurs above us, and hence we waste our work.
1683 for (SplitPoint* p = th->activeSplitPoint; p; p = p->parentSplitPoint)
1686 if (level < minLevel)
1698 // Recheck the conditions under lock protection
1699 sp->spinlock.acquire();
1701 if ( sp->allSlavesSearching
1702 && sp->slavesMask.count() < MAX_SLAVES_PER_SPLITPOINT)
1708 sp->slavesMask.set(idx);
1709 activeSplitPoint = sp;
1716 sp->spinlock.release();
1720 // If search is finished then sleep, otherwise just yield
1721 if (!Threads.main()->thinking)
1725 std::unique_lock<Mutex> lk(mutex);
1726 while (!exit && !Threads.main()->thinking)
1727 sleepCondition.wait(lk);
1730 std::this_thread::yield(); // Wait for a new job or for our slaves to finish
1735 /// check_time() is called by the timer thread when the timer triggers. It is
1736 /// used to print debug info and, more importantly, to detect when we are out of
1737 /// available time and thus stop the search.
1741 static TimePoint lastInfoTime = now();
1742 int elapsed = Time.elapsed();
1744 if (now() - lastInfoTime >= 1000)
1746 lastInfoTime = now();
1750 // An engine may not stop pondering until told so by the GUI
1754 if (Limits.use_time_management())
1756 bool stillAtFirstMove = Signals.firstRootMove
1757 && !Signals.failedLowAtRoot
1758 && elapsed > Time.available() * 75 / 100;
1760 if ( stillAtFirstMove
1761 || elapsed > Time.maximum() - 2 * TimerThread::Resolution)
1762 Signals.stop = true;
1764 else if (Limits.movetime && elapsed >= Limits.movetime)
1765 Signals.stop = true;
1767 else if (Limits.nodes)
1769 int64_t nodes = RootPos.nodes_searched();
1771 // Loop across all split points and sum accumulated SplitPoint nodes plus
1772 // all the currently active positions nodes.
1774 for (Thread* th : Threads)
1775 for (size_t i = 0; i < th->splitPointsSize; ++i)
1777 SplitPoint& sp = th->splitPoints[i];
1779 sp.spinlock.acquire();
1783 for (size_t idx = 0; idx < Threads.size(); ++idx)
1784 if (sp.slavesMask.test(idx) && Threads[idx]->activePosition)
1785 nodes += Threads[idx]->activePosition->nodes_searched();
1787 sp.spinlock.release();
1790 if (nodes >= Limits.nodes)
1791 Signals.stop = true;