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;
45 StateStackPtr SetupStates;
48 namespace Tablebases {
58 namespace TB = Tablebases;
62 using namespace Search;
66 // Different node types, used as template parameter
67 enum NodeType { Root, PV, NonPV };
69 // Razoring and futility margin based on depth
70 inline Value razor_margin(Depth d) { return Value(512 + 32 * d); }
71 inline Value futility_margin(Depth d) { return Value(200 * d); }
73 // Futility and reductions lookup tables, initialized at startup
74 int FutilityMoveCounts[2][16]; // [improving][depth]
75 Depth Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
77 template <bool PvNode> inline Depth reduction(bool i, Depth d, int mn) {
78 return Reductions[PvNode][i][std::min(d, 63 * ONE_PLY)][std::min(mn, 63)];
81 // Skill struct is used to implement strength limiting
83 Skill(int l) : level(l) {}
84 bool enabled() const { return level < 20; }
85 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
86 Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
87 Move pick_best(size_t multiPV);
90 Move best = MOVE_NONE;
93 // EasyMoveManager struct is used to detect a so called 'easy move'; when PV is
94 // stable across multiple search iterations we can fast return the best move.
95 struct EasyMoveManager {
100 pv[0] = pv[1] = pv[2] = MOVE_NONE;
103 Move get(Key key) const {
104 return expectedPosKey == key ? pv[2] : MOVE_NONE;
107 void update(Position& pos, const std::vector<Move>& newPv) {
109 assert(newPv.size() >= 3);
111 // Keep track of how many times in a row 3rd ply remains stable
112 stableCnt = (newPv[2] == pv[2]) ? stableCnt + 1 : 0;
114 if (!std::equal(newPv.begin(), newPv.begin() + 3, pv))
116 std::copy(newPv.begin(), newPv.begin() + 3, pv);
119 pos.do_move(newPv[0], st[0], pos.gives_check(newPv[0], CheckInfo(pos)));
120 pos.do_move(newPv[1], st[1], pos.gives_check(newPv[1], CheckInfo(pos)));
121 expectedPosKey = pos.key();
122 pos.undo_move(newPv[1]);
123 pos.undo_move(newPv[0]);
134 EasyMoveManager EasyMove;
135 double BestMoveChanges;
136 Value DrawValue[COLOR_NB];
137 HistoryStats History;
138 CounterMovesHistoryStats CounterMovesHistory;
140 MovesStats Countermoves, Followupmoves;
142 template <NodeType NT, bool SpNode>
143 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
145 template <NodeType NT, bool InCheck>
146 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
148 void id_loop(Position& pos);
149 Value value_to_tt(Value v, int ply);
150 Value value_from_tt(Value v, int ply);
151 void update_pv(Move* pv, Move move, Move* childPv);
152 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
157 /// Search::init() is called during startup to initialize various lookup tables
159 void Search::init() {
161 const double K[][2] = {{ 0.83, 2.25 }, { 0.50, 3.00 }};
163 for (int pv = 0; pv <= 1; ++pv)
164 for (int imp = 0; imp <= 1; ++imp)
165 for (int d = 1; d < 64; ++d)
166 for (int mc = 1; mc < 64; ++mc)
168 double r = K[pv][0] + log(d) * log(mc) / K[pv][1];
171 Reductions[pv][imp][d][mc] = int(r) * ONE_PLY;
173 // Increase reduction when eval is not improving
174 if (!pv && !imp && Reductions[pv][imp][d][mc] >= 2 * ONE_PLY)
175 Reductions[pv][imp][d][mc] += ONE_PLY;
178 for (int d = 0; d < 16; ++d)
180 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
181 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
186 /// Search::perft() is our utility to verify move generation. All the leaf nodes
187 /// up to the given depth are generated and counted and the sum returned.
189 uint64_t Search::perft(Position& pos, Depth depth) {
192 uint64_t cnt, nodes = 0;
194 const bool leaf = (depth == 2 * ONE_PLY);
196 for (const auto& m : MoveList<LEGAL>(pos))
198 if (Root && depth <= ONE_PLY)
202 pos.do_move(m, st, pos.gives_check(m, ci));
203 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
208 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
213 template uint64_t Search::perft<true>(Position& pos, Depth depth);
216 /// Search::think() is the external interface to Stockfish's search, and is
217 /// called by the main thread when the program receives the UCI 'go' command. It
218 /// searches from RootPos and at the end prints the "bestmove" to output.
220 void Search::think() {
222 TimeMgr.init(Limits, RootPos.side_to_move(), RootPos.game_ply());
224 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
225 DrawValue[ RootPos.side_to_move()] = VALUE_DRAW - Value(contempt);
226 DrawValue[~RootPos.side_to_move()] = VALUE_DRAW + Value(contempt);
229 TB::RootInTB = false;
230 TB::UseRule50 = Options["Syzygy50MoveRule"];
231 TB::ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
232 TB::Cardinality = Options["SyzygyProbeLimit"];
234 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
235 if (TB::Cardinality > TB::MaxCardinality)
237 TB::Cardinality = TB::MaxCardinality;
238 TB::ProbeDepth = DEPTH_ZERO;
241 if (RootMoves.empty())
243 RootMoves.push_back(RootMove(MOVE_NONE));
244 sync_cout << "info depth 0 score "
245 << UCI::value(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
250 if (TB::Cardinality >= RootPos.count<ALL_PIECES>(WHITE)
251 + RootPos.count<ALL_PIECES>(BLACK))
253 // If the current root position is in the tablebases then RootMoves
254 // contains only moves that preserve the draw or win.
255 TB::RootInTB = Tablebases::root_probe(RootPos, RootMoves, TB::Score);
258 TB::Cardinality = 0; // Do not probe tablebases during the search
260 else // If DTZ tables are missing, use WDL tables as a fallback
262 // Filter out moves that do not preserve a draw or win
263 TB::RootInTB = Tablebases::root_probe_wdl(RootPos, RootMoves, TB::Score);
265 // Only probe during search if winning
266 if (TB::Score <= VALUE_DRAW)
272 TB::Hits = RootMoves.size();
275 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
276 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
281 for (Thread* th : Threads)
284 th->notify_one(); // Wake up all the threads
287 Threads.timer->run = true;
288 Threads.timer->notify_one(); // Start the recurring timer
290 id_loop(RootPos); // Let's start searching !
292 Threads.timer->run = false;
295 // When we reach the maximum depth, we can arrive here without a raise of
296 // Signals.stop. However, if we are pondering or in an infinite search,
297 // the UCI protocol states that we shouldn't print the best move before the
298 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
299 // until the GUI sends one of those commands (which also raises Signals.stop).
300 if (!Signals.stop && (Limits.ponder || Limits.infinite))
302 Signals.stopOnPonderhit = true;
303 RootPos.this_thread()->wait_for(Signals.stop);
306 sync_cout << "bestmove " << UCI::move(RootMoves[0].pv[0], RootPos.is_chess960());
308 if (RootMoves[0].pv.size() > 1 || RootMoves[0].extract_ponder_from_tt(RootPos))
309 std::cout << " ponder " << UCI::move(RootMoves[0].pv[1], RootPos.is_chess960());
311 std::cout << sync_endl;
317 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
318 // with increasing depth until the allocated thinking time has been consumed,
319 // user stops the search, or the maximum search depth is reached.
321 void id_loop(Position& pos) {
323 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
325 Value bestValue, alpha, beta, delta;
327 Move easyMove = EasyMove.get(pos.key());
330 std::memset(ss-2, 0, 5 * sizeof(Stack));
334 bestValue = delta = alpha = -VALUE_INFINITE;
335 beta = VALUE_INFINITE;
339 CounterMovesHistory.clear();
341 Countermoves.clear();
342 Followupmoves.clear();
344 size_t multiPV = Options["MultiPV"];
345 Skill skill(Options["Skill Level"]);
347 // When playing with strength handicap enable MultiPV search that we will
348 // use behind the scenes to retrieve a set of possible moves.
350 multiPV = std::max(multiPV, (size_t)4);
352 multiPV = std::min(multiPV, RootMoves.size());
354 // Iterative deepening loop until requested to stop or target depth reached
355 while (++depth < DEPTH_MAX && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
357 // Age out PV variability metric
358 BestMoveChanges *= 0.5;
360 // Save the last iteration's scores before first PV line is searched and
361 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
362 for (RootMove& rm : RootMoves)
363 rm.previousScore = rm.score;
365 // MultiPV loop. We perform a full root search for each PV line
366 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
368 // Reset aspiration window starting size
369 if (depth >= 5 * ONE_PLY)
372 alpha = std::max(RootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
373 beta = std::min(RootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
376 // Start with a small aspiration window and, in the case of a fail
377 // high/low, re-search with a bigger window until we're not failing
381 bestValue = search<Root, false>(pos, ss, alpha, beta, depth, false);
383 // Bring the best move to the front. It is critical that sorting
384 // is done with a stable algorithm because all the values but the
385 // first and eventually the new best one are set to -VALUE_INFINITE
386 // and we want to keep the same order for all the moves except the
387 // new PV that goes to the front. Note that in case of MultiPV
388 // search the already searched PV lines are preserved.
389 std::stable_sort(RootMoves.begin() + PVIdx, RootMoves.end());
391 // Write PV back to transposition table in case the relevant
392 // entries have been overwritten during the search.
393 for (size_t i = 0; i <= PVIdx; ++i)
394 RootMoves[i].insert_pv_in_tt(pos);
396 // If search has been stopped break immediately. Sorting and
397 // writing PV back to TT is safe because RootMoves is still
398 // valid, although it refers to previous iteration.
402 // When failing high/low give some update (without cluttering
403 // the UI) before a re-search.
405 && (bestValue <= alpha || bestValue >= beta)
406 && now() - SearchTime > 3000)
407 sync_cout << UCI::pv(pos, depth, alpha, beta) << sync_endl;
409 // In case of failing low/high increase aspiration window and
410 // re-search, otherwise exit the loop.
411 if (bestValue <= alpha)
413 beta = (alpha + beta) / 2;
414 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
416 Signals.failedLowAtRoot = true;
417 Signals.stopOnPonderhit = false;
419 else if (bestValue >= beta)
421 alpha = (alpha + beta) / 2;
422 beta = std::min(bestValue + delta, VALUE_INFINITE);
429 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
432 // Sort the PV lines searched so far and update the GUI
433 std::stable_sort(RootMoves.begin(), RootMoves.begin() + PVIdx + 1);
436 sync_cout << "info nodes " << RootPos.nodes_searched()
437 << " time " << now() - SearchTime << sync_endl;
439 else if (PVIdx + 1 == multiPV || now() - SearchTime > 3000)
440 sync_cout << UCI::pv(pos, depth, alpha, beta) << sync_endl;
443 // If skill level is enabled and time is up, pick a sub-optimal best move
444 if (skill.enabled() && skill.time_to_pick(depth))
445 skill.pick_best(multiPV);
447 // Have we found a "mate in x"?
449 && bestValue >= VALUE_MATE_IN_MAX_PLY
450 && VALUE_MATE - bestValue <= 2 * Limits.mate)
453 // Do we have time for the next iteration? Can we stop searching now?
454 if (Limits.use_time_management())
456 if (!Signals.stop && !Signals.stopOnPonderhit)
458 // Take some extra time if the best move has changed
459 if (depth > 4 * ONE_PLY && multiPV == 1)
460 TimeMgr.pv_instability(BestMoveChanges);
462 // Stop the search if only one legal move is available or all
463 // of the available time has been used or we matched an easyMove
464 // from the previous search and just did a fast verification.
465 if ( RootMoves.size() == 1
466 || now() - SearchTime > TimeMgr.available_time()
467 || ( RootMoves[0].pv[0] == easyMove
468 && BestMoveChanges < 0.03
469 && now() - SearchTime > TimeMgr.available_time() / 10))
471 // If we are allowed to ponder do not stop the search now but
472 // keep pondering until the GUI sends "ponderhit" or "stop".
474 Signals.stopOnPonderhit = true;
480 if (RootMoves[0].pv.size() >= 3)
481 EasyMove.update(pos, RootMoves[0].pv);
487 // Clear any candidate easy move that wasn't stable for the last search
488 // iterations; the second condition prevents consecutive fast moves.
489 if (EasyMove.stableCnt < 6 || now() - SearchTime < TimeMgr.available_time())
492 // If skill level is enabled, swap best PV line with the sub-optimal one
494 std::swap(RootMoves[0], *std::find(RootMoves.begin(),
495 RootMoves.end(), skill.best_move(multiPV)));
499 // search<>() is the main search function for both PV and non-PV nodes and for
500 // normal and SplitPoint nodes. When called just after a split point the search
501 // is simpler because we have already probed the hash table, done a null move
502 // search, and searched the first move before splitting, so we don't have to
503 // repeat all this work again. We also don't need to store anything to the hash
504 // table here: This is taken care of after we return from the split point.
506 template <NodeType NT, bool SpNode>
507 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
509 const bool RootNode = NT == Root;
510 const bool PvNode = NT == PV || NT == Root;
512 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
513 assert(PvNode || (alpha == beta - 1));
514 assert(depth > DEPTH_ZERO);
516 Move pv[MAX_PLY+1], quietsSearched[64];
519 SplitPoint* splitPoint;
521 Move ttMove, move, excludedMove, bestMove;
522 Depth extension, newDepth, predictedDepth;
523 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
524 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
525 bool captureOrPromotion, dangerous, doFullDepthSearch;
526 int moveCount, quietCount;
528 // Step 1. Initialize node
529 Thread* thisThread = pos.this_thread();
530 inCheck = pos.checkers();
534 splitPoint = ss->splitPoint;
535 bestMove = splitPoint->bestMove;
536 bestValue = splitPoint->bestValue;
539 ttMove = excludedMove = MOVE_NONE;
540 ttValue = VALUE_NONE;
542 assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
547 moveCount = quietCount = 0;
548 bestValue = -VALUE_INFINITE;
549 ss->ply = (ss-1)->ply + 1;
551 // Used to send selDepth info to GUI
552 if (PvNode && thisThread->maxPly < ss->ply)
553 thisThread->maxPly = ss->ply;
557 // Step 2. Check for aborted search and immediate draw
558 if (Signals.stop || pos.is_draw() || ss->ply >= MAX_PLY)
559 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
561 // Step 3. Mate distance pruning. Even if we mate at the next move our score
562 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
563 // a shorter mate was found upward in the tree then there is no need to search
564 // because we will never beat the current alpha. Same logic but with reversed
565 // signs applies also in the opposite condition of being mated instead of giving
566 // mate. In this case return a fail-high score.
567 alpha = std::max(mated_in(ss->ply), alpha);
568 beta = std::min(mate_in(ss->ply+1), beta);
573 assert(0 <= ss->ply && ss->ply < MAX_PLY);
575 ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
576 (ss+1)->skipEarlyPruning = false; (ss+1)->reduction = DEPTH_ZERO;
577 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
579 // Step 4. Transposition table lookup
580 // We don't want the score of a partial search to overwrite a previous full search
581 // TT value, so we use a different position key in case of an excluded move.
582 excludedMove = ss->excludedMove;
583 posKey = excludedMove ? pos.exclusion_key() : pos.key();
584 tte = TT.probe(posKey, ttHit);
585 ss->ttMove = ttMove = RootNode ? RootMoves[PVIdx].pv[0] : ttHit ? tte->move() : MOVE_NONE;
586 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
588 // At non-PV nodes we check for a fail high/low. We don't probe at PV nodes
591 && tte->depth() >= depth
592 && ttValue != VALUE_NONE // Only in case of TT access race
593 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
594 : (tte->bound() & BOUND_UPPER)))
596 ss->currentMove = ttMove; // Can be MOVE_NONE
598 // If ttMove is quiet, update killers, history, counter move and followup move on TT hit
599 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove) && !inCheck)
600 update_stats(pos, ss, ttMove, depth, nullptr, 0);
605 // Step 4a. Tablebase probe
606 if (!RootNode && TB::Cardinality)
608 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
610 if ( piecesCnt <= TB::Cardinality
611 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
612 && pos.rule50_count() == 0)
614 int found, v = Tablebases::probe_wdl(pos, &found);
620 int drawScore = TB::UseRule50 ? 1 : 0;
622 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
623 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
624 : VALUE_DRAW + 2 * v * drawScore;
626 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
627 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
628 MOVE_NONE, VALUE_NONE, TT.generation());
635 // Step 5. Evaluate the position statically and update parent's gain statistics
638 ss->staticEval = eval = VALUE_NONE;
644 // Never assume anything on values stored in TT
645 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
646 eval = ss->staticEval = evaluate(pos);
648 // Can ttValue be used as a better position evaluation?
649 if (ttValue != VALUE_NONE)
650 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
655 eval = ss->staticEval =
656 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
658 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
661 if (ss->skipEarlyPruning)
664 if ( !pos.captured_piece_type()
665 && ss->staticEval != VALUE_NONE
666 && (ss-1)->staticEval != VALUE_NONE
667 && (move = (ss-1)->currentMove) != MOVE_NULL
669 && type_of(move) == NORMAL)
671 Square to = to_sq(move);
672 Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
675 // Step 6. Razoring (skipped when in check)
677 && depth < 4 * ONE_PLY
678 && eval + razor_margin(depth) <= alpha
679 && ttMove == MOVE_NONE
680 && !pos.pawn_on_7th(pos.side_to_move()))
682 if ( depth <= ONE_PLY
683 && eval + razor_margin(3 * ONE_PLY) <= alpha)
684 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
686 Value ralpha = alpha - razor_margin(depth);
687 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
692 // Step 7. Futility pruning: child node (skipped when in check)
694 && depth < 7 * ONE_PLY
695 && eval - futility_margin(depth) >= beta
696 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
697 && pos.non_pawn_material(pos.side_to_move()))
698 return eval - futility_margin(depth);
700 // Step 8. Null move search with verification search (is omitted in PV nodes)
702 && depth >= 2 * ONE_PLY
704 && pos.non_pawn_material(pos.side_to_move()))
706 ss->currentMove = MOVE_NULL;
708 assert(eval - beta >= 0);
710 // Null move dynamic reduction based on depth and value
711 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
713 pos.do_null_move(st);
714 (ss+1)->skipEarlyPruning = true;
715 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
716 : - search<NonPV, false>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
717 (ss+1)->skipEarlyPruning = false;
718 pos.undo_null_move();
720 if (nullValue >= beta)
722 // Do not return unproven mate scores
723 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
726 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
729 // Do verification search at high depths
730 ss->skipEarlyPruning = true;
731 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
732 : search<NonPV, false>(pos, ss, beta-1, beta, depth-R, false);
733 ss->skipEarlyPruning = false;
740 // Step 9. ProbCut (skipped when in check)
741 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
742 // and a reduced search returns a value much above beta, we can (almost) safely
743 // prune the previous move.
745 && depth >= 5 * ONE_PLY
746 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
748 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
749 Depth rdepth = depth - 4 * ONE_PLY;
751 assert(rdepth >= ONE_PLY);
752 assert((ss-1)->currentMove != MOVE_NONE);
753 assert((ss-1)->currentMove != MOVE_NULL);
755 MovePicker mp(pos, ttMove, History, CounterMovesHistory, pos.captured_piece_type());
758 while ((move = mp.next_move<false>()) != MOVE_NONE)
759 if (pos.legal(move, ci.pinned))
761 ss->currentMove = move;
762 pos.do_move(move, st, pos.gives_check(move, ci));
763 value = -search<NonPV, false>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
770 // Step 10. Internal iterative deepening (skipped when in check)
771 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
773 && (PvNode || ss->staticEval + 256 >= beta))
775 Depth d = 2 * (depth - 2 * ONE_PLY) - (PvNode ? DEPTH_ZERO : depth / 2);
776 ss->skipEarlyPruning = true;
777 search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
778 ss->skipEarlyPruning = false;
780 tte = TT.probe(posKey, ttHit);
781 ttMove = ttHit ? tte->move() : MOVE_NONE;
784 moves_loop: // When in check and at SpNode search starts from here
786 Square prevMoveSq = to_sq((ss-1)->currentMove);
787 Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
788 Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
790 Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
791 Move followupmoves[] = { Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].first,
792 Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].second };
794 MovePicker mp(pos, ttMove, depth, History, CounterMovesHistory, countermoves, followupmoves, ss);
796 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
797 improving = ss->staticEval >= (ss-2)->staticEval
798 || ss->staticEval == VALUE_NONE
799 ||(ss-2)->staticEval == VALUE_NONE;
801 singularExtensionNode = !RootNode
803 && depth >= 8 * ONE_PLY
804 && ttMove != MOVE_NONE
805 /* && ttValue != VALUE_NONE Already implicit in the next condition */
806 && abs(ttValue) < VALUE_KNOWN_WIN
807 && !excludedMove // Recursive singular search is not allowed
808 && (tte->bound() & BOUND_LOWER)
809 && tte->depth() >= depth - 3 * ONE_PLY;
811 // Step 11. Loop through moves
812 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
813 while ((move = mp.next_move<SpNode>()) != MOVE_NONE)
817 if (move == excludedMove)
820 // At root obey the "searchmoves" option and skip moves not listed in Root
821 // Move List. As a consequence any illegal move is also skipped. In MultiPV
822 // mode we also skip PV moves which have been already searched.
823 if (RootNode && !std::count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
828 // Shared counter cannot be decremented later if the move turns out to be illegal
829 if (!pos.legal(move, ci.pinned))
832 moveCount = ++splitPoint->moveCount;
833 splitPoint->spinlock.release();
840 Signals.firstRootMove = (moveCount == 1);
842 if (thisThread == Threads.main() && now() - SearchTime > 3000)
843 sync_cout << "info depth " << depth / ONE_PLY
844 << " currmove " << UCI::move(move, pos.is_chess960())
845 << " currmovenumber " << moveCount + PVIdx << sync_endl;
849 (ss+1)->pv = nullptr;
851 extension = DEPTH_ZERO;
852 captureOrPromotion = pos.capture_or_promotion(move);
854 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
855 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
856 : pos.gives_check(move, ci);
858 dangerous = givesCheck
859 || type_of(move) != NORMAL
860 || pos.advanced_pawn_push(move);
862 // Step 12. Extend checks
863 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
866 // Singular extension search. If all moves but one fail low on a search of
867 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
868 // is singular and should be extended. To verify this we do a reduced search
869 // on all the other moves but the ttMove and if the result is lower than
870 // ttValue minus a margin then we extend the ttMove.
871 if ( singularExtensionNode
874 && pos.legal(move, ci.pinned))
876 Value rBeta = ttValue - 2 * depth / ONE_PLY;
877 ss->excludedMove = move;
878 ss->skipEarlyPruning = true;
879 value = search<NonPV, false>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
880 ss->skipEarlyPruning = false;
881 ss->excludedMove = MOVE_NONE;
887 // Update the current move (this must be done after singular extension search)
888 newDepth = depth - ONE_PLY + extension;
890 // Step 13. Pruning at shallow depth
892 && !captureOrPromotion
895 && bestValue > VALUE_MATED_IN_MAX_PLY)
897 // Move count based pruning
898 if ( depth < 16 * ONE_PLY
899 && moveCount >= FutilityMoveCounts[improving][depth])
902 splitPoint->spinlock.acquire();
907 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
909 // Futility pruning: parent node
910 if (predictedDepth < 7 * ONE_PLY)
912 futilityValue = ss->staticEval + futility_margin(predictedDepth)
913 + 128 + Gains[pos.moved_piece(move)][to_sq(move)];
915 if (futilityValue <= alpha)
917 bestValue = std::max(bestValue, futilityValue);
921 splitPoint->spinlock.acquire();
922 if (bestValue > splitPoint->bestValue)
923 splitPoint->bestValue = bestValue;
929 // Prune moves with negative SEE at low depths
930 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
933 splitPoint->spinlock.acquire();
939 // Speculative prefetch as early as possible
940 prefetch(TT.first_entry(pos.key_after(move)));
942 // Check for legality just before making the move
943 if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
949 ss->currentMove = move;
950 if (!SpNode && !captureOrPromotion && quietCount < 64)
951 quietsSearched[quietCount++] = move;
953 // Step 14. Make the move
954 pos.do_move(move, st, givesCheck);
956 // Step 15. Reduced depth search (LMR). If the move fails high it will be
957 // re-searched at full depth.
958 if ( depth >= 3 * ONE_PLY
960 && !captureOrPromotion
961 && move != ss->killers[0]
962 && move != ss->killers[1])
964 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
966 if ( (!PvNode && cutNode)
967 || History[pos.piece_on(to_sq(move))][to_sq(move)] < VALUE_ZERO)
968 ss->reduction += ONE_PLY;
970 if (move == countermoves[0] || move == countermoves[1])
971 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
973 // Decrease reduction for moves that escape a capture
975 && type_of(move) == NORMAL
976 && type_of(pos.piece_on(to_sq(move))) != PAWN
977 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
978 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
980 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
982 alpha = splitPoint->alpha;
984 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d, true);
986 // Re-search at intermediate depth if reduction is very high
987 if (value > alpha && ss->reduction >= 4 * ONE_PLY)
989 Depth d2 = std::max(newDepth - 2 * ONE_PLY, ONE_PLY);
990 value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d2, true);
993 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
994 ss->reduction = DEPTH_ZERO;
997 doFullDepthSearch = !PvNode || moveCount > 1;
999 // Step 16. Full depth search, when LMR is skipped or fails high
1000 if (doFullDepthSearch)
1003 alpha = splitPoint->alpha;
1005 value = newDepth < ONE_PLY ?
1006 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1007 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1008 : - search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1011 // For PV nodes only, do a full PV search on the first move or after a fail
1012 // high (in the latter case search only if value < beta), otherwise let the
1013 // parent node fail low with value <= alpha and to try another move.
1014 if (PvNode && (moveCount == 1 || (value > alpha && (RootNode || value < beta))))
1017 (ss+1)->pv[0] = MOVE_NONE;
1019 value = newDepth < ONE_PLY ?
1020 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1021 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1022 : - search<PV, false>(pos, ss+1, -beta, -alpha, newDepth, false);
1025 // Step 17. Undo move
1026 pos.undo_move(move);
1028 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1030 // Step 18. Check for new best move
1033 splitPoint->spinlock.acquire();
1034 bestValue = splitPoint->bestValue;
1035 alpha = splitPoint->alpha;
1038 // Finished searching the move. If a stop or a cutoff occurred, the return
1039 // value of the search cannot be trusted, and we return immediately without
1040 // updating best move, PV and TT.
1041 if (Signals.stop || thisThread->cutoff_occurred())
1046 RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
1048 // PV move or new best move ?
1049 if (moveCount == 1 || value > alpha)
1056 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1057 rm.pv.push_back(*m);
1059 // We record how often the best move has been changed in each
1060 // iteration. This information is used for time management: When
1061 // the best move changes frequently, we allocate some more time.
1066 // All other moves but the PV are set to the lowest value: this is
1067 // not a problem when sorting because the sort is stable and the
1068 // move position in the list is preserved - just the PV is pushed up.
1069 rm.score = -VALUE_INFINITE;
1072 if (value > bestValue)
1074 bestValue = SpNode ? splitPoint->bestValue = value : value;
1078 // If there is an easy move for this position, clear it if unstable
1080 && EasyMove.get(pos.key())
1081 && (move != EasyMove.get(pos.key()) || moveCount > 1))
1084 bestMove = SpNode ? splitPoint->bestMove = move : move;
1086 if (PvNode && !RootNode) // Update pv even in fail-high case
1087 update_pv(SpNode ? splitPoint->ss->pv : ss->pv, move, (ss+1)->pv);
1089 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1090 alpha = SpNode ? splitPoint->alpha = value : value;
1093 assert(value >= beta); // Fail high
1096 splitPoint->cutoff = true;
1103 // Step 19. Check for splitting the search
1105 && Threads.size() >= 2
1106 && depth >= Threads.minimumSplitDepth
1107 && ( !thisThread->activeSplitPoint
1108 || !thisThread->activeSplitPoint->allSlavesSearching
1109 || ( Threads.size() > MAX_SLAVES_PER_SPLITPOINT
1110 && thisThread->activeSplitPoint->slavesMask.count() == MAX_SLAVES_PER_SPLITPOINT))
1111 && thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
1113 assert(bestValue > -VALUE_INFINITE && bestValue < beta);
1115 thisThread->split(pos, ss, alpha, beta, &bestValue, &bestMove,
1116 depth, moveCount, &mp, NT, cutNode);
1118 if (Signals.stop || thisThread->cutoff_occurred())
1121 if (bestValue >= beta)
1129 // Following condition would detect a stop or a cutoff set only after move
1130 // loop has been completed. But in this case bestValue is valid because we
1131 // have fully searched our subtree, and we can anyhow save the result in TT.
1133 if (Signals.stop || thisThread->cutoff_occurred())
1137 // Step 20. Check for mate and stalemate
1138 // All legal moves have been searched and if there are no legal moves, it
1139 // must be mate or stalemate. If we are in a singular extension search then
1140 // return a fail low score.
1142 bestValue = excludedMove ? alpha
1143 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1145 // Quiet best move: update killers, history, countermoves and followupmoves
1146 else if (bestValue >= beta && !pos.capture_or_promotion(bestMove) && !inCheck)
1147 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount - 1);
1149 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1150 bestValue >= beta ? BOUND_LOWER :
1151 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1152 depth, bestMove, ss->staticEval, TT.generation());
1154 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1160 // qsearch() is the quiescence search function, which is called by the main
1161 // search function when the remaining depth is zero (or, to be more precise,
1162 // less than ONE_PLY).
1164 template <NodeType NT, bool InCheck>
1165 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1167 const bool PvNode = NT == PV;
1169 assert(NT == PV || NT == NonPV);
1170 assert(InCheck == !!pos.checkers());
1171 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1172 assert(PvNode || (alpha == beta - 1));
1173 assert(depth <= DEPTH_ZERO);
1179 Move ttMove, move, bestMove;
1180 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1181 bool ttHit, givesCheck, evasionPrunable;
1186 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1188 ss->pv[0] = MOVE_NONE;
1191 ss->currentMove = bestMove = MOVE_NONE;
1192 ss->ply = (ss-1)->ply + 1;
1194 // Check for an instant draw or if the maximum ply has been reached
1195 if (pos.is_draw() || ss->ply >= MAX_PLY)
1196 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
1198 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1200 // Decide whether or not to include checks: this fixes also the type of
1201 // TT entry depth that we are going to use. Note that in qsearch we use
1202 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1203 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1204 : DEPTH_QS_NO_CHECKS;
1206 // Transposition table lookup
1208 tte = TT.probe(posKey, ttHit);
1209 ttMove = ttHit ? tte->move() : MOVE_NONE;
1210 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1214 && tte->depth() >= ttDepth
1215 && ttValue != VALUE_NONE // Only in case of TT access race
1216 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1217 : (tte->bound() & BOUND_UPPER)))
1219 ss->currentMove = ttMove; // Can be MOVE_NONE
1223 // Evaluate the position statically
1226 ss->staticEval = VALUE_NONE;
1227 bestValue = futilityBase = -VALUE_INFINITE;
1233 // Never assume anything on values stored in TT
1234 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1235 ss->staticEval = bestValue = evaluate(pos);
1237 // Can ttValue be used as a better position evaluation?
1238 if (ttValue != VALUE_NONE)
1239 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1240 bestValue = ttValue;
1243 ss->staticEval = bestValue =
1244 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
1246 // Stand pat. Return immediately if static value is at least beta
1247 if (bestValue >= beta)
1250 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1251 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1256 if (PvNode && bestValue > alpha)
1259 futilityBase = bestValue + 128;
1262 // Initialize a MovePicker object for the current position, and prepare
1263 // to search the moves. Because the depth is <= 0 here, only captures,
1264 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1266 MovePicker mp(pos, ttMove, depth, History, CounterMovesHistory, to_sq((ss-1)->currentMove));
1269 // Loop through the moves until no moves remain or a beta cutoff occurs
1270 while ((move = mp.next_move<false>()) != MOVE_NONE)
1272 assert(is_ok(move));
1274 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1275 ? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1276 : pos.gives_check(move, ci);
1281 && futilityBase > -VALUE_KNOWN_WIN
1282 && !pos.advanced_pawn_push(move))
1284 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1286 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1288 if (futilityValue <= alpha)
1290 bestValue = std::max(bestValue, futilityValue);
1294 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1296 bestValue = std::max(bestValue, futilityBase);
1301 // Detect non-capture evasions that are candidates to be pruned
1302 evasionPrunable = InCheck
1303 && bestValue > VALUE_MATED_IN_MAX_PLY
1304 && !pos.capture(move)
1305 && !pos.can_castle(pos.side_to_move());
1307 // Don't search moves with negative SEE values
1308 if ( (!InCheck || evasionPrunable)
1309 && type_of(move) != PROMOTION
1310 && pos.see_sign(move) < VALUE_ZERO)
1313 // Speculative prefetch as early as possible
1314 prefetch(TT.first_entry(pos.key_after(move)));
1316 // Check for legality just before making the move
1317 if (!pos.legal(move, ci.pinned))
1320 ss->currentMove = move;
1322 // Make and search the move
1323 pos.do_move(move, st, givesCheck);
1324 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1325 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1326 pos.undo_move(move);
1328 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1330 // Check for new best move
1331 if (value > bestValue)
1337 if (PvNode) // Update pv even in fail-high case
1338 update_pv(ss->pv, move, (ss+1)->pv);
1340 if (PvNode && value < beta) // Update alpha here! Always alpha < beta
1347 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1348 ttDepth, move, ss->staticEval, TT.generation());
1356 // All legal moves have been searched. A special case: If we're in check
1357 // and no legal moves were found, it is checkmate.
1358 if (InCheck && bestValue == -VALUE_INFINITE)
1359 return mated_in(ss->ply); // Plies to mate from the root
1361 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1362 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1363 ttDepth, bestMove, ss->staticEval, TT.generation());
1365 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1371 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1372 // "plies to mate from the current position". Non-mate scores are unchanged.
1373 // The function is called before storing a value in the transposition table.
1375 Value value_to_tt(Value v, int ply) {
1377 assert(v != VALUE_NONE);
1379 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1380 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1384 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1385 // from the transposition table (which refers to the plies to mate/be mated
1386 // from current position) to "plies to mate/be mated from the root".
1388 Value value_from_tt(Value v, int ply) {
1390 return v == VALUE_NONE ? VALUE_NONE
1391 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1392 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1396 // update_pv() adds current move and appends child pv[]
1398 void update_pv(Move* pv, Move move, Move* childPv) {
1400 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1405 // update_stats() updates killers, history, countermoves and followupmoves
1406 // stats after a fail-high of a quiet move.
1408 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt) {
1410 if (ss->killers[0] != move)
1412 ss->killers[1] = ss->killers[0];
1413 ss->killers[0] = move;
1416 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY));
1418 Square prevSq = to_sq((ss-1)->currentMove);
1419 HistoryStats& cmh = CounterMovesHistory[pos.piece_on(prevSq)][prevSq];
1421 History.update(pos.moved_piece(move), to_sq(move), bonus);
1423 if (is_ok((ss-1)->currentMove))
1425 Countermoves.update(pos.piece_on(prevSq), prevSq, move);
1426 cmh.update(pos.moved_piece(move), to_sq(move), bonus);
1429 // Decrease all the other played quiet moves
1430 for (int i = 0; i < quietsCnt; ++i)
1432 History.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1434 if (is_ok((ss-1)->currentMove))
1435 cmh.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1438 if (is_ok((ss-2)->currentMove) && (ss-1)->currentMove == (ss-1)->ttMove)
1440 Square prevPrevSq = to_sq((ss-2)->currentMove);
1441 Followupmoves.update(pos.piece_on(prevPrevSq), prevPrevSq, move);
1446 // When playing with strength handicap, choose best move among a set of RootMoves
1447 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1449 Move Skill::pick_best(size_t multiPV) {
1451 // PRNG sequence should be non-deterministic, so we seed it with the time at init
1452 static PRNG rng(now());
1454 // RootMoves are already sorted by score in descending order
1455 int variance = std::min(RootMoves[0].score - RootMoves[multiPV - 1].score, PawnValueMg);
1456 int weakness = 120 - 2 * level;
1457 int maxScore = -VALUE_INFINITE;
1459 // Choose best move. For each move score we add two terms both dependent on
1460 // weakness. One deterministic and bigger for weaker levels, and one random,
1461 // then we choose the move with the resulting highest score.
1462 for (size_t i = 0; i < multiPV; ++i)
1464 // This is our magic formula
1465 int push = ( weakness * int(RootMoves[0].score - RootMoves[i].score)
1466 + variance * (rng.rand<unsigned>() % weakness)) / 128;
1468 if (RootMoves[i].score + push > maxScore)
1470 maxScore = RootMoves[i].score + push;
1471 best = RootMoves[i].pv[0];
1480 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1481 /// that all (if any) unsearched PV lines are sent using a previous search score.
1483 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1485 std::stringstream ss;
1486 TimePoint elapsed = now() - SearchTime + 1;
1487 size_t multiPV = std::min((size_t)Options["MultiPV"], RootMoves.size());
1490 for (Thread* th : Threads)
1491 if (th->maxPly > selDepth)
1492 selDepth = th->maxPly;
1494 for (size_t i = 0; i < multiPV; ++i)
1496 bool updated = (i <= PVIdx);
1498 if (depth == ONE_PLY && !updated)
1501 Depth d = updated ? depth : depth - ONE_PLY;
1502 Value v = updated ? RootMoves[i].score : RootMoves[i].previousScore;
1504 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1505 v = tb ? TB::Score : v;
1507 if (ss.rdbuf()->in_avail()) // Not at first line
1511 << " depth " << d / ONE_PLY
1512 << " seldepth " << selDepth
1513 << " multipv " << i + 1
1514 << " score " << UCI::value(v);
1516 if (!tb && i == PVIdx)
1517 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1519 ss << " nodes " << pos.nodes_searched()
1520 << " nps " << pos.nodes_searched() * 1000 / elapsed;
1522 if (elapsed > 1000) // Earlier makes little sense
1523 ss << " hashfull " << TT.hashfull();
1525 ss << " tbhits " << TB::Hits
1526 << " time " << elapsed
1529 for (Move m : RootMoves[i].pv)
1530 ss << " " << UCI::move(m, pos.is_chess960());
1537 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1538 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1539 /// first, even if the old TT entries have been overwritten.
1541 void RootMove::insert_pv_in_tt(Position& pos) {
1543 StateInfo state[MAX_PLY], *st = state;
1548 assert(MoveList<LEGAL>(pos).contains(m));
1550 TTEntry* tte = TT.probe(pos.key(), ttHit);
1552 if (!ttHit || tte->move() != m) // Don't overwrite correct entries
1553 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, m, VALUE_NONE, TT.generation());
1555 pos.do_move(m, *st++, pos.gives_check(m, CheckInfo(pos)));
1558 for (size_t i = pv.size(); i > 0; )
1559 pos.undo_move(pv[--i]);
1563 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move before
1564 /// exiting the search, for instance in case we stop the search during a fail high at
1565 /// root. We try hard to have a ponder move to return to the GUI, otherwise in case of
1566 /// 'ponder on' we have nothing to think on.
1568 bool RootMove::extract_ponder_from_tt(Position& pos)
1573 assert(pv.size() == 1);
1575 pos.do_move(pv[0], st, pos.gives_check(pv[0], CheckInfo(pos)));
1576 TTEntry* tte = TT.probe(pos.key(), ttHit);
1577 pos.undo_move(pv[0]);
1581 Move m = tte->move(); // Local copy to be SMP safe
1582 if (MoveList<LEGAL>(pos).contains(m))
1583 return pv.push_back(m), true;
1590 /// Thread::idle_loop() is where the thread is parked when it has no work to do
1592 void Thread::idle_loop() {
1594 // Pointer 'this_sp' is not null only if we are called from split(), and not
1595 // at the thread creation. This means we are the split point's master.
1596 SplitPoint* this_sp = activeSplitPoint;
1598 assert(!this_sp || (this_sp->master == this && searching));
1600 while (!exit && !(this_sp && this_sp->slavesMask.none()))
1602 // If this thread has been assigned work, launch a search
1607 assert(activeSplitPoint);
1608 SplitPoint* sp = activeSplitPoint;
1612 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
1613 Position pos(*sp->pos, this);
1615 std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
1616 ss->splitPoint = sp;
1618 sp->spinlock.acquire();
1620 assert(activePosition == nullptr);
1622 activePosition = &pos;
1624 if (sp->nodeType == NonPV)
1625 search<NonPV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1627 else if (sp->nodeType == PV)
1628 search<PV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1630 else if (sp->nodeType == Root)
1631 search<Root, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
1639 activePosition = nullptr;
1640 sp->slavesMask.reset(idx);
1641 sp->allSlavesSearching = false;
1642 sp->nodes += pos.nodes_searched();
1644 // After releasing the lock we can't access any SplitPoint related data
1645 // in a safe way because it could have been released under our feet by
1647 sp->spinlock.release();
1649 // Try to late join to another split point if none of its slaves has
1650 // already finished.
1651 SplitPoint* bestSp = NULL;
1652 int minLevel = INT_MAX;
1654 for (Thread* th : Threads)
1656 const size_t size = th->splitPointsSize; // Local copy
1657 sp = size ? &th->splitPoints[size - 1] : nullptr;
1660 && sp->allSlavesSearching
1661 && sp->slavesMask.count() < MAX_SLAVES_PER_SPLITPOINT
1665 assert(!(this_sp && this_sp->slavesMask.none()));
1666 assert(Threads.size() > 2);
1668 // Prefer to join to SP with few parents to reduce the probability
1669 // that a cut-off occurs above us, and hence we waste our work.
1671 for (SplitPoint* p = th->activeSplitPoint; p; p = p->parentSplitPoint)
1674 if (level < minLevel)
1686 // Recheck the conditions under lock protection
1687 sp->spinlock.acquire();
1689 if ( sp->allSlavesSearching
1690 && sp->slavesMask.count() < MAX_SLAVES_PER_SPLITPOINT)
1696 sp->slavesMask.set(idx);
1697 activeSplitPoint = sp;
1704 sp->spinlock.release();
1708 // If search is finished then sleep, otherwise just yield
1709 if (!Threads.main()->thinking)
1713 std::unique_lock<Mutex> lk(mutex);
1714 while (!exit && !Threads.main()->thinking)
1715 sleepCondition.wait(lk);
1718 std::this_thread::yield(); // Wait for a new job or for our slaves to finish
1723 /// check_time() is called by the timer thread when the timer triggers. It is
1724 /// used to print debug info and, more importantly, to detect when we are out of
1725 /// available time and thus stop the search.
1729 static TimePoint lastInfoTime = now();
1730 TimePoint elapsed = now() - SearchTime;
1732 if (now() - lastInfoTime >= 1000)
1734 lastInfoTime = now();
1738 // An engine may not stop pondering until told so by the GUI
1742 if (Limits.use_time_management())
1744 bool stillAtFirstMove = Signals.firstRootMove
1745 && !Signals.failedLowAtRoot
1746 && elapsed > TimeMgr.available_time() * 75 / 100;
1748 if ( stillAtFirstMove
1749 || elapsed > TimeMgr.maximum_time() - 2 * TimerThread::Resolution)
1750 Signals.stop = true;
1752 else if (Limits.movetime && elapsed >= Limits.movetime)
1753 Signals.stop = true;
1755 else if (Limits.nodes)
1757 int64_t nodes = RootPos.nodes_searched();
1759 // Loop across all split points and sum accumulated SplitPoint nodes plus
1760 // all the currently active positions nodes.
1762 for (Thread* th : Threads)
1763 for (size_t i = 0; i < th->splitPointsSize; ++i)
1765 SplitPoint& sp = th->splitPoints[i];
1767 sp.spinlock.acquire();
1771 for (size_t idx = 0; idx < Threads.size(); ++idx)
1772 if (sp.slavesMask.test(idx) && Threads[idx]->activePosition)
1773 nodes += Threads[idx]->activePosition->nodes_searched();
1775 sp.spinlock.release();
1778 if (nodes >= Limits.nodes)
1779 Signals.stop = true;