2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2016 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
37 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
55 namespace TB = Tablebases;
59 using namespace Search;
63 // Different node types, used as a template parameter
64 enum NodeType { NonPV, PV };
66 // Razoring and futility margin based on depth
67 const int razor_margin[4] = { 483, 570, 603, 554 };
68 Value futility_margin(Depth d) { return Value(200 * d); }
70 // Futility and reductions lookup tables, initialized at startup
71 int FutilityMoveCounts[2][16]; // [improving][depth]
72 Depth Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
74 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
75 return Reductions[PvNode][i][std::min(d, 63 * ONE_PLY)][std::min(mn, 63)];
78 // Skill structure is used to implement strength limit
80 Skill(int l) : level(l) {}
81 bool enabled() const { return level < 20; }
82 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
83 Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
84 Move pick_best(size_t multiPV);
87 Move best = MOVE_NONE;
90 // EasyMoveManager structure is used to detect an 'easy move'. When the PV is
91 // stable across multiple search iterations, we can quickly return the best move.
92 struct EasyMoveManager {
97 pv[0] = pv[1] = pv[2] = MOVE_NONE;
100 Move get(Key key) const {
101 return expectedPosKey == key ? pv[2] : MOVE_NONE;
104 void update(Position& pos, const std::vector<Move>& newPv) {
106 assert(newPv.size() >= 3);
108 // Keep track of how many times in a row the 3rd ply remains stable
109 stableCnt = (newPv[2] == pv[2]) ? stableCnt + 1 : 0;
111 if (!std::equal(newPv.begin(), newPv.begin() + 3, pv))
113 std::copy(newPv.begin(), newPv.begin() + 3, pv);
116 pos.do_move(newPv[0], st[0], pos.gives_check(newPv[0], CheckInfo(pos)));
117 pos.do_move(newPv[1], st[1], pos.gives_check(newPv[1], CheckInfo(pos)));
118 expectedPosKey = pos.key();
119 pos.undo_move(newPv[1]);
120 pos.undo_move(newPv[0]);
129 // Set of rows with half bits set to 1 and half to 0. It is used to allocate
130 // the search depths across the threads.
131 typedef std::vector<int> Row;
133 const Row HalfDensity[] = {
146 {0, 0, 0, 0, 1, 1, 1, 1},
147 {0, 0, 0, 1, 1, 1, 1, 0},
148 {0, 0, 1, 1, 1, 1, 0 ,0},
149 {0, 1, 1, 1, 1, 0, 0 ,0},
150 {1, 1, 1, 1, 0, 0, 0 ,0},
151 {1, 1, 1, 0, 0, 0, 0 ,1},
152 {1, 1, 0, 0, 0, 0, 1 ,1},
153 {1, 0, 0, 0, 0, 1, 1 ,1},
156 const size_t HalfDensitySize = std::extent<decltype(HalfDensity)>::value;
158 EasyMoveManager EasyMove;
159 Value DrawValue[COLOR_NB];
160 CounterMoveHistoryStats CounterMoveHistory;
162 template <NodeType NT>
163 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
165 template <NodeType NT, bool InCheck>
166 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
168 Value value_to_tt(Value v, int ply);
169 Value value_from_tt(Value v, int ply);
170 void update_pv(Move* pv, Move move, Move* childPv);
171 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
177 /// Search::init() is called during startup to initialize various lookup tables
179 void Search::init() {
181 for (int imp = 0; imp <= 1; ++imp)
182 for (int d = 1; d < 64; ++d)
183 for (int mc = 1; mc < 64; ++mc)
185 double r = log(d) * log(mc) / 2;
189 Reductions[NonPV][imp][d][mc] = int(std::round(r)) * ONE_PLY;
190 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - ONE_PLY, DEPTH_ZERO);
192 // Increase reduction for non-PV nodes when eval is not improving
193 if (!imp && Reductions[NonPV][imp][d][mc] >= 2 * ONE_PLY)
194 Reductions[NonPV][imp][d][mc] += ONE_PLY;
197 for (int d = 0; d < 16; ++d)
199 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
200 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
205 /// Search::clear() resets search state to zero, to obtain reproducible results
207 void Search::clear() {
210 CounterMoveHistory.clear();
212 for (Thread* th : Threads)
215 th->counterMoves.clear();
218 Threads.main()->previousScore = VALUE_INFINITE;
222 /// Search::perft() is our utility to verify move generation. All the leaf nodes
223 /// up to the given depth are generated and counted, and the sum is returned.
225 uint64_t Search::perft(Position& pos, Depth depth) {
228 uint64_t cnt, nodes = 0;
230 const bool leaf = (depth == 2 * ONE_PLY);
232 for (const auto& m : MoveList<LEGAL>(pos))
234 if (Root && depth <= ONE_PLY)
238 pos.do_move(m, st, pos.gives_check(m, ci));
239 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
244 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
249 template uint64_t Search::perft<true>(Position&, Depth);
252 /// MainThread::search() is called by the main thread when the program receives
253 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
255 void MainThread::search() {
257 Color us = rootPos.side_to_move();
258 Time.init(Limits, us, rootPos.game_ply());
260 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
261 DrawValue[ us] = VALUE_DRAW - Value(contempt);
262 DrawValue[~us] = VALUE_DRAW + Value(contempt);
264 if (rootMoves.empty())
266 rootMoves.push_back(RootMove(MOVE_NONE));
267 sync_cout << "info depth 0 score "
268 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
273 for (Thread* th : Threads)
275 th->start_searching();
277 Thread::search(); // Let's start searching!
280 // When playing in 'nodes as time' mode, subtract the searched nodes from
281 // the available ones before exiting.
283 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
285 // When we reach the maximum depth, we can arrive here without a raise of
286 // Signals.stop. However, if we are pondering or in an infinite search,
287 // the UCI protocol states that we shouldn't print the best move before the
288 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
289 // until the GUI sends one of those commands (which also raises Signals.stop).
290 if (!Signals.stop && (Limits.ponder || Limits.infinite))
292 Signals.stopOnPonderhit = true;
296 // Stop the threads if not already stopped
299 // Wait until all threads have finished
300 for (Thread* th : Threads)
302 th->wait_for_search_finished();
304 // Check if there are threads with a better score than main thread
305 Thread* bestThread = this;
306 if ( !this->easyMovePlayed
307 && Options["MultiPV"] == 1
309 && !Skill(Options["Skill Level"]).enabled()
310 && rootMoves[0].pv[0] != MOVE_NONE)
312 for (Thread* th : Threads)
313 if ( th->completedDepth > bestThread->completedDepth
314 && th->rootMoves[0].score > bestThread->rootMoves[0].score)
318 previousScore = bestThread->rootMoves[0].score;
320 // Send new PV when needed
321 if (bestThread != this)
322 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
324 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
326 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
327 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
329 std::cout << sync_endl;
333 // Thread::search() is the main iterative deepening loop. It calls search()
334 // repeatedly with increasing depth until the allocated thinking time has been
335 // consumed, the user stops the search, or the maximum search depth is reached.
337 void Thread::search() {
339 Stack stack[MAX_PLY+7], *ss = stack+5; // To allow referencing (ss-5) and (ss+2)
340 Value bestValue, alpha, beta, delta;
341 Move easyMove = MOVE_NONE;
342 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
344 std::memset(ss-5, 0, 8 * sizeof(Stack));
346 bestValue = delta = alpha = -VALUE_INFINITE;
347 beta = VALUE_INFINITE;
348 completedDepth = DEPTH_ZERO;
352 easyMove = EasyMove.get(rootPos.key());
354 mainThread->easyMovePlayed = mainThread->failedLow = false;
355 mainThread->bestMoveChanges = 0;
359 size_t multiPV = Options["MultiPV"];
360 Skill skill(Options["Skill Level"]);
362 // When playing with strength handicap enable MultiPV search that we will
363 // use behind the scenes to retrieve a set of possible moves.
365 multiPV = std::max(multiPV, (size_t)4);
367 multiPV = std::min(multiPV, rootMoves.size());
369 // Iterative deepening loop until requested to stop or the target depth is reached.
370 while (++rootDepth < DEPTH_MAX && !Signals.stop && (!Limits.depth || Threads.main()->rootDepth <= Limits.depth))
372 // Set up the new depths for the helper threads skipping on average every
373 // 2nd ply (using a half-density matrix).
376 const Row& row = HalfDensity[(idx - 1) % HalfDensitySize];
377 if (row[(rootDepth + rootPos.game_ply()) % row.size()])
381 // Age out PV variability metric
383 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
385 // Save the last iteration's scores before first PV line is searched and
386 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
387 for (RootMove& rm : rootMoves)
388 rm.previousScore = rm.score;
390 // MultiPV loop. We perform a full root search for each PV line
391 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
393 // Reset aspiration window starting size
394 if (rootDepth >= 5 * ONE_PLY)
397 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
398 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
401 // Start with a small aspiration window and, in the case of a fail
402 // high/low, re-search with a bigger window until we're not failing
406 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false);
408 // Bring the best move to the front. It is critical that sorting
409 // is done with a stable algorithm because all the values but the
410 // first and eventually the new best one are set to -VALUE_INFINITE
411 // and we want to keep the same order for all the moves except the
412 // new PV that goes to the front. Note that in case of MultiPV
413 // search the already searched PV lines are preserved.
414 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
416 // Write PV back to the transposition table in case the relevant
417 // entries have been overwritten during the search.
418 for (size_t i = 0; i <= PVIdx; ++i)
419 rootMoves[i].insert_pv_in_tt(rootPos);
421 // If search has been stopped, break immediately. Sorting and
422 // writing PV back to TT is safe because RootMoves is still
423 // valid, although it refers to the previous iteration.
427 // When failing high/low give some update (without cluttering
428 // the UI) before a re-search.
431 && (bestValue <= alpha || bestValue >= beta)
432 && Time.elapsed() > 3000)
433 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
435 // In case of failing low/high increase aspiration window and
436 // re-search, otherwise exit the loop.
437 if (bestValue <= alpha)
439 beta = (alpha + beta) / 2;
440 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
444 mainThread->failedLow = true;
445 Signals.stopOnPonderhit = false;
448 else if (bestValue >= beta)
450 alpha = (alpha + beta) / 2;
451 beta = std::min(bestValue + delta, VALUE_INFINITE);
456 delta += delta / 4 + 5;
458 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
461 // Sort the PV lines searched so far and update the GUI
462 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
468 sync_cout << "info nodes " << Threads.nodes_searched()
469 << " time " << Time.elapsed() << sync_endl;
471 else if (PVIdx + 1 == multiPV || Time.elapsed() > 3000)
472 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
476 completedDepth = rootDepth;
481 // If skill level is enabled and time is up, pick a sub-optimal best move
482 if (skill.enabled() && skill.time_to_pick(rootDepth))
483 skill.pick_best(multiPV);
485 // Have we found a "mate in x"?
487 && bestValue >= VALUE_MATE_IN_MAX_PLY
488 && VALUE_MATE - bestValue <= 2 * Limits.mate)
491 // Do we have time for the next iteration? Can we stop searching now?
492 if (Limits.use_time_management())
494 if (!Signals.stop && !Signals.stopOnPonderhit)
496 // Stop the search if only one legal move is available, or if all
497 // of the available time has been used, or if we matched an easyMove
498 // from the previous search and just did a fast verification.
499 const int F[] = { mainThread->failedLow,
500 bestValue - mainThread->previousScore };
502 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
503 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
505 bool doEasyMove = rootMoves[0].pv[0] == easyMove
506 && mainThread->bestMoveChanges < 0.03
507 && Time.elapsed() > Time.optimum() * 5 / 42;
509 if ( rootMoves.size() == 1
510 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628
511 || (mainThread->easyMovePlayed = doEasyMove))
513 // If we are allowed to ponder do not stop the search now but
514 // keep pondering until the GUI sends "ponderhit" or "stop".
516 Signals.stopOnPonderhit = true;
522 if (rootMoves[0].pv.size() >= 3)
523 EasyMove.update(rootPos, rootMoves[0].pv);
532 // Clear any candidate easy move that wasn't stable for the last search
533 // iterations; the second condition prevents consecutive fast moves.
534 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
537 // If skill level is enabled, swap best PV line with the sub-optimal one
539 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
540 rootMoves.end(), skill.best_move(multiPV)));
546 // search<>() is the main search function for both PV and non-PV nodes
548 template <NodeType NT>
549 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
551 const bool PvNode = NT == PV;
552 const bool rootNode = PvNode && (ss-1)->ply == 0;
554 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
555 assert(PvNode || (alpha == beta - 1));
556 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
558 Move pv[MAX_PLY+1], quietsSearched[64];
562 Move ttMove, move, excludedMove, bestMove;
563 Depth extension, newDepth, predictedDepth;
564 Value bestValue, value, ttValue, eval, nullValue;
565 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
566 bool captureOrPromotion, doFullDepthSearch, moveCountPruning;
568 int moveCount, quietCount;
570 // Step 1. Initialize node
571 Thread* thisThread = pos.this_thread();
572 inCheck = pos.checkers();
573 moveCount = quietCount = ss->moveCount = 0;
574 bestValue = -VALUE_INFINITE;
575 ss->ply = (ss-1)->ply + 1;
577 // Check for the available remaining time
578 if (thisThread->resetCalls.load(std::memory_order_relaxed))
580 thisThread->resetCalls = false;
581 thisThread->callsCnt = 0;
583 if (++thisThread->callsCnt > 4096)
585 for (Thread* th : Threads)
586 th->resetCalls = true;
591 // Used to send selDepth info to GUI
592 if (PvNode && thisThread->maxPly < ss->ply)
593 thisThread->maxPly = ss->ply;
597 // Step 2. Check for aborted search and immediate draw
598 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw() || ss->ply >= MAX_PLY)
599 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
600 : DrawValue[pos.side_to_move()];
602 // Step 3. Mate distance pruning. Even if we mate at the next move our score
603 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
604 // a shorter mate was found upward in the tree then there is no need to search
605 // because we will never beat the current alpha. Same logic but with reversed
606 // signs applies also in the opposite condition of being mated instead of giving
607 // mate. In this case return a fail-high score.
608 alpha = std::max(mated_in(ss->ply), alpha);
609 beta = std::min(mate_in(ss->ply+1), beta);
614 assert(0 <= ss->ply && ss->ply < MAX_PLY);
616 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
617 ss->counterMoves = nullptr;
618 (ss+1)->skipEarlyPruning = false;
619 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
621 // Step 4. Transposition table lookup. We don't want the score of a partial
622 // search to overwrite a previous full search TT value, so we use a different
623 // position key in case of an excluded move.
624 excludedMove = ss->excludedMove;
625 posKey = excludedMove ? pos.exclusion_key() : pos.key();
626 tte = TT.probe(posKey, ttHit);
627 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
628 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
629 : ttHit ? tte->move() : MOVE_NONE;
631 // At non-PV nodes we check for an early TT cutoff
634 && tte->depth() >= depth
635 && ttValue != VALUE_NONE // Possible in case of TT access race
636 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
637 : (tte->bound() & BOUND_UPPER)))
639 ss->currentMove = ttMove; // Can be MOVE_NONE
641 // If ttMove is quiet, update killers, history, counter move on TT hit
642 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove))
643 update_stats(pos, ss, ttMove, depth, nullptr, 0);
648 // Step 4a. Tablebase probe
649 if (!rootNode && TB::Cardinality)
651 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
653 if ( piecesCnt <= TB::Cardinality
654 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
655 && pos.rule50_count() == 0
656 && !pos.can_castle(ANY_CASTLING))
658 int found, v = Tablebases::probe_wdl(pos, &found);
664 int drawScore = TB::UseRule50 ? 1 : 0;
666 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
667 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
668 : VALUE_DRAW + 2 * v * drawScore;
670 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
671 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
672 MOVE_NONE, VALUE_NONE, TT.generation());
679 // Step 5. Evaluate the position statically
682 ss->staticEval = eval = VALUE_NONE;
688 // Never assume anything on values stored in TT
689 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
690 eval = ss->staticEval = evaluate(pos);
692 // Can ttValue be used as a better position evaluation?
693 if (ttValue != VALUE_NONE)
694 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
699 eval = ss->staticEval =
700 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
701 : -(ss-1)->staticEval + 2 * Eval::Tempo;
703 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
704 ss->staticEval, TT.generation());
707 if (ss->skipEarlyPruning)
710 // Step 6. Razoring (skipped when in check)
712 && depth < 4 * ONE_PLY
713 && eval + razor_margin[depth] <= alpha
714 && ttMove == MOVE_NONE)
716 if ( depth <= ONE_PLY
717 && eval + razor_margin[3 * ONE_PLY] <= alpha)
718 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
720 Value ralpha = alpha - razor_margin[depth];
721 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
726 // Step 7. Futility pruning: child node (skipped when in check)
728 && depth < 7 * ONE_PLY
729 && eval - futility_margin(depth) >= beta
730 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
731 && pos.non_pawn_material(pos.side_to_move()))
732 return eval - futility_margin(depth);
734 // Step 8. Null move search with verification search (is omitted in PV nodes)
736 && depth >= 2 * ONE_PLY
738 && pos.non_pawn_material(pos.side_to_move()))
740 ss->currentMove = MOVE_NULL;
741 ss->counterMoves = nullptr;
743 assert(eval - beta >= 0);
745 // Null move dynamic reduction based on depth and value
746 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
748 pos.do_null_move(st);
749 (ss+1)->skipEarlyPruning = true;
750 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
751 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
752 (ss+1)->skipEarlyPruning = false;
753 pos.undo_null_move();
755 if (nullValue >= beta)
757 // Do not return unproven mate scores
758 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
761 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
764 // Do verification search at high depths
765 ss->skipEarlyPruning = true;
766 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
767 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
768 ss->skipEarlyPruning = false;
775 // Step 9. ProbCut (skipped when in check)
776 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
777 // and a reduced search returns a value much above beta, we can (almost)
778 // safely prune the previous move.
780 && depth >= 5 * ONE_PLY
781 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
783 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
784 Depth rdepth = depth - 4 * ONE_PLY;
786 assert(rdepth >= ONE_PLY);
787 assert((ss-1)->currentMove != MOVE_NONE);
788 assert((ss-1)->currentMove != MOVE_NULL);
790 MovePicker mp(pos, ttMove, PieceValue[MG][pos.captured_piece_type()]);
793 while ((move = mp.next_move()) != MOVE_NONE)
794 if (pos.legal(move, ci.pinned))
796 ss->currentMove = move;
797 ss->counterMoves = &CounterMoveHistory[pos.moved_piece(move)][to_sq(move)];
798 pos.do_move(move, st, pos.gives_check(move, ci));
799 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
806 // Step 10. Internal iterative deepening (skipped when in check)
807 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
809 && (PvNode || ss->staticEval + 256 >= beta))
811 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
812 ss->skipEarlyPruning = true;
813 search<NT>(pos, ss, alpha, beta, d, true);
814 ss->skipEarlyPruning = false;
816 tte = TT.probe(posKey, ttHit);
817 ttMove = ttHit ? tte->move() : MOVE_NONE;
820 moves_loop: // When in check search starts from here
822 Square prevSq = to_sq((ss-1)->currentMove);
823 const CounterMoveStats* cmh = (ss-1)->counterMoves;
824 const CounterMoveStats* fmh = (ss-2)->counterMoves;
825 const CounterMoveStats* fmh2 = (ss-4)->counterMoves;
827 MovePicker mp(pos, ttMove, depth, ss);
829 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
830 improving = ss->staticEval >= (ss-2)->staticEval
831 || ss->staticEval == VALUE_NONE
832 ||(ss-2)->staticEval == VALUE_NONE;
834 singularExtensionNode = !rootNode
835 && depth >= 8 * ONE_PLY
836 && ttMove != MOVE_NONE
837 /* && ttValue != VALUE_NONE Already implicit in the next condition */
838 && abs(ttValue) < VALUE_KNOWN_WIN
839 && !excludedMove // Recursive singular search is not allowed
840 && (tte->bound() & BOUND_LOWER)
841 && tte->depth() >= depth - 3 * ONE_PLY;
843 // Step 11. Loop through moves
844 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
845 while ((move = mp.next_move()) != MOVE_NONE)
849 if (move == excludedMove)
852 // At root obey the "searchmoves" option and skip moves not listed in Root
853 // Move List. As a consequence any illegal move is also skipped. In MultiPV
854 // mode we also skip PV moves which have been already searched.
855 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
856 thisThread->rootMoves.end(), move))
859 ss->moveCount = ++moveCount;
861 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
862 sync_cout << "info depth " << depth / ONE_PLY
863 << " currmove " << UCI::move(move, pos.is_chess960())
864 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
867 (ss+1)->pv = nullptr;
869 extension = DEPTH_ZERO;
870 captureOrPromotion = pos.capture_or_promotion(move);
871 moved_piece = pos.moved_piece(move);
873 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
874 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
875 : pos.gives_check(move, ci);
877 moveCountPruning = depth < 16 * ONE_PLY
878 && moveCount >= FutilityMoveCounts[improving][depth];
880 // Step 12. Extend checks
883 || (!moveCountPruning && pos.see_sign(move) >= VALUE_ZERO)))
886 // Singular extension search. If all moves but one fail low on a search of
887 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
888 // is singular and should be extended. To verify this we do a reduced search
889 // on all the other moves but the ttMove and if the result is lower than
890 // ttValue minus a margin then we extend the ttMove.
891 if ( singularExtensionNode
894 && pos.legal(move, ci.pinned))
896 Value rBeta = ttValue - 2 * depth / ONE_PLY;
897 ss->excludedMove = move;
898 ss->skipEarlyPruning = true;
899 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
900 ss->skipEarlyPruning = false;
901 ss->excludedMove = MOVE_NONE;
907 // Update the current move (this must be done after singular extension search)
908 newDepth = depth - ONE_PLY + extension;
910 // Step 13. Pruning at shallow depth
912 && !captureOrPromotion
915 && !pos.advanced_pawn_push(move)
916 && bestValue > VALUE_MATED_IN_MAX_PLY)
918 // Move count based pruning
919 if (moveCountPruning)
922 // Countermoves based pruning
923 if ( depth <= 4 * ONE_PLY
924 && move != ss->killers[0]
925 && (!cmh || (*cmh )[moved_piece][to_sq(move)] < VALUE_ZERO)
926 && (!fmh || (*fmh )[moved_piece][to_sq(move)] < VALUE_ZERO)
927 && (!fmh2 || (*fmh2)[moved_piece][to_sq(move)] < VALUE_ZERO || (cmh && fmh)))
930 predictedDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO);
932 // Futility pruning: parent node
933 if ( predictedDepth < 7 * ONE_PLY
934 && ss->staticEval + futility_margin(predictedDepth) + 256 <= alpha)
937 // Prune moves with negative SEE at low depths
938 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
942 // Speculative prefetch as early as possible
943 prefetch(TT.first_entry(pos.key_after(move)));
945 // Check for legality just before making the move
946 if (!rootNode && !pos.legal(move, ci.pinned))
948 ss->moveCount = --moveCount;
952 ss->currentMove = move;
953 ss->counterMoves = &CounterMoveHistory[moved_piece][to_sq(move)];
955 // Step 14. Make the move
956 pos.do_move(move, st, givesCheck);
958 // Step 15. Reduced depth search (LMR). If the move fails high it will be
959 // re-searched at full depth.
960 if ( depth >= 3 * ONE_PLY
962 && !captureOrPromotion)
964 Depth r = reduction<PvNode>(improving, depth, moveCount);
965 Value val = thisThread->history[moved_piece][to_sq(move)]
966 + (cmh ? (*cmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
967 + (fmh ? (*fmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
968 + (fmh2 ? (*fmh2)[moved_piece][to_sq(move)] : VALUE_ZERO);
970 // Increase reduction for cut nodes
971 if (!PvNode && cutNode)
974 // Decrease reduction for moves that escape a capture. Filter out
975 // castling moves, because they are coded as "king captures rook" and
976 // hence break make_move(). Also use see() instead of see_sign(),
977 // because the destination square is empty.
978 else if ( type_of(move) == NORMAL
979 && type_of(pos.piece_on(to_sq(move))) != PAWN
980 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
983 // Decrease/increase reduction for moves with a good/bad history
984 int rHist = (val - 10000) / 20000;
985 r = std::max(DEPTH_ZERO, r - rHist * ONE_PLY);
987 Depth d = std::max(newDepth - r, ONE_PLY);
989 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
991 doFullDepthSearch = (value > alpha && r != DEPTH_ZERO);
994 doFullDepthSearch = !PvNode || moveCount > 1;
996 // Step 16. Full depth search when LMR is skipped or fails high
997 if (doFullDepthSearch)
998 value = newDepth < ONE_PLY ?
999 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1000 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1001 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1003 // For PV nodes only, do a full PV search on the first move or after a fail
1004 // high (in the latter case search only if value < beta), otherwise let the
1005 // parent node fail low with value <= alpha and try another move.
1006 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1009 (ss+1)->pv[0] = MOVE_NONE;
1011 value = newDepth < ONE_PLY ?
1012 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1013 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1014 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1017 // Step 17. Undo move
1018 pos.undo_move(move);
1020 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1022 // Step 18. Check for a new best move
1023 // Finished searching the move. If a stop occurred, the return value of
1024 // the search cannot be trusted, and we return immediately without
1025 // updating best move, PV and TT.
1026 if (Signals.stop.load(std::memory_order_relaxed))
1031 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1032 thisThread->rootMoves.end(), move);
1034 // PV move or new best move ?
1035 if (moveCount == 1 || value > alpha)
1042 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1043 rm.pv.push_back(*m);
1045 // We record how often the best move has been changed in each
1046 // iteration. This information is used for time management: When
1047 // the best move changes frequently, we allocate some more time.
1048 if (moveCount > 1 && thisThread == Threads.main())
1049 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1052 // All other moves but the PV are set to the lowest value: this is
1053 // not a problem when sorting because the sort is stable and the
1054 // move position in the list is preserved - just the PV is pushed up.
1055 rm.score = -VALUE_INFINITE;
1058 if (value > bestValue)
1064 // If there is an easy move for this position, clear it if unstable
1066 && thisThread == Threads.main()
1067 && EasyMove.get(pos.key())
1068 && (move != EasyMove.get(pos.key()) || moveCount > 1))
1073 if (PvNode && !rootNode) // Update pv even in fail-high case
1074 update_pv(ss->pv, move, (ss+1)->pv);
1076 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1080 assert(value >= beta); // Fail high
1086 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1087 quietsSearched[quietCount++] = move;
1090 // The following condition would detect a stop only after move loop has been
1091 // completed. But in this case bestValue is valid because we have fully
1092 // searched our subtree, and we can anyhow save the result in TT.
1098 // Step 20. Check for mate and stalemate
1099 // All legal moves have been searched and if there are no legal moves, it
1100 // must be a mate or a stalemate. If we are in a singular extension search then
1101 // return a fail low score.
1103 bestValue = excludedMove ? alpha
1104 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1106 // Quiet best move: update killers, history and countermoves
1107 else if (bestMove && !pos.capture_or_promotion(bestMove))
1108 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount);
1110 // Bonus for prior countermove that caused the fail low
1111 else if ( depth >= 3 * ONE_PLY
1114 && !pos.captured_piece_type()
1115 && is_ok((ss-1)->currentMove))
1117 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + 2 * depth / ONE_PLY - 2);
1118 if ((ss-2)->counterMoves)
1119 (ss-2)->counterMoves->update(pos.piece_on(prevSq), prevSq, bonus);
1121 if ((ss-3)->counterMoves)
1122 (ss-3)->counterMoves->update(pos.piece_on(prevSq), prevSq, bonus);
1124 if ((ss-5)->counterMoves)
1125 (ss-5)->counterMoves->update(pos.piece_on(prevSq), prevSq, bonus);
1128 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1129 bestValue >= beta ? BOUND_LOWER :
1130 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1131 depth, bestMove, ss->staticEval, TT.generation());
1133 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1139 // qsearch() is the quiescence search function, which is called by the main
1140 // search function when the remaining depth is zero (or, to be more precise,
1141 // less than ONE_PLY).
1143 template <NodeType NT, bool InCheck>
1144 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1146 const bool PvNode = NT == PV;
1148 assert(InCheck == !!pos.checkers());
1149 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1150 assert(PvNode || (alpha == beta - 1));
1151 assert(depth <= DEPTH_ZERO);
1157 Move ttMove, move, bestMove;
1158 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1159 bool ttHit, givesCheck, evasionPrunable;
1164 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1166 ss->pv[0] = MOVE_NONE;
1169 ss->currentMove = bestMove = MOVE_NONE;
1170 ss->ply = (ss-1)->ply + 1;
1172 // Check for an instant draw or if the maximum ply has been reached
1173 if (pos.is_draw() || ss->ply >= MAX_PLY)
1174 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1175 : DrawValue[pos.side_to_move()];
1177 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1179 // Decide whether or not to include checks: this fixes also the type of
1180 // TT entry depth that we are going to use. Note that in qsearch we use
1181 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1182 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1183 : DEPTH_QS_NO_CHECKS;
1185 // Transposition table lookup
1187 tte = TT.probe(posKey, ttHit);
1188 ttMove = ttHit ? tte->move() : MOVE_NONE;
1189 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1193 && tte->depth() >= ttDepth
1194 && ttValue != VALUE_NONE // Only in case of TT access race
1195 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1196 : (tte->bound() & BOUND_UPPER)))
1198 ss->currentMove = ttMove; // Can be MOVE_NONE
1202 // Evaluate the position statically
1205 ss->staticEval = VALUE_NONE;
1206 bestValue = futilityBase = -VALUE_INFINITE;
1212 // Never assume anything on values stored in TT
1213 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1214 ss->staticEval = bestValue = evaluate(pos);
1216 // Can ttValue be used as a better position evaluation?
1217 if (ttValue != VALUE_NONE)
1218 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1219 bestValue = ttValue;
1222 ss->staticEval = bestValue =
1223 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1224 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1226 // Stand pat. Return immediately if static value is at least beta
1227 if (bestValue >= beta)
1230 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1231 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1236 if (PvNode && bestValue > alpha)
1239 futilityBase = bestValue + 128;
1242 // Initialize a MovePicker object for the current position, and prepare
1243 // to search the moves. Because the depth is <= 0 here, only captures,
1244 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1246 MovePicker mp(pos, ttMove, depth, to_sq((ss-1)->currentMove));
1249 // Loop through the moves until no moves remain or a beta cutoff occurs
1250 while ((move = mp.next_move()) != MOVE_NONE)
1252 assert(is_ok(move));
1254 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1255 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1256 : pos.gives_check(move, ci);
1261 && futilityBase > -VALUE_KNOWN_WIN
1262 && !pos.advanced_pawn_push(move))
1264 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1266 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1268 if (futilityValue <= alpha)
1270 bestValue = std::max(bestValue, futilityValue);
1274 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1276 bestValue = std::max(bestValue, futilityBase);
1281 // Detect non-capture evasions that are candidates to be pruned
1282 evasionPrunable = InCheck
1283 && bestValue > VALUE_MATED_IN_MAX_PLY
1284 && !pos.capture(move);
1286 // Don't search moves with negative SEE values
1287 if ( (!InCheck || evasionPrunable)
1288 && type_of(move) != PROMOTION
1289 && pos.see_sign(move) < VALUE_ZERO)
1292 // Speculative prefetch as early as possible
1293 prefetch(TT.first_entry(pos.key_after(move)));
1295 // Check for legality just before making the move
1296 if (!pos.legal(move, ci.pinned))
1299 ss->currentMove = move;
1301 // Make and search the move
1302 pos.do_move(move, st, givesCheck);
1303 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1304 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1305 pos.undo_move(move);
1307 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1309 // Check for a new best move
1310 if (value > bestValue)
1316 if (PvNode) // Update pv even in fail-high case
1317 update_pv(ss->pv, move, (ss+1)->pv);
1319 if (PvNode && value < beta) // Update alpha here!
1326 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1327 ttDepth, move, ss->staticEval, TT.generation());
1335 // All legal moves have been searched. A special case: If we're in check
1336 // and no legal moves were found, it is checkmate.
1337 if (InCheck && bestValue == -VALUE_INFINITE)
1338 return mated_in(ss->ply); // Plies to mate from the root
1340 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1341 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1342 ttDepth, bestMove, ss->staticEval, TT.generation());
1344 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1350 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1351 // "plies to mate from the current position". Non-mate scores are unchanged.
1352 // The function is called before storing a value in the transposition table.
1354 Value value_to_tt(Value v, int ply) {
1356 assert(v != VALUE_NONE);
1358 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1359 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1363 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1364 // from the transposition table (which refers to the plies to mate/be mated
1365 // from current position) to "plies to mate/be mated from the root".
1367 Value value_from_tt(Value v, int ply) {
1369 return v == VALUE_NONE ? VALUE_NONE
1370 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1371 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1375 // update_pv() adds current move and appends child pv[]
1377 void update_pv(Move* pv, Move move, Move* childPv) {
1379 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1385 // update_stats() updates killers, history, countermove and countermove plus
1386 // follow-up move history when a new quiet best move is found.
1388 void update_stats(const Position& pos, Stack* ss, Move move,
1389 Depth depth, Move* quiets, int quietsCnt) {
1391 if (ss->killers[0] != move)
1393 ss->killers[1] = ss->killers[0];
1394 ss->killers[0] = move;
1397 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + 2 * depth / ONE_PLY - 2);
1399 Square prevSq = to_sq((ss-1)->currentMove);
1400 CounterMoveStats* cmh = (ss-1)->counterMoves;
1401 CounterMoveStats* fmh = (ss-2)->counterMoves;
1402 CounterMoveStats* fmh2 = (ss-4)->counterMoves;
1403 Thread* thisThread = pos.this_thread();
1405 thisThread->history.update(pos.moved_piece(move), to_sq(move), bonus);
1409 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1410 cmh->update(pos.moved_piece(move), to_sq(move), bonus);
1414 fmh->update(pos.moved_piece(move), to_sq(move), bonus);
1417 fmh2->update(pos.moved_piece(move), to_sq(move), bonus);
1419 // Decrease all the other played quiet moves
1420 for (int i = 0; i < quietsCnt; ++i)
1422 thisThread->history.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1425 cmh->update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1428 fmh->update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1431 fmh2->update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1434 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1435 if ((ss-1)->moveCount == 1 && !pos.captured_piece_type())
1437 if ((ss-2)->counterMoves)
1438 (ss-2)->counterMoves->update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY - 1);
1440 if ((ss-3)->counterMoves)
1441 (ss-3)->counterMoves->update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY - 1);
1443 if ((ss-5)->counterMoves)
1444 (ss-5)->counterMoves->update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY - 1);
1449 // When playing with strength handicap, choose best move among a set of RootMoves
1450 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1452 Move Skill::pick_best(size_t multiPV) {
1454 const RootMoves& rootMoves = Threads.main()->rootMoves;
1455 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1457 // RootMoves are already sorted by score in descending order
1458 Value topScore = rootMoves[0].score;
1459 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1460 int weakness = 120 - 2 * level;
1461 int maxScore = -VALUE_INFINITE;
1463 // Choose best move. For each move score we add two terms, both dependent on
1464 // weakness. One is deterministic and bigger for weaker levels, and one is
1465 // random. Then we choose the move with the resulting highest score.
1466 for (size_t i = 0; i < multiPV; ++i)
1468 // This is our magic formula
1469 int push = ( weakness * int(topScore - rootMoves[i].score)
1470 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1472 if (rootMoves[i].score + push > maxScore)
1474 maxScore = rootMoves[i].score + push;
1475 best = rootMoves[i].pv[0];
1483 // check_time() is used to print debug info and, more importantly, to detect
1484 // when we are out of available time and thus stop the search.
1488 static TimePoint lastInfoTime = now();
1490 int elapsed = Time.elapsed();
1491 TimePoint tick = Limits.startTime + elapsed;
1493 if (tick - lastInfoTime >= 1000)
1495 lastInfoTime = tick;
1499 // An engine may not stop pondering until told so by the GUI
1503 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1504 || (Limits.movetime && elapsed >= Limits.movetime)
1505 || (Limits.nodes && Threads.nodes_searched() >= Limits.nodes))
1506 Signals.stop = true;
1512 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1513 /// that all (if any) unsearched PV lines are sent using a previous search score.
1515 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1517 std::stringstream ss;
1518 int elapsed = Time.elapsed() + 1;
1519 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1520 size_t PVIdx = pos.this_thread()->PVIdx;
1521 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1522 uint64_t nodes_searched = Threads.nodes_searched();
1524 for (size_t i = 0; i < multiPV; ++i)
1526 bool updated = (i <= PVIdx);
1528 if (depth == ONE_PLY && !updated)
1531 Depth d = updated ? depth : depth - ONE_PLY;
1532 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1534 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1535 v = tb ? TB::Score : v;
1537 if (ss.rdbuf()->in_avail()) // Not at first line
1541 << " depth " << d / ONE_PLY
1542 << " seldepth " << pos.this_thread()->maxPly
1543 << " multipv " << i + 1
1544 << " score " << UCI::value(v);
1546 if (!tb && i == PVIdx)
1547 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1549 ss << " nodes " << nodes_searched
1550 << " nps " << nodes_searched * 1000 / elapsed;
1552 if (elapsed > 1000) // Earlier makes little sense
1553 ss << " hashfull " << TT.hashfull();
1555 ss << " tbhits " << TB::Hits
1556 << " time " << elapsed
1559 for (Move m : rootMoves[i].pv)
1560 ss << " " << UCI::move(m, pos.is_chess960());
1567 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1568 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1569 /// first, even if the old TT entries have been overwritten.
1571 void RootMove::insert_pv_in_tt(Position& pos) {
1573 StateInfo state[MAX_PLY], *st = state;
1578 assert(MoveList<LEGAL>(pos).contains(m));
1580 TTEntry* tte = TT.probe(pos.key(), ttHit);
1582 if (!ttHit || tte->move() != m) // Don't overwrite correct entries
1583 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE,
1584 m, VALUE_NONE, TT.generation());
1586 pos.do_move(m, *st++, pos.gives_check(m, CheckInfo(pos)));
1589 for (size_t i = pv.size(); i > 0; )
1590 pos.undo_move(pv[--i]);
1594 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1595 /// before exiting the search, for instance, in case we stop the search during a
1596 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1597 /// otherwise in case of 'ponder on' we have nothing to think on.
1599 bool RootMove::extract_ponder_from_tt(Position& pos)
1604 assert(pv.size() == 1);
1606 pos.do_move(pv[0], st, pos.gives_check(pv[0], CheckInfo(pos)));
1607 TTEntry* tte = TT.probe(pos.key(), ttHit);
1608 pos.undo_move(pv[0]);
1612 Move m = tte->move(); // Local copy to be SMP safe
1613 if (MoveList<LEGAL>(pos).contains(m))
1614 return pv.push_back(m), true;
1620 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1624 UseRule50 = Options["Syzygy50MoveRule"];
1625 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1626 Cardinality = Options["SyzygyProbeLimit"];
1628 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1629 if (Cardinality > MaxCardinality)
1631 Cardinality = MaxCardinality;
1632 ProbeDepth = DEPTH_ZERO;
1635 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1638 // If the current root position is in the tablebases, then RootMoves
1639 // contains only moves that preserve the draw or the win.
1640 RootInTB = root_probe(pos, rootMoves, TB::Score);
1643 Cardinality = 0; // Do not probe tablebases during the search
1645 else // If DTZ tables are missing, use WDL tables as a fallback
1647 // Filter out moves that do not preserve the draw or the win.
1648 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1650 // Only probe during search if winning
1651 if (TB::Score <= VALUE_DRAW)
1657 Hits = rootMoves.size();
1660 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1661 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1