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 inline Value razor_margin(Depth d) { return Value(512 + 32 * d); }
70 inline 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> inline 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::perft() is our utility to verify move generation. All the leaf nodes
185 /// up to the given depth are generated and counted and the sum returned.
187 uint64_t Search::perft(Position& pos, Depth depth) {
190 uint64_t cnt, nodes = 0;
192 const bool leaf = (depth == 2 * ONE_PLY);
194 for (const auto& m : MoveList<LEGAL>(pos))
196 if (Root && depth <= ONE_PLY)
200 pos.do_move(m, st, pos.gives_check(m, ci));
201 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
206 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
211 template uint64_t Search::perft<true>(Position& pos, Depth depth);
214 /// Search::think() is the external interface to Stockfish's search, and is
215 /// called by the main thread when the program receives the UCI 'go' command. It
216 /// searches from RootPos and at the end prints the "bestmove" to output.
218 void Search::think() {
220 Color us = RootPos.side_to_move();
221 Time.init(Limits, us, RootPos.game_ply(), now());
223 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
224 DrawValue[ us] = VALUE_DRAW - Value(contempt);
225 DrawValue[~us] = VALUE_DRAW + Value(contempt);
228 TB::RootInTB = false;
229 TB::UseRule50 = Options["Syzygy50MoveRule"];
230 TB::ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
231 TB::Cardinality = Options["SyzygyProbeLimit"];
233 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
234 if (TB::Cardinality > TB::MaxCardinality)
236 TB::Cardinality = TB::MaxCardinality;
237 TB::ProbeDepth = DEPTH_ZERO;
240 if (RootMoves.empty())
242 RootMoves.push_back(RootMove(MOVE_NONE));
243 sync_cout << "info depth 0 score "
244 << UCI::value(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
249 if (TB::Cardinality >= RootPos.count<ALL_PIECES>(WHITE)
250 + RootPos.count<ALL_PIECES>(BLACK))
252 // If the current root position is in the tablebases then RootMoves
253 // contains only moves that preserve the draw or win.
254 TB::RootInTB = Tablebases::root_probe(RootPos, RootMoves, TB::Score);
257 TB::Cardinality = 0; // Do not probe tablebases during the search
259 else // If DTZ tables are missing, use WDL tables as a fallback
261 // Filter out moves that do not preserve a draw or win
262 TB::RootInTB = Tablebases::root_probe_wdl(RootPos, RootMoves, TB::Score);
264 // Only probe during search if winning
265 if (TB::Score <= VALUE_DRAW)
271 TB::Hits = RootMoves.size();
274 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
275 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
280 for (Thread* th : Threads)
283 th->notify_one(); // Wake up all the threads
286 Threads.timer->run = true;
287 Threads.timer->notify_one(); // Start the recurring timer
289 id_loop(RootPos); // Let's start searching !
291 Threads.timer->run = false;
294 // When playing in 'nodes as time' mode, subtract the searched nodes from
295 // the available ones before to exit.
297 Time.availableNodes += Limits.inc[us] - RootPos.nodes_searched();
299 // When we reach the maximum depth, we can arrive here without a raise of
300 // Signals.stop. However, if we are pondering or in an infinite search,
301 // the UCI protocol states that we shouldn't print the best move before the
302 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
303 // until the GUI sends one of those commands (which also raises Signals.stop).
304 if (!Signals.stop && (Limits.ponder || Limits.infinite))
306 Signals.stopOnPonderhit = true;
307 RootPos.this_thread()->wait_for(Signals.stop);
310 sync_cout << "bestmove " << UCI::move(RootMoves[0].pv[0], RootPos.is_chess960());
312 if (RootMoves[0].pv.size() > 1 || RootMoves[0].extract_ponder_from_tt(RootPos))
313 std::cout << " ponder " << UCI::move(RootMoves[0].pv[1], RootPos.is_chess960());
315 std::cout << sync_endl;
321 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
322 // with increasing depth until the allocated thinking time has been consumed,
323 // user stops the search, or the maximum search depth is reached.
325 void id_loop(Position& pos) {
327 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
329 Value bestValue, alpha, beta, delta;
331 Move easyMove = EasyMove.get(pos.key());
334 std::memset(ss-2, 0, 5 * sizeof(Stack));
338 bestValue = delta = alpha = -VALUE_INFINITE;
339 beta = VALUE_INFINITE;
343 CounterMovesHistory.clear();
345 Countermoves.clear();
347 size_t multiPV = Options["MultiPV"];
348 Skill skill(Options["Skill Level"]);
350 // When playing with strength handicap enable MultiPV search that we will
351 // use behind the scenes to retrieve a set of possible moves.
353 multiPV = std::max(multiPV, (size_t)4);
355 multiPV = std::min(multiPV, RootMoves.size());
357 // Iterative deepening loop until requested to stop or target depth reached
358 while (++depth < DEPTH_MAX && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
360 // Age out PV variability metric
361 BestMoveChanges *= 0.5;
363 // Save the last iteration's scores before first PV line is searched and
364 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
365 for (RootMove& rm : RootMoves)
366 rm.previousScore = rm.score;
368 // MultiPV loop. We perform a full root search for each PV line
369 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
371 // Reset aspiration window starting size
372 if (depth >= 5 * ONE_PLY)
375 alpha = std::max(RootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
376 beta = std::min(RootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
379 // Start with a small aspiration window and, in the case of a fail
380 // high/low, re-search with a bigger window until we're not failing
384 bestValue = search<Root, false>(pos, ss, alpha, beta, depth, false);
386 // Bring the best move to the front. It is critical that sorting
387 // is done with a stable algorithm because all the values but the
388 // first and eventually the new best one are set to -VALUE_INFINITE
389 // and we want to keep the same order for all the moves except the
390 // new PV that goes to the front. Note that in case of MultiPV
391 // search the already searched PV lines are preserved.
392 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
394 // Write PV back to transposition table in case the relevant
395 // entries have been overwritten during the search.
396 for (size_t i = 0; i <= PVIdx; ++i)
397 RootMoves[i].insert_pv_in_tt(pos);
399 // If search has been stopped break immediately. Sorting and
400 // writing PV back to TT is safe because RootMoves is still
401 // valid, although it refers to previous iteration.
405 // When failing high/low give some update (without cluttering
406 // the UI) before a re-search.
408 && (bestValue <= alpha || bestValue >= beta)
409 && Time.elapsed() > 3000)
410 sync_cout << UCI::pv(pos, depth, alpha, beta) << sync_endl;
412 // In case of failing low/high increase aspiration window and
413 // re-search, otherwise exit the loop.
414 if (bestValue <= alpha)
416 beta = (alpha + beta) / 2;
417 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
419 Signals.failedLowAtRoot = true;
420 Signals.stopOnPonderhit = false;
422 else if (bestValue >= beta)
424 alpha = (alpha + beta) / 2;
425 beta = std::min(bestValue + delta, VALUE_INFINITE);
432 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
435 // Sort the PV lines searched so far and update the GUI
436 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
439 sync_cout << "info nodes " << RootPos.nodes_searched()
440 << " time " << Time.elapsed() << sync_endl;
442 else if (PVIdx + 1 == multiPV || Time.elapsed() > 3000)
443 sync_cout << UCI::pv(pos, depth, alpha, beta) << sync_endl;
446 // If skill level is enabled and time is up, pick a sub-optimal best move
447 if (skill.enabled() && skill.time_to_pick(depth))
448 skill.pick_best(multiPV);
450 // Have we found a "mate in x"?
452 && bestValue >= VALUE_MATE_IN_MAX_PLY
453 && VALUE_MATE - bestValue <= 2 * Limits.mate)
456 // Do we have time for the next iteration? Can we stop searching now?
457 if (Limits.use_time_management())
459 if (!Signals.stop && !Signals.stopOnPonderhit)
461 // Take some extra time if the best move has changed
462 if (depth > 4 * ONE_PLY && multiPV == 1)
463 Time.pv_instability(BestMoveChanges);
465 // Stop the search if only one legal move is available or all
466 // of the available time has been used or we matched an easyMove
467 // from the previous search and just did a fast verification.
468 if ( RootMoves.size() == 1
469 || Time.elapsed() > Time.available()
470 || ( RootMoves[0].pv[0] == easyMove
471 && BestMoveChanges < 0.03
472 && Time.elapsed() > Time.available() / 10))
474 // If we are allowed to ponder do not stop the search now but
475 // keep pondering until the GUI sends "ponderhit" or "stop".
477 Signals.stopOnPonderhit = true;
483 if (RootMoves[0].pv.size() >= 3)
484 EasyMove.update(pos, RootMoves[0].pv);
490 // Clear any candidate easy move that wasn't stable for the last search
491 // iterations; the second condition prevents consecutive fast moves.
492 if (EasyMove.stableCnt < 6 || Time.elapsed() < Time.available())
495 // If skill level is enabled, swap best PV line with the sub-optimal one
497 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
498 RootMoves.end(), skill.best_move(multiPV)));
502 // search<>() is the main search function for both PV and non-PV nodes and for
503 // normal and SplitPoint nodes. When called just after a split point the search
504 // is simpler because we have already probed the hash table, done a null move
505 // search, and searched the first move before splitting, so we don't have to
506 // repeat all this work again. We also don't need to store anything to the hash
507 // table here: This is taken care of after we return from the split point.
509 template <NodeType NT, bool SpNode>
510 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
512 const bool RootNode = NT == Root;
513 const bool PvNode = NT == PV || NT == Root;
515 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
516 assert(PvNode || (alpha == beta - 1));
517 assert(depth > DEPTH_ZERO);
519 Move pv[MAX_PLY+1], quietsSearched[64];
522 SplitPoint* splitPoint;
524 Move ttMove, move, excludedMove, bestMove;
525 Depth extension, newDepth, predictedDepth;
526 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
527 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
528 bool captureOrPromotion, dangerous, doFullDepthSearch;
529 int moveCount, quietCount;
531 // Step 1. Initialize node
532 Thread* thisThread = pos.this_thread();
533 inCheck = pos.checkers();
537 splitPoint = ss->splitPoint;
538 bestMove = splitPoint->bestMove;
539 bestValue = splitPoint->bestValue;
542 ttMove = excludedMove = MOVE_NONE;
543 ttValue = VALUE_NONE;
545 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
550 moveCount = quietCount = 0;
551 bestValue = -VALUE_INFINITE;
552 ss->ply = (ss-1)->ply + 1;
554 // Used to send selDepth info to GUI
555 if (PvNode && thisThread->maxPly < ss->ply)
556 thisThread->maxPly = ss->ply;
560 // Step 2. Check for aborted search and immediate draw
561 if (Signals.stop || pos.is_draw() || ss->ply >= MAX_PLY)
562 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
564 // Step 3. Mate distance pruning. Even if we mate at the next move our score
565 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
566 // a shorter mate was found upward in the tree then there is no need to search
567 // because we will never beat the current alpha. Same logic but with reversed
568 // signs applies also in the opposite condition of being mated instead of giving
569 // mate. In this case return a fail-high score.
570 alpha = std::max(mated_in(ss->ply), alpha);
571 beta = std::min(mate_in(ss->ply+1), beta);
576 assert(0 <= ss->ply && ss->ply < MAX_PLY);
578 ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
579 (ss+1)->skipEarlyPruning = false; (ss+1)->reduction = DEPTH_ZERO;
580 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
582 // Step 4. Transposition table lookup
583 // We don't want the score of a partial search to overwrite a previous full search
584 // TT value, so we use a different position key in case of an excluded move.
585 excludedMove = ss->excludedMove;
586 posKey = excludedMove ? pos.exclusion_key() : pos.key();
587 tte = TT.probe(posKey, ttHit);
588 ss->ttMove = ttMove = RootNode ? RootMoves[PVIdx].pv[0] : ttHit ? tte->move() : MOVE_NONE;
589 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
591 // At non-PV nodes we check for a fail high/low. We don't prune at PV nodes
594 && tte->depth() >= depth
595 && ttValue != VALUE_NONE // Only in case of TT access race
596 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
597 : (tte->bound() & BOUND_UPPER)))
599 ss->currentMove = ttMove; // Can be MOVE_NONE
601 // If ttMove is quiet, update killers, history, counter move on TT hit
602 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove))
603 update_stats(pos, ss, ttMove, depth, nullptr, 0);
608 // Step 4a. Tablebase probe
609 if (!RootNode && TB::Cardinality)
611 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
613 if ( piecesCnt <= TB::Cardinality
614 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
615 && pos.rule50_count() == 0)
617 int found, v = Tablebases::probe_wdl(pos, &found);
623 int drawScore = TB::UseRule50 ? 1 : 0;
625 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
626 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
627 : VALUE_DRAW + 2 * v * drawScore;
629 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
630 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
631 MOVE_NONE, VALUE_NONE, TT.generation());
638 // Step 5. Evaluate the position statically and update parent's gain statistics
641 ss->staticEval = eval = VALUE_NONE;
647 // Never assume anything on values stored in TT
648 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
649 eval = ss->staticEval = evaluate(pos);
651 // Can ttValue be used as a better position evaluation?
652 if (ttValue != VALUE_NONE)
653 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
658 eval = ss->staticEval =
659 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
661 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
664 if (ss->skipEarlyPruning)
667 if ( !pos.captured_piece_type()
668 && ss->staticEval != VALUE_NONE
669 && (ss-1)->staticEval != VALUE_NONE
670 && (move = (ss-1)->currentMove) != MOVE_NULL
672 && type_of(move) == NORMAL)
674 Square to = to_sq(move);
675 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
678 // Step 6. Razoring (skipped when in check)
680 && depth < 4 * ONE_PLY
681 && eval + razor_margin(depth) <= alpha
682 && ttMove == MOVE_NONE
683 && !pos.pawn_on_7th(pos.side_to_move()))
685 if ( depth <= ONE_PLY
686 && eval + razor_margin(3 * ONE_PLY) <= alpha)
687 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
689 Value ralpha = alpha - razor_margin(depth);
690 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
695 // Step 7. Futility pruning: child node (skipped when in check)
697 && depth < 7 * ONE_PLY
698 && eval - futility_margin(depth) >= beta
699 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
700 && pos.non_pawn_material(pos.side_to_move()))
701 return eval - futility_margin(depth);
703 // Step 8. Null move search with verification search (is omitted in PV nodes)
705 && depth >= 2 * ONE_PLY
707 && pos.non_pawn_material(pos.side_to_move()))
709 ss->currentMove = MOVE_NULL;
711 assert(eval - beta >= 0);
713 // Null move dynamic reduction based on depth and value
714 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
716 pos.do_null_move(st);
717 (ss+1)->skipEarlyPruning = true;
718 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
719 : - search<NonPV, false>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
720 (ss+1)->skipEarlyPruning = false;
721 pos.undo_null_move();
723 if (nullValue >= beta)
725 // Do not return unproven mate scores
726 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
729 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
732 // Do verification search at high depths
733 ss->skipEarlyPruning = true;
734 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
735 : search<NonPV, false>(pos, ss, beta-1, beta, depth-R, false);
736 ss->skipEarlyPruning = false;
743 // Step 9. ProbCut (skipped when in check)
744 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
745 // and a reduced search returns a value much above beta, we can (almost) safely
746 // prune the previous move.
748 && depth >= 5 * ONE_PLY
749 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
751 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
752 Depth rdepth = depth - 4 * ONE_PLY;
754 assert(rdepth >= ONE_PLY);
755 assert((ss-1)->currentMove != MOVE_NONE);
756 assert((ss-1)->currentMove != MOVE_NULL);
758 MovePicker mp(pos, ttMove, History, CounterMovesHistory, pos.captured_piece_type());
761 while ((move = mp.next_move<false>()) != MOVE_NONE)
762 if (pos.legal(move, ci.pinned))
764 ss->currentMove = move;
765 pos.do_move(move, st, pos.gives_check(move, ci));
766 value = -search<NonPV, false>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
773 // Step 10. Internal iterative deepening (skipped when in check)
774 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
776 && (PvNode || ss->staticEval + 256 >= beta))
778 Depth d = 2 * (depth - 2 * ONE_PLY) - (PvNode ? DEPTH_ZERO : depth / 2);
779 ss->skipEarlyPruning = true;
780 search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
781 ss->skipEarlyPruning = false;
783 tte = TT.probe(posKey, ttHit);
784 ttMove = ttHit ? tte->move() : MOVE_NONE;
787 moves_loop: // When in check and at SpNode search starts from here
789 Square prevMoveSq = to_sq((ss-1)->currentMove);
790 Move countermove = Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq];
792 MovePicker mp(pos, ttMove, depth, History, CounterMovesHistory, countermove, ss);
794 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
795 improving = ss->staticEval >= (ss-2)->staticEval
796 || ss->staticEval == VALUE_NONE
797 ||(ss-2)->staticEval == VALUE_NONE;
799 singularExtensionNode = !RootNode
801 && depth >= 8 * ONE_PLY
802 && ttMove != MOVE_NONE
803 /* && ttValue != VALUE_NONE Already implicit in the next condition */
804 && abs(ttValue) < VALUE_KNOWN_WIN
805 && !excludedMove // Recursive singular search is not allowed
806 && (tte->bound() & BOUND_LOWER)
807 && tte->depth() >= depth - 3 * ONE_PLY;
809 // Step 11. Loop through moves
810 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
811 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
815 if (move == excludedMove)
818 // At root obey the "searchmoves" option and skip moves not listed in Root
819 // Move List. As a consequence any illegal move is also skipped. In MultiPV
820 // mode we also skip PV moves which have been already searched.
821 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
826 // Shared counter cannot be decremented later if the move turns out to be illegal
827 if (!pos.legal(move, ci.pinned))
830 moveCount = ++splitPoint->moveCount;
831 splitPoint->spinlock.release();
838 Signals.firstRootMove = (moveCount == 1);
840 if (thisThread == Threads.main() && Time.elapsed() > 3000)
841 sync_cout << "info depth " << depth / ONE_PLY
842 << " currmove " << UCI::move(move, pos.is_chess960())
843 << " currmovenumber " << moveCount + PVIdx << sync_endl;
847 (ss+1)->pv = nullptr;
849 extension = DEPTH_ZERO;
850 captureOrPromotion = pos.capture_or_promotion(move);
852 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
853 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
854 : pos.gives_check(move, ci);
856 dangerous = givesCheck
857 || type_of(move) != NORMAL
858 || pos.advanced_pawn_push(move);
860 // Step 12. Extend checks
861 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
864 // Singular extension search. If all moves but one fail low on a search of
865 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
866 // is singular and should be extended. To verify this we do a reduced search
867 // on all the other moves but the ttMove and if the result is lower than
868 // ttValue minus a margin then we extend the ttMove.
869 if ( singularExtensionNode
872 && pos.legal(move, ci.pinned))
874 Value rBeta = ttValue - 2 * depth / ONE_PLY;
875 ss->excludedMove = move;
876 ss->skipEarlyPruning = true;
877 value = search<NonPV, false>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
878 ss->skipEarlyPruning = false;
879 ss->excludedMove = MOVE_NONE;
885 // Update the current move (this must be done after singular extension search)
886 newDepth = depth - ONE_PLY + extension;
888 // Step 13. Pruning at shallow depth
890 && !captureOrPromotion
893 && bestValue > VALUE_MATED_IN_MAX_PLY)
895 // Move count based pruning
896 if ( depth < 16 * ONE_PLY
897 && moveCount >= FutilityMoveCounts[improving][depth])
900 splitPoint->spinlock.acquire();
905 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
907 // Futility pruning: parent node
908 if (predictedDepth < 7 * ONE_PLY)
910 futilityValue = ss->staticEval + futility_margin(predictedDepth)
911 + 128 + Gains[pos.moved_piece(move)][to_sq(move)];
913 if (futilityValue <= alpha)
915 bestValue = std::max(bestValue, futilityValue);
919 splitPoint->spinlock.acquire();
920 if (bestValue > splitPoint->bestValue)
921 splitPoint->bestValue = bestValue;
927 // Prune moves with negative SEE at low depths
928 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
931 splitPoint->spinlock.acquire();
937 // Speculative prefetch as early as possible
938 prefetch(TT.first_entry(pos.key_after(move)));
940 // Check for legality just before making the move
941 if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
947 ss->currentMove = move;
949 // Step 14. Make the move
950 pos.do_move(move, st, givesCheck);
952 // Step 15. Reduced depth search (LMR). If the move fails high it will be
953 // re-searched at full depth.
954 if ( depth >= 3 * ONE_PLY
956 && !captureOrPromotion
957 && move != ss->killers[0]
958 && move != ss->killers[1])
960 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
962 if ( (!PvNode && cutNode)
963 || History[pos.piece_on(to_sq(move))][to_sq(move)] < VALUE_ZERO
964 || ( History[pos.piece_on(to_sq(move))][to_sq(move)]
965 + CounterMovesHistory[pos.piece_on(prevMoveSq)][prevMoveSq]
966 [pos.piece_on(to_sq(move))][to_sq(move)] < VALUE_ZERO))
967 ss->reduction += ONE_PLY;
969 if (move == countermove)
970 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
972 // Decrease reduction for moves that escape a capture
974 && type_of(move) == NORMAL
975 && type_of(pos.piece_on(to_sq(move))) != PAWN
976 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
977 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
979 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
981 alpha = splitPoint->alpha;
983 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d, true);
985 // Re-search at intermediate depth if reduction is very high
986 if (value > alpha && ss->reduction >= 4 * ONE_PLY)
988 Depth d2 = std::max(newDepth - 2 * ONE_PLY, ONE_PLY);
989 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d2, true);
992 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
993 ss->reduction = DEPTH_ZERO;
996 doFullDepthSearch = !PvNode || moveCount > 1;
998 // Step 16. Full depth search, when LMR is skipped or fails high
999 if (doFullDepthSearch)
1002 alpha = splitPoint->alpha;
1004 value = newDepth < ONE_PLY ?
1005 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1006 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1007 : - search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1010 // For PV nodes only, do a full PV search on the first move or after a fail
1011 // high (in the latter case search only if value < beta), otherwise let the
1012 // parent node fail low with value <= alpha and to try another move.
1013 if (PvNode && (moveCount == 1 || (value > alpha && (RootNode || value < beta))))
1016 (ss+1)->pv[0] = MOVE_NONE;
1018 value = newDepth < ONE_PLY ?
1019 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1020 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1021 : - search<PV, false>(pos, ss+1, -beta, -alpha, newDepth, false);
1024 // Step 17. Undo move
1025 pos.undo_move(move);
1027 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1029 // Step 18. Check for new best move
1032 splitPoint->spinlock.acquire();
1033 bestValue = splitPoint->bestValue;
1034 alpha = splitPoint->alpha;
1037 // Finished searching the move. If a stop or a cutoff occurred, the return
1038 // value of the search cannot be trusted, and we return immediately without
1039 // updating best move, PV and TT.
1040 if (Signals.stop || thisThread->cutoff_occurred())
1045 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
1047 // PV move or new best move ?
1048 if (moveCount == 1 || value > alpha)
1055 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1056 rm.pv.push_back(*m);
1058 // We record how often the best move has been changed in each
1059 // iteration. This information is used for time management: When
1060 // the best move changes frequently, we allocate some more time.
1065 // All other moves but the PV are set to the lowest value: this is
1066 // not a problem when sorting because the sort is stable and the
1067 // move position in the list is preserved - just the PV is pushed up.
1068 rm.score = -VALUE_INFINITE;
1071 if (value > bestValue)
1073 bestValue = SpNode ? splitPoint->bestValue = value : value;
1077 // If there is an easy move for this position, clear it if unstable
1079 && EasyMove.get(pos.key())
1080 && (move != EasyMove.get(pos.key()) || moveCount > 1))
1083 bestMove = SpNode ? splitPoint->bestMove = move : move;
1085 if (PvNode && !RootNode) // Update pv even in fail-high case
1086 update_pv(SpNode ? splitPoint->ss->pv : ss->pv, move, (ss+1)->pv);
1088 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1089 alpha = SpNode ? splitPoint->alpha = value : value;
1092 assert(value >= beta); // Fail high
1095 splitPoint->cutoff = true;
1102 if (!SpNode && !captureOrPromotion && move != bestMove && quietCount < 64)
1103 quietsSearched[quietCount++] = move;
1105 // Step 19. Check for splitting the search
1107 && Threads.size() >= 2
1108 && depth >= Threads.minimumSplitDepth
1109 && ( !thisThread->activeSplitPoint
1110 || !thisThread->activeSplitPoint->allSlavesSearching
1111 || ( Threads.size() > MAX_SLAVES_PER_SPLITPOINT
1112 && thisThread->activeSplitPoint->slavesMask.count() == MAX_SLAVES_PER_SPLITPOINT))
1113 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1115 assert(bestValue > -VALUE_INFINITE && bestValue < beta);
1117 thisThread->split(pos, ss, alpha, beta, &bestValue, &bestMove,
1118 depth, moveCount, &mp, NT, cutNode);
1120 if (Signals.stop || thisThread->cutoff_occurred())
1123 if (bestValue >= beta)
1131 // Following condition would detect a stop or a cutoff set only after move
1132 // loop has been completed. But in this case bestValue is valid because we
1133 // have fully searched our subtree, and we can anyhow save the result in TT.
1135 if (Signals.stop || thisThread->cutoff_occurred())
1139 // Step 20. Check for mate and stalemate
1140 // All legal moves have been searched and if there are no legal moves, it
1141 // must be mate or stalemate. If we are in a singular extension search then
1142 // return a fail low score.
1144 bestValue = excludedMove ? alpha
1145 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1147 // Quiet best move: update killers, history and countermoves
1148 else if (bestMove && !pos.capture_or_promotion(bestMove))
1149 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount);
1151 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1152 bestValue >= beta ? BOUND_LOWER :
1153 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1154 depth, bestMove, ss->staticEval, TT.generation());
1156 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1162 // qsearch() is the quiescence search function, which is called by the main
1163 // search function when the remaining depth is zero (or, to be more precise,
1164 // less than ONE_PLY).
1166 template <NodeType NT, bool InCheck>
1167 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1169 const bool PvNode = NT == PV;
1171 assert(NT == PV || NT == NonPV);
1172 assert(InCheck == !!pos.checkers());
1173 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1174 assert(PvNode || (alpha == beta - 1));
1175 assert(depth <= DEPTH_ZERO);
1181 Move ttMove, move, bestMove;
1182 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1183 bool ttHit, givesCheck, evasionPrunable;
1188 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1190 ss->pv[0] = MOVE_NONE;
1193 ss->currentMove = bestMove = MOVE_NONE;
1194 ss->ply = (ss-1)->ply + 1;
1196 // Check for an instant draw or if the maximum ply has been reached
1197 if (pos.is_draw() || ss->ply >= MAX_PLY)
1198 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
1200 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1202 // Decide whether or not to include checks: this fixes also the type of
1203 // TT entry depth that we are going to use. Note that in qsearch we use
1204 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1205 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1206 : DEPTH_QS_NO_CHECKS;
1208 // Transposition table lookup
1210 tte = TT.probe(posKey, ttHit);
1211 ttMove = ttHit ? tte->move() : MOVE_NONE;
1212 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1216 && tte->depth() >= ttDepth
1217 && ttValue != VALUE_NONE // Only in case of TT access race
1218 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1219 : (tte->bound() & BOUND_UPPER)))
1221 ss->currentMove = ttMove; // Can be MOVE_NONE
1225 // Evaluate the position statically
1228 ss->staticEval = VALUE_NONE;
1229 bestValue = futilityBase = -VALUE_INFINITE;
1235 // Never assume anything on values stored in TT
1236 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1237 ss->staticEval = bestValue = evaluate(pos);
1239 // Can ttValue be used as a better position evaluation?
1240 if (ttValue != VALUE_NONE)
1241 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1242 bestValue = ttValue;
1245 ss->staticEval = bestValue =
1246 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
1248 // Stand pat. Return immediately if static value is at least beta
1249 if (bestValue >= beta)
1252 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1253 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1258 if (PvNode && bestValue > alpha)
1261 futilityBase = bestValue + 128;
1264 // Initialize a MovePicker object for the current position, and prepare
1265 // to search the moves. Because the depth is <= 0 here, only captures,
1266 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1268 MovePicker mp(pos, ttMove, depth, History, CounterMovesHistory, to_sq((ss-1)->currentMove));
1271 // Loop through the moves until no moves remain or a beta cutoff occurs
1272 while ((move = mp.next_move<false>()) != MOVE_NONE)
1274 assert(is_ok(move));
1276 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1277 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1278 : pos.gives_check(move, ci);
1283 && futilityBase > -VALUE_KNOWN_WIN
1284 && !pos.advanced_pawn_push(move))
1286 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1288 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1290 if (futilityValue <= alpha)
1292 bestValue = std::max(bestValue, futilityValue);
1296 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1298 bestValue = std::max(bestValue, futilityBase);
1303 // Detect non-capture evasions that are candidates to be pruned
1304 evasionPrunable = InCheck
1305 && bestValue > VALUE_MATED_IN_MAX_PLY
1306 && !pos.capture(move);
1308 // Don't search moves with negative SEE values
1309 if ( (!InCheck || evasionPrunable)
1310 && type_of(move) != PROMOTION
1311 && pos.see_sign(move) < VALUE_ZERO)
1314 // Speculative prefetch as early as possible
1315 prefetch(TT.first_entry(pos.key_after(move)));
1317 // Check for legality just before making the move
1318 if (!pos.legal(move, ci.pinned))
1321 ss->currentMove = move;
1323 // Make and search the move
1324 pos.do_move(move, st, givesCheck);
1325 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1326 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1327 pos.undo_move(move);
1329 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1331 // Check for new best move
1332 if (value > bestValue)
1338 if (PvNode) // Update pv even in fail-high case
1339 update_pv(ss->pv, move, (ss+1)->pv);
1341 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1348 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1349 ttDepth, move, ss->staticEval, TT.generation());
1357 // All legal moves have been searched. A special case: If we're in check
1358 // and no legal moves were found, it is checkmate.
1359 if (InCheck && bestValue == -VALUE_INFINITE)
1360 return mated_in(ss->ply); // Plies to mate from the root
1362 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1363 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1364 ttDepth, bestMove, ss->staticEval, TT.generation());
1366 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1372 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1373 // "plies to mate from the current position". Non-mate scores are unchanged.
1374 // The function is called before storing a value in the transposition table.
1376 Value value_to_tt(Value v, int ply) {
1378 assert(v != VALUE_NONE);
1380 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1381 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1385 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1386 // from the transposition table (which refers to the plies to mate/be mated
1387 // from current position) to "plies to mate/be mated from the root".
1389 Value value_from_tt(Value v, int ply) {
1391 return v == VALUE_NONE ? VALUE_NONE
1392 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1393 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1397 // update_pv() adds current move and appends child pv[]
1399 void update_pv(Move* pv, Move move, Move* childPv) {
1401 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1407 // update_stats() updates killers, history, countermove history and
1408 // countermoves stats for a quiet best move.
1410 void update_stats(const Position& pos, Stack* ss, Move move,
1411 Depth depth, Move* quiets, int quietsCnt) {
1413 if (ss->killers[0] != move)
1415 ss->killers[1] = ss->killers[0];
1416 ss->killers[0] = move;
1419 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY));
1421 Square prevSq = to_sq((ss-1)->currentMove);
1422 HistoryStats& cmh = CounterMovesHistory[pos.piece_on(prevSq)][prevSq];
1424 History.update(pos.moved_piece(move), to_sq(move), bonus);
1426 if (is_ok((ss-1)->currentMove))
1428 Countermoves.update(pos.piece_on(prevSq), prevSq, move);
1429 cmh.update(pos.moved_piece(move), to_sq(move), bonus);
1432 // Decrease all the other played quiet moves
1433 for (int i = 0; i < quietsCnt; ++i)
1435 History.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1437 if (is_ok((ss-1)->currentMove))
1438 cmh.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1441 // Extra penalty for TT move in previous ply when it gets refuted
1442 if (is_ok((ss-2)->currentMove) && (ss-1)->currentMove == (ss-1)->ttMove)
1444 Square prevPrevSq = to_sq((ss-2)->currentMove);
1445 HistoryStats& ttMoveCmh = CounterMovesHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
1446 ttMoveCmh.update(pos.piece_on(prevSq), prevSq, -bonus - 2 * depth / ONE_PLY - 1);
1451 // When playing with strength handicap, choose best move among a set of RootMoves
1452 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1454 Move Skill::pick_best(size_t multiPV) {
1456 // PRNG sequence should be non-deterministic, so we seed it with the time at init
1457 static PRNG rng(now());
1459 // RootMoves are already sorted by score in descending order
1460 int variance = std::min(RootMoves[0].score - RootMoves[multiPV - 1].score, PawnValueMg);
1461 int weakness = 120 - 2 * level;
1462 int maxScore = -VALUE_INFINITE;
1464 // Choose best move. For each move score we add two terms both dependent on
1465 // weakness. One deterministic and bigger for weaker levels, and one random,
1466 // then we choose the move with the resulting highest score.
1467 for (size_t i = 0; i < multiPV; ++i)
1469 // This is our magic formula
1470 int push = ( weakness * int(RootMoves[0].score - RootMoves[i].score)
1471 + variance * (rng.rand<unsigned>() % weakness)) / 128;
1473 if (RootMoves[i].score + push > maxScore)
1475 maxScore = RootMoves[i].score + push;
1476 best = RootMoves[i].pv[0];
1485 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1486 /// that all (if any) unsearched PV lines are sent using a previous search score.
1488 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1490 std::stringstream ss;
1491 int elapsed = Time.elapsed() + 1;
1492 size_t multiPV = std::min((size_t)Options["MultiPV"], RootMoves.size());
1495 for (Thread* th : Threads)
1496 if (th->maxPly > selDepth)
1497 selDepth = th->maxPly;
1499 for (size_t i = 0; i < multiPV; ++i)
1501 bool updated = (i <= PVIdx);
1503 if (depth == ONE_PLY && !updated)
1506 Depth d = updated ? depth : depth - ONE_PLY;
1507 Value v = updated ? RootMoves[i].score : RootMoves[i].previousScore;
1509 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1510 v = tb ? TB::Score : v;
1512 if (ss.rdbuf()->in_avail()) // Not at first line
1516 << " depth " << d / ONE_PLY
1517 << " seldepth " << selDepth
1518 << " multipv " << i + 1
1519 << " score " << UCI::value(v);
1521 if (!tb && i == PVIdx)
1522 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1524 ss << " nodes " << pos.nodes_searched()
1525 << " nps " << pos.nodes_searched() * 1000 / elapsed;
1527 if (elapsed > 1000) // Earlier makes little sense
1528 ss << " hashfull " << TT.hashfull();
1530 ss << " tbhits " << TB::Hits
1531 << " time " << elapsed
1534 for (Move m : RootMoves[i].pv)
1535 ss << " " << UCI::move(m, pos.is_chess960());
1542 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1543 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1544 /// first, even if the old TT entries have been overwritten.
1546 void RootMove::insert_pv_in_tt(Position& pos) {
1548 StateInfo state[MAX_PLY], *st = state;
1553 assert(MoveList<LEGAL>(pos).contains(m));
1555 TTEntry* tte = TT.probe(pos.key(), ttHit);
1557 if (!ttHit || tte->move() != m) // Don't overwrite correct entries
1558 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, m, VALUE_NONE, TT.generation());
1560 pos.do_move(m, *st++, pos.gives_check(m, CheckInfo(pos)));
1563 for (size_t i = pv.size(); i > 0; )
1564 pos.undo_move(pv[--i]);
1568 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move before
1569 /// exiting the search, for instance in case we stop the search during a fail high at
1570 /// root. We try hard to have a ponder move to return to the GUI, otherwise in case of
1571 /// 'ponder on' we have nothing to think on.
1573 bool RootMove::extract_ponder_from_tt(Position& pos)
1578 assert(pv.size() == 1);
1580 pos.do_move(pv[0], st, pos.gives_check(pv[0], CheckInfo(pos)));
1581 TTEntry* tte = TT.probe(pos.key(), ttHit);
1582 pos.undo_move(pv[0]);
1586 Move m = tte->move(); // Local copy to be SMP safe
1587 if (MoveList<LEGAL>(pos).contains(m))
1588 return pv.push_back(m), true;
1595 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1597 void Thread::idle_loop() {
1599 // Pointer 'this_sp' is not null only if we are called from split(), and not
1600 // at the thread creation. This means we are the split point's master.
1601 SplitPoint* this_sp = activeSplitPoint;
1603 assert(!this_sp || (this_sp->master == this && searching));
1605 while (!exit && !(this_sp && this_sp->slavesMask.none()))
1607 // If this thread has been assigned work, launch a search
1612 assert(activeSplitPoint);
1613 SplitPoint* sp = activeSplitPoint;
1617 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
1618 Position pos(*sp->pos, this);
1620 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1621 ss->splitPoint = sp;
1623 sp->spinlock.acquire();
1625 assert(activePosition == nullptr);
1627 activePosition = &pos;
1629 if (sp->nodeType == NonPV)
1630 search<NonPV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1632 else if (sp->nodeType == PV)
1633 search<PV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1635 else if (sp->nodeType == Root)
1636 search<Root, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1644 activePosition = nullptr;
1645 sp->slavesMask.reset(idx);
1646 sp->allSlavesSearching = false;
1647 sp->nodes += pos.nodes_searched();
1649 // After releasing the lock we can't access any SplitPoint related data
1650 // in a safe way because it could have been released under our feet by
1652 sp->spinlock.release();
1654 // Try to late join to another split point if none of its slaves has
1655 // already finished.
1656 SplitPoint* bestSp = NULL;
1657 int minLevel = INT_MAX;
1659 for (Thread* th : Threads)
1661 const size_t size = th->splitPointsSize; // Local copy
1662 sp = size ? &th->splitPoints[size - 1] : nullptr;
1665 && sp->allSlavesSearching
1666 && sp->slavesMask.count() < MAX_SLAVES_PER_SPLITPOINT
1670 assert(!(this_sp && this_sp->slavesMask.none()));
1671 assert(Threads.size() > 2);
1673 // Prefer to join to SP with few parents to reduce the probability
1674 // that a cut-off occurs above us, and hence we waste our work.
1676 for (SplitPoint* p = th->activeSplitPoint; p; p = p->parentSplitPoint)
1679 if (level < minLevel)
1691 // Recheck the conditions under lock protection
1692 sp->spinlock.acquire();
1694 if ( sp->allSlavesSearching
1695 && sp->slavesMask.count() < MAX_SLAVES_PER_SPLITPOINT)
1701 sp->slavesMask.set(idx);
1702 activeSplitPoint = sp;
1709 sp->spinlock.release();
1713 // If search is finished then sleep, otherwise just yield
1714 if (!Threads.main()->thinking)
1718 std::unique_lock<Mutex> lk(mutex);
1719 while (!exit && !Threads.main()->thinking)
1720 sleepCondition.wait(lk);
1723 std::this_thread::yield(); // Wait for a new job or for our slaves to finish
1728 /// check_time() is called by the timer thread when the timer triggers. It is
1729 /// used to print debug info and, more importantly, to detect when we are out of
1730 /// available time and thus stop the search.
1734 static TimePoint lastInfoTime = now();
1735 int elapsed = Time.elapsed();
1737 if (now() - lastInfoTime >= 1000)
1739 lastInfoTime = now();
1743 // An engine may not stop pondering until told so by the GUI
1747 if (Limits.use_time_management())
1749 bool stillAtFirstMove = Signals.firstRootMove
1750 && !Signals.failedLowAtRoot
1751 && elapsed > Time.available() * 75 / 100;
1753 if ( stillAtFirstMove
1754 || elapsed > Time.maximum() - 2 * TimerThread::Resolution)
1755 Signals.stop = true;
1757 else if (Limits.movetime && elapsed >= Limits.movetime)
1758 Signals.stop = true;
1760 else if (Limits.nodes)
1762 int64_t nodes = RootPos.nodes_searched();
1764 // Loop across all split points and sum accumulated SplitPoint nodes plus
1765 // all the currently active positions nodes.
1767 for (Thread* th : Threads)
1768 for (size_t i = 0; i < th->splitPointsSize; ++i)
1770 SplitPoint& sp = th->splitPoints[i];
1772 sp.spinlock.acquire();
1776 for (size_t idx = 0; idx < Threads.size(); ++idx)
1777 if (sp.slavesMask.test(idx) && Threads[idx]->activePosition)
1778 nodes += Threads[idx]->activePosition->nodes_searched();
1780 sp.spinlock.release();
1783 if (nodes >= Limits.nodes)
1784 Signals.stop = true;