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);
265 TB::RootInTB = false;
266 TB::UseRule50 = Options["Syzygy50MoveRule"];
267 TB::ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
268 TB::Cardinality = Options["SyzygyProbeLimit"];
270 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
271 if (TB::Cardinality > TB::MaxCardinality)
273 TB::Cardinality = TB::MaxCardinality;
274 TB::ProbeDepth = DEPTH_ZERO;
277 if (rootMoves.empty())
279 rootMoves.push_back(RootMove(MOVE_NONE));
280 sync_cout << "info depth 0 score "
281 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
286 if ( TB::Cardinality >= rootPos.count<ALL_PIECES>(WHITE)
287 + rootPos.count<ALL_PIECES>(BLACK)
288 && !rootPos.can_castle(ANY_CASTLING))
290 // If the current root position is in the tablebases, then RootMoves
291 // contains only moves that preserve the draw or the win.
292 TB::RootInTB = Tablebases::root_probe(rootPos, rootMoves, TB::Score);
295 TB::Cardinality = 0; // Do not probe tablebases during the search
297 else // If DTZ tables are missing, use WDL tables as a fallback
299 // Filter out moves that do not preserve the draw or the win.
300 TB::RootInTB = Tablebases::root_probe_wdl(rootPos, rootMoves, TB::Score);
302 // Only probe during search if winning
303 if (TB::Score <= VALUE_DRAW)
309 TB::Hits = rootMoves.size();
312 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
313 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
318 for (Thread* th : Threads)
320 th->start_searching();
322 Thread::search(); // Let's start searching!
325 // When playing in 'nodes as time' mode, subtract the searched nodes from
326 // the available ones before exiting.
328 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
330 // When we reach the maximum depth, we can arrive here without a raise of
331 // Signals.stop. However, if we are pondering or in an infinite search,
332 // the UCI protocol states that we shouldn't print the best move before the
333 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
334 // until the GUI sends one of those commands (which also raises Signals.stop).
335 if (!Signals.stop && (Limits.ponder || Limits.infinite))
337 Signals.stopOnPonderhit = true;
341 // Stop the threads if not already stopped
344 // Wait until all threads have finished
345 for (Thread* th : Threads)
347 th->wait_for_search_finished();
349 // Check if there are threads with a better score than main thread
350 Thread* bestThread = this;
351 if ( !this->easyMovePlayed
352 && Options["MultiPV"] == 1
353 && !Skill(Options["Skill Level"]).enabled()
354 && rootMoves[0].pv[0] != MOVE_NONE)
356 for (Thread* th : Threads)
357 if ( th->completedDepth > bestThread->completedDepth
358 && th->rootMoves[0].score > bestThread->rootMoves[0].score)
362 previousScore = bestThread->rootMoves[0].score;
364 // Send new PV when needed
365 if (bestThread != this)
366 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
368 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
370 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
371 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
373 std::cout << sync_endl;
377 // Thread::search() is the main iterative deepening loop. It calls search()
378 // repeatedly with increasing depth until the allocated thinking time has been
379 // consumed, the user stops the search, or the maximum search depth is reached.
381 void Thread::search() {
383 Stack stack[MAX_PLY+7], *ss = stack+5; // To allow referencing (ss-5) and (ss+2)
384 Value bestValue, alpha, beta, delta;
385 Move easyMove = MOVE_NONE;
386 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
388 std::memset(ss-5, 0, 8 * sizeof(Stack));
390 bestValue = delta = alpha = -VALUE_INFINITE;
391 beta = VALUE_INFINITE;
392 completedDepth = DEPTH_ZERO;
396 easyMove = EasyMove.get(rootPos.key());
398 mainThread->easyMovePlayed = mainThread->failedLow = false;
399 mainThread->bestMoveChanges = 0;
403 size_t multiPV = Options["MultiPV"];
404 Skill skill(Options["Skill Level"]);
406 // When playing with strength handicap enable MultiPV search that we will
407 // use behind the scenes to retrieve a set of possible moves.
409 multiPV = std::max(multiPV, (size_t)4);
411 multiPV = std::min(multiPV, rootMoves.size());
413 // Iterative deepening loop until requested to stop or the target depth is reached.
414 while (++rootDepth < DEPTH_MAX && !Signals.stop && (!Limits.depth || rootDepth <= Limits.depth))
416 // Set up the new depths for the helper threads skipping on average every
417 // 2nd ply (using a half-density matrix).
420 const Row& row = HalfDensity[(idx - 1) % HalfDensitySize];
421 if (row[(rootDepth + rootPos.game_ply()) % row.size()])
425 // Age out PV variability metric
427 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
429 // Save the last iteration's scores before first PV line is searched and
430 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
431 for (RootMove& rm : rootMoves)
432 rm.previousScore = rm.score;
434 // MultiPV loop. We perform a full root search for each PV line
435 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
437 // Reset aspiration window starting size
438 if (rootDepth >= 5 * ONE_PLY)
441 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
442 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
445 // Start with a small aspiration window and, in the case of a fail
446 // high/low, re-search with a bigger window until we're not failing
450 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false);
452 // Bring the best move to the front. It is critical that sorting
453 // is done with a stable algorithm because all the values but the
454 // first and eventually the new best one are set to -VALUE_INFINITE
455 // and we want to keep the same order for all the moves except the
456 // new PV that goes to the front. Note that in case of MultiPV
457 // search the already searched PV lines are preserved.
458 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
460 // Write PV back to the transposition table in case the relevant
461 // entries have been overwritten during the search.
462 for (size_t i = 0; i <= PVIdx; ++i)
463 rootMoves[i].insert_pv_in_tt(rootPos);
465 // If search has been stopped, break immediately. Sorting and
466 // writing PV back to TT is safe because RootMoves is still
467 // valid, although it refers to the previous iteration.
471 // When failing high/low give some update (without cluttering
472 // the UI) before a re-search.
475 && (bestValue <= alpha || bestValue >= beta)
476 && Time.elapsed() > 3000)
477 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
479 // In case of failing low/high increase aspiration window and
480 // re-search, otherwise exit the loop.
481 if (bestValue <= alpha)
483 beta = (alpha + beta) / 2;
484 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
488 mainThread->failedLow = true;
489 Signals.stopOnPonderhit = false;
492 else if (bestValue >= beta)
494 alpha = (alpha + beta) / 2;
495 beta = std::min(bestValue + delta, VALUE_INFINITE);
500 delta += delta / 4 + 5;
502 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
505 // Sort the PV lines searched so far and update the GUI
506 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
512 sync_cout << "info nodes " << Threads.nodes_searched()
513 << " time " << Time.elapsed() << sync_endl;
515 else if (PVIdx + 1 == multiPV || Time.elapsed() > 3000)
516 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
520 completedDepth = rootDepth;
525 // If skill level is enabled and time is up, pick a sub-optimal best move
526 if (skill.enabled() && skill.time_to_pick(rootDepth))
527 skill.pick_best(multiPV);
529 // Have we found a "mate in x"?
531 && bestValue >= VALUE_MATE_IN_MAX_PLY
532 && VALUE_MATE - bestValue <= 2 * Limits.mate)
535 // Do we have time for the next iteration? Can we stop searching now?
536 if (Limits.use_time_management())
538 if (!Signals.stop && !Signals.stopOnPonderhit)
540 // Stop the search if only one legal move is available, or if all
541 // of the available time has been used, or if we matched an easyMove
542 // from the previous search and just did a fast verification.
543 const bool F[] = { !mainThread->failedLow,
544 bestValue >= mainThread->previousScore };
546 int improvingFactor = 640 - 160*F[0] - 126*F[1] - 124*F[0]*F[1];
547 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
549 bool doEasyMove = rootMoves[0].pv[0] == easyMove
550 && mainThread->bestMoveChanges < 0.03
551 && Time.elapsed() > Time.optimum() * 25 / 204;
553 if ( rootMoves.size() == 1
554 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 634
555 || (mainThread->easyMovePlayed = doEasyMove))
557 // If we are allowed to ponder do not stop the search now but
558 // keep pondering until the GUI sends "ponderhit" or "stop".
560 Signals.stopOnPonderhit = true;
566 if (rootMoves[0].pv.size() >= 3)
567 EasyMove.update(rootPos, rootMoves[0].pv);
576 // Clear any candidate easy move that wasn't stable for the last search
577 // iterations; the second condition prevents consecutive fast moves.
578 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
581 // If skill level is enabled, swap best PV line with the sub-optimal one
583 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
584 rootMoves.end(), skill.best_move(multiPV)));
590 // search<>() is the main search function for both PV and non-PV nodes
592 template <NodeType NT>
593 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
595 const bool PvNode = NT == PV;
596 const bool rootNode = PvNode && (ss-1)->ply == 0;
598 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
599 assert(PvNode || (alpha == beta - 1));
600 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
602 Move pv[MAX_PLY+1], quietsSearched[64];
606 Move ttMove, move, excludedMove, bestMove;
607 Depth extension, newDepth, predictedDepth;
608 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
609 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
610 bool captureOrPromotion, doFullDepthSearch;
611 int moveCount, quietCount;
613 // Step 1. Initialize node
614 Thread* thisThread = pos.this_thread();
615 inCheck = pos.checkers();
616 moveCount = quietCount = ss->moveCount = 0;
617 bestValue = -VALUE_INFINITE;
618 ss->ply = (ss-1)->ply + 1;
620 // Check for the available remaining time
621 if (thisThread->resetCalls.load(std::memory_order_relaxed))
623 thisThread->resetCalls = false;
624 thisThread->callsCnt = 0;
626 if (++thisThread->callsCnt > 4096)
628 for (Thread* th : Threads)
629 th->resetCalls = true;
634 // Used to send selDepth info to GUI
635 if (PvNode && thisThread->maxPly < ss->ply)
636 thisThread->maxPly = ss->ply;
640 // Step 2. Check for aborted search and immediate draw
641 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw() || ss->ply >= MAX_PLY)
642 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
643 : DrawValue[pos.side_to_move()];
645 // Step 3. Mate distance pruning. Even if we mate at the next move our score
646 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
647 // a shorter mate was found upward in the tree then there is no need to search
648 // because we will never beat the current alpha. Same logic but with reversed
649 // signs applies also in the opposite condition of being mated instead of giving
650 // mate. In this case return a fail-high score.
651 alpha = std::max(mated_in(ss->ply), alpha);
652 beta = std::min(mate_in(ss->ply+1), beta);
657 assert(0 <= ss->ply && ss->ply < MAX_PLY);
659 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
660 ss->counterMoves = nullptr;
661 (ss+1)->skipEarlyPruning = false;
662 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
664 // Step 4. Transposition table lookup. We don't want the score of a partial
665 // search to overwrite a previous full search TT value, so we use a different
666 // position key in case of an excluded move.
667 excludedMove = ss->excludedMove;
668 posKey = excludedMove ? pos.exclusion_key() : pos.key();
669 tte = TT.probe(posKey, ttHit);
670 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
671 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
672 : ttHit ? tte->move() : MOVE_NONE;
674 // At non-PV nodes we check for an early TT cutoff
677 && tte->depth() >= depth
678 && ttValue != VALUE_NONE // Possible in case of TT access race
679 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
680 : (tte->bound() & BOUND_UPPER)))
682 ss->currentMove = ttMove; // Can be MOVE_NONE
684 // If ttMove is quiet, update killers, history, counter move on TT hit
685 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove))
686 update_stats(pos, ss, ttMove, depth, nullptr, 0);
691 // Step 4a. Tablebase probe
692 if (!rootNode && TB::Cardinality)
694 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
696 if ( piecesCnt <= TB::Cardinality
697 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
698 && pos.rule50_count() == 0
699 && !pos.can_castle(ANY_CASTLING))
701 int found, v = Tablebases::probe_wdl(pos, &found);
707 int drawScore = TB::UseRule50 ? 1 : 0;
709 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
710 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
711 : VALUE_DRAW + 2 * v * drawScore;
713 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
714 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
715 MOVE_NONE, VALUE_NONE, TT.generation());
722 // Step 5. Evaluate the position statically
725 ss->staticEval = eval = VALUE_NONE;
731 // Never assume anything on values stored in TT
732 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
733 eval = ss->staticEval = evaluate(pos);
735 // Can ttValue be used as a better position evaluation?
736 if (ttValue != VALUE_NONE)
737 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
742 eval = ss->staticEval =
743 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
744 : -(ss-1)->staticEval + 2 * Eval::Tempo;
746 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
747 ss->staticEval, TT.generation());
750 if (ss->skipEarlyPruning)
753 // Step 6. Razoring (skipped when in check)
755 && depth < 4 * ONE_PLY
756 && eval + razor_margin[depth] <= alpha
757 && ttMove == MOVE_NONE)
759 if ( depth <= ONE_PLY
760 && eval + razor_margin[3 * ONE_PLY] <= alpha)
761 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
763 Value ralpha = alpha - razor_margin[depth];
764 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
769 // Step 7. Futility pruning: child node (skipped when in check)
771 && depth < 7 * ONE_PLY
772 && eval - futility_margin(depth) >= beta
773 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
774 && pos.non_pawn_material(pos.side_to_move()))
775 return eval - futility_margin(depth);
777 // Step 8. Null move search with verification search (is omitted in PV nodes)
779 && depth >= 2 * ONE_PLY
781 && pos.non_pawn_material(pos.side_to_move()))
783 ss->currentMove = MOVE_NULL;
784 ss->counterMoves = nullptr;
786 assert(eval - beta >= 0);
788 // Null move dynamic reduction based on depth and value
789 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
791 pos.do_null_move(st);
792 (ss+1)->skipEarlyPruning = true;
793 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
794 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
795 (ss+1)->skipEarlyPruning = false;
796 pos.undo_null_move();
798 if (nullValue >= beta)
800 // Do not return unproven mate scores
801 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
804 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
807 // Do verification search at high depths
808 ss->skipEarlyPruning = true;
809 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
810 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
811 ss->skipEarlyPruning = false;
818 // Step 9. ProbCut (skipped when in check)
819 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
820 // and a reduced search returns a value much above beta, we can (almost)
821 // safely prune the previous move.
823 && depth >= 5 * ONE_PLY
824 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
826 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
827 Depth rdepth = depth - 4 * ONE_PLY;
829 assert(rdepth >= ONE_PLY);
830 assert((ss-1)->currentMove != MOVE_NONE);
831 assert((ss-1)->currentMove != MOVE_NULL);
833 MovePicker mp(pos, ttMove, PieceValue[MG][pos.captured_piece_type()]);
836 while ((move = mp.next_move()) != MOVE_NONE)
837 if (pos.legal(move, ci.pinned))
839 ss->currentMove = move;
840 ss->counterMoves = &CounterMoveHistory[pos.moved_piece(move)][to_sq(move)];
841 pos.do_move(move, st, pos.gives_check(move, ci));
842 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
849 // Step 10. Internal iterative deepening (skipped when in check)
850 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
852 && (PvNode || ss->staticEval + 256 >= beta))
854 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
855 ss->skipEarlyPruning = true;
856 search<NT>(pos, ss, alpha, beta, d, true);
857 ss->skipEarlyPruning = false;
859 tte = TT.probe(posKey, ttHit);
860 ttMove = ttHit ? tte->move() : MOVE_NONE;
863 moves_loop: // When in check search starts from here
865 Square prevSq = to_sq((ss-1)->currentMove);
866 const CounterMoveStats& cmh = CounterMoveHistory[pos.piece_on(prevSq)][prevSq];
868 MovePicker mp(pos, ttMove, depth, ss);
870 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
871 improving = ss->staticEval >= (ss-2)->staticEval
872 || ss->staticEval == VALUE_NONE
873 ||(ss-2)->staticEval == VALUE_NONE;
875 singularExtensionNode = !rootNode
876 && depth >= 8 * ONE_PLY
877 && ttMove != MOVE_NONE
878 /* && ttValue != VALUE_NONE Already implicit in the next condition */
879 && abs(ttValue) < VALUE_KNOWN_WIN
880 && !excludedMove // Recursive singular search is not allowed
881 && (tte->bound() & BOUND_LOWER)
882 && tte->depth() >= depth - 3 * ONE_PLY;
884 // Step 11. Loop through moves
885 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
886 while ((move = mp.next_move()) != MOVE_NONE)
890 if (move == excludedMove)
893 // At root obey the "searchmoves" option and skip moves not listed in Root
894 // Move List. As a consequence any illegal move is also skipped. In MultiPV
895 // mode we also skip PV moves which have been already searched.
896 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
897 thisThread->rootMoves.end(), move))
900 ss->moveCount = ++moveCount;
902 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
903 sync_cout << "info depth " << depth / ONE_PLY
904 << " currmove " << UCI::move(move, pos.is_chess960())
905 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
908 (ss+1)->pv = nullptr;
910 extension = DEPTH_ZERO;
911 captureOrPromotion = pos.capture_or_promotion(move);
913 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
914 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
915 : pos.gives_check(move, ci);
917 // Step 12. Extend checks
918 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
921 // Singular extension search. If all moves but one fail low on a search of
922 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
923 // is singular and should be extended. To verify this we do a reduced search
924 // on all the other moves but the ttMove and if the result is lower than
925 // ttValue minus a margin then we extend the ttMove.
926 if ( singularExtensionNode
929 && pos.legal(move, ci.pinned))
931 Value rBeta = ttValue - 2 * depth / ONE_PLY;
932 ss->excludedMove = move;
933 ss->skipEarlyPruning = true;
934 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
935 ss->skipEarlyPruning = false;
936 ss->excludedMove = MOVE_NONE;
942 // Update the current move (this must be done after singular extension search)
943 newDepth = depth - ONE_PLY + extension;
945 // Step 13. Pruning at shallow depth
947 && !captureOrPromotion
950 && !pos.advanced_pawn_push(move)
951 && bestValue > VALUE_MATED_IN_MAX_PLY)
953 // Move count based pruning
954 if ( depth < 16 * ONE_PLY
955 && moveCount >= FutilityMoveCounts[improving][depth])
958 // History based pruning
959 if ( depth <= 4 * ONE_PLY
960 && move != ss->killers[0]
961 && thisThread->history[pos.moved_piece(move)][to_sq(move)] < VALUE_ZERO
962 && cmh[pos.moved_piece(move)][to_sq(move)] < VALUE_ZERO)
965 predictedDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO);
967 // Futility pruning: parent node
968 if (predictedDepth < 7 * ONE_PLY)
970 futilityValue = ss->staticEval + futility_margin(predictedDepth) + 256;
972 if (futilityValue <= alpha)
974 bestValue = std::max(bestValue, futilityValue);
979 // Prune moves with negative SEE at low depths
980 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
984 // Speculative prefetch as early as possible
985 prefetch(TT.first_entry(pos.key_after(move)));
987 // Check for legality just before making the move
988 if (!rootNode && !pos.legal(move, ci.pinned))
990 ss->moveCount = --moveCount;
994 ss->currentMove = move;
995 ss->counterMoves = &CounterMoveHistory[pos.moved_piece(move)][to_sq(move)];
997 // Step 14. Make the move
998 pos.do_move(move, st, givesCheck);
1000 // Step 15. Reduced depth search (LMR). If the move fails high it will be
1001 // re-searched at full depth.
1002 if ( depth >= 3 * ONE_PLY
1004 && !captureOrPromotion)
1006 Depth r = reduction<PvNode>(improving, depth, moveCount);
1007 Value hValue = thisThread->history[pos.piece_on(to_sq(move))][to_sq(move)];
1008 Value cmhValue = cmh[pos.piece_on(to_sq(move))][to_sq(move)];
1010 // Increase reduction for cut nodes and moves with a bad history
1011 if ( (!PvNode && cutNode)
1012 || (hValue < VALUE_ZERO && cmhValue <= VALUE_ZERO))
1015 // Decrease/increase reduction for moves with a good/bad history
1016 int rHist = (hValue + cmhValue) / 14980;
1017 r = std::max(DEPTH_ZERO, r - rHist * ONE_PLY);
1019 // Decrease reduction for moves that escape a capture. Filter out
1020 // castling moves, because they are coded as "king captures rook" and
1021 // hence break make_move(). Also use see() instead of see_sign(),
1022 // because the destination square is empty.
1024 && type_of(move) == NORMAL
1025 && type_of(pos.piece_on(to_sq(move))) != PAWN
1026 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
1027 r = std::max(DEPTH_ZERO, r - ONE_PLY);
1029 Depth d = std::max(newDepth - r, ONE_PLY);
1031 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1033 doFullDepthSearch = (value > alpha && r != DEPTH_ZERO);
1036 doFullDepthSearch = !PvNode || moveCount > 1;
1038 // Step 16. Full depth search when LMR is skipped or fails high
1039 if (doFullDepthSearch)
1040 value = newDepth < ONE_PLY ?
1041 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1042 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1043 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1045 // For PV nodes only, do a full PV search on the first move or after a fail
1046 // high (in the latter case search only if value < beta), otherwise let the
1047 // parent node fail low with value <= alpha and try another move.
1048 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1051 (ss+1)->pv[0] = MOVE_NONE;
1053 value = newDepth < ONE_PLY ?
1054 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1055 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1056 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1059 // Step 17. Undo move
1060 pos.undo_move(move);
1062 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1064 // Step 18. Check for a new best move
1065 // Finished searching the move. If a stop occurred, the return value of
1066 // the search cannot be trusted, and we return immediately without
1067 // updating best move, PV and TT.
1068 if (Signals.stop.load(std::memory_order_relaxed))
1073 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1074 thisThread->rootMoves.end(), move);
1076 // PV move or new best move ?
1077 if (moveCount == 1 || value > alpha)
1084 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1085 rm.pv.push_back(*m);
1087 // We record how often the best move has been changed in each
1088 // iteration. This information is used for time management: When
1089 // the best move changes frequently, we allocate some more time.
1090 if (moveCount > 1 && thisThread == Threads.main())
1091 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1094 // All other moves but the PV are set to the lowest value: this is
1095 // not a problem when sorting because the sort is stable and the
1096 // move position in the list is preserved - just the PV is pushed up.
1097 rm.score = -VALUE_INFINITE;
1100 if (value > bestValue)
1106 // If there is an easy move for this position, clear it if unstable
1108 && thisThread == Threads.main()
1109 && EasyMove.get(pos.key())
1110 && (move != EasyMove.get(pos.key()) || moveCount > 1))
1115 if (PvNode && !rootNode) // Update pv even in fail-high case
1116 update_pv(ss->pv, move, (ss+1)->pv);
1118 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1122 assert(value >= beta); // Fail high
1128 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1129 quietsSearched[quietCount++] = move;
1132 // The following condition would detect a stop only after move loop has been
1133 // completed. But in this case bestValue is valid because we have fully
1134 // searched our subtree, and we can anyhow save the result in TT.
1140 // Step 20. Check for mate and stalemate
1141 // All legal moves have been searched and if there are no legal moves, it
1142 // must be a mate or a stalemate. If we are in a singular extension search then
1143 // return a fail low score.
1145 bestValue = excludedMove ? alpha
1146 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1148 // Quiet best move: update killers, history and countermoves
1149 else if (bestMove && !pos.capture_or_promotion(bestMove))
1150 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount);
1152 // Bonus for prior countermove that caused the fail low
1153 else if ( depth >= 3 * ONE_PLY
1156 && !pos.captured_piece_type()
1157 && is_ok((ss-1)->currentMove))
1159 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY - 1);
1160 if ((ss-2)->counterMoves)
1161 (ss-2)->counterMoves->update(pos.piece_on(prevSq), prevSq, bonus);
1163 if ((ss-3)->counterMoves)
1164 (ss-3)->counterMoves->update(pos.piece_on(prevSq), prevSq, bonus);
1166 if ((ss-5)->counterMoves)
1167 (ss-5)->counterMoves->update(pos.piece_on(prevSq), prevSq, bonus);
1170 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1171 bestValue >= beta ? BOUND_LOWER :
1172 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1173 depth, bestMove, ss->staticEval, TT.generation());
1175 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1181 // qsearch() is the quiescence search function, which is called by the main
1182 // search function when the remaining depth is zero (or, to be more precise,
1183 // less than ONE_PLY).
1185 template <NodeType NT, bool InCheck>
1186 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1188 const bool PvNode = NT == PV;
1190 assert(InCheck == !!pos.checkers());
1191 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1192 assert(PvNode || (alpha == beta - 1));
1193 assert(depth <= DEPTH_ZERO);
1199 Move ttMove, move, bestMove;
1200 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1201 bool ttHit, givesCheck, evasionPrunable;
1206 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1208 ss->pv[0] = MOVE_NONE;
1211 ss->currentMove = bestMove = MOVE_NONE;
1212 ss->ply = (ss-1)->ply + 1;
1214 // Check for an instant draw or if the maximum ply has been reached
1215 if (pos.is_draw() || ss->ply >= MAX_PLY)
1216 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1217 : DrawValue[pos.side_to_move()];
1219 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1221 // Decide whether or not to include checks: this fixes also the type of
1222 // TT entry depth that we are going to use. Note that in qsearch we use
1223 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1224 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1225 : DEPTH_QS_NO_CHECKS;
1227 // Transposition table lookup
1229 tte = TT.probe(posKey, ttHit);
1230 ttMove = ttHit ? tte->move() : MOVE_NONE;
1231 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1235 && tte->depth() >= ttDepth
1236 && ttValue != VALUE_NONE // Only in case of TT access race
1237 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1238 : (tte->bound() & BOUND_UPPER)))
1240 ss->currentMove = ttMove; // Can be MOVE_NONE
1244 // Evaluate the position statically
1247 ss->staticEval = VALUE_NONE;
1248 bestValue = futilityBase = -VALUE_INFINITE;
1254 // Never assume anything on values stored in TT
1255 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1256 ss->staticEval = bestValue = evaluate(pos);
1258 // Can ttValue be used as a better position evaluation?
1259 if (ttValue != VALUE_NONE)
1260 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1261 bestValue = ttValue;
1264 ss->staticEval = bestValue =
1265 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1266 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1268 // Stand pat. Return immediately if static value is at least beta
1269 if (bestValue >= beta)
1272 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1273 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1278 if (PvNode && bestValue > alpha)
1281 futilityBase = bestValue + 128;
1284 // Initialize a MovePicker object for the current position, and prepare
1285 // to search the moves. Because the depth is <= 0 here, only captures,
1286 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1288 MovePicker mp(pos, ttMove, depth, to_sq((ss-1)->currentMove));
1291 // Loop through the moves until no moves remain or a beta cutoff occurs
1292 while ((move = mp.next_move()) != MOVE_NONE)
1294 assert(is_ok(move));
1296 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1297 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1298 : pos.gives_check(move, ci);
1303 && futilityBase > -VALUE_KNOWN_WIN
1304 && !pos.advanced_pawn_push(move))
1306 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1308 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1310 if (futilityValue <= alpha)
1312 bestValue = std::max(bestValue, futilityValue);
1316 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1318 bestValue = std::max(bestValue, futilityBase);
1323 // Detect non-capture evasions that are candidates to be pruned
1324 evasionPrunable = InCheck
1325 && bestValue > VALUE_MATED_IN_MAX_PLY
1326 && !pos.capture(move);
1328 // Don't search moves with negative SEE values
1329 if ( (!InCheck || evasionPrunable)
1330 && type_of(move) != PROMOTION
1331 && pos.see_sign(move) < VALUE_ZERO)
1334 // Speculative prefetch as early as possible
1335 prefetch(TT.first_entry(pos.key_after(move)));
1337 // Check for legality just before making the move
1338 if (!pos.legal(move, ci.pinned))
1341 ss->currentMove = move;
1343 // Make and search the move
1344 pos.do_move(move, st, givesCheck);
1345 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1346 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1347 pos.undo_move(move);
1349 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1351 // Check for a new best move
1352 if (value > bestValue)
1358 if (PvNode) // Update pv even in fail-high case
1359 update_pv(ss->pv, move, (ss+1)->pv);
1361 if (PvNode && value < beta) // Update alpha here!
1368 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1369 ttDepth, move, ss->staticEval, TT.generation());
1377 // All legal moves have been searched. A special case: If we're in check
1378 // and no legal moves were found, it is checkmate.
1379 if (InCheck && bestValue == -VALUE_INFINITE)
1380 return mated_in(ss->ply); // Plies to mate from the root
1382 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1383 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1384 ttDepth, bestMove, ss->staticEval, TT.generation());
1386 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1392 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1393 // "plies to mate from the current position". Non-mate scores are unchanged.
1394 // The function is called before storing a value in the transposition table.
1396 Value value_to_tt(Value v, int ply) {
1398 assert(v != VALUE_NONE);
1400 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1401 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1405 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1406 // from the transposition table (which refers to the plies to mate/be mated
1407 // from current position) to "plies to mate/be mated from the root".
1409 Value value_from_tt(Value v, int ply) {
1411 return v == VALUE_NONE ? VALUE_NONE
1412 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1413 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1417 // update_pv() adds current move and appends child pv[]
1419 void update_pv(Move* pv, Move move, Move* childPv) {
1421 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1427 // update_stats() updates killers, history, countermove and countermove plus
1428 // follow-up move history when a new quiet best move is found.
1430 void update_stats(const Position& pos, Stack* ss, Move move,
1431 Depth depth, Move* quiets, int quietsCnt) {
1433 if (ss->killers[0] != move)
1435 ss->killers[1] = ss->killers[0];
1436 ss->killers[0] = move;
1439 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY - 1);
1441 Square prevSq = to_sq((ss-1)->currentMove);
1442 CounterMoveStats* cmh = (ss-1)->counterMoves;
1443 CounterMoveStats* fmh = (ss-2)->counterMoves;
1444 CounterMoveStats* fmh2 = (ss-4)->counterMoves;
1445 Thread* thisThread = pos.this_thread();
1447 thisThread->history.update(pos.moved_piece(move), to_sq(move), bonus);
1451 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1452 cmh->update(pos.moved_piece(move), to_sq(move), bonus);
1456 fmh->update(pos.moved_piece(move), to_sq(move), bonus);
1459 fmh2->update(pos.moved_piece(move), to_sq(move), bonus);
1461 // Decrease all the other played quiet moves
1462 for (int i = 0; i < quietsCnt; ++i)
1464 thisThread->history.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1467 cmh->update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1470 fmh->update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1473 fmh2->update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1476 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1477 if ((ss-1)->moveCount == 1 && !pos.captured_piece_type())
1479 if ((ss-2)->counterMoves)
1480 (ss-2)->counterMoves->update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY);
1482 if ((ss-3)->counterMoves)
1483 (ss-3)->counterMoves->update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY);
1485 if ((ss-5)->counterMoves)
1486 (ss-5)->counterMoves->update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY);
1491 // When playing with strength handicap, choose best move among a set of RootMoves
1492 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1494 Move Skill::pick_best(size_t multiPV) {
1496 const RootMoves& rootMoves = Threads.main()->rootMoves;
1497 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1499 // RootMoves are already sorted by score in descending order
1500 Value topScore = rootMoves[0].score;
1501 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1502 int weakness = 120 - 2 * level;
1503 int maxScore = -VALUE_INFINITE;
1505 // Choose best move. For each move score we add two terms, both dependent on
1506 // weakness. One is deterministic and bigger for weaker levels, and one is
1507 // random. Then we choose the move with the resulting highest score.
1508 for (size_t i = 0; i < multiPV; ++i)
1510 // This is our magic formula
1511 int push = ( weakness * int(topScore - rootMoves[i].score)
1512 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1514 if (rootMoves[i].score + push > maxScore)
1516 maxScore = rootMoves[i].score + push;
1517 best = rootMoves[i].pv[0];
1525 // check_time() is used to print debug info and, more importantly, to detect
1526 // when we are out of available time and thus stop the search.
1530 static TimePoint lastInfoTime = now();
1532 int elapsed = Time.elapsed();
1533 TimePoint tick = Limits.startTime + elapsed;
1535 if (tick - lastInfoTime >= 1000)
1537 lastInfoTime = tick;
1541 // An engine may not stop pondering until told so by the GUI
1545 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1546 || (Limits.movetime && elapsed >= Limits.movetime)
1547 || (Limits.nodes && Threads.nodes_searched() >= Limits.nodes))
1548 Signals.stop = true;
1554 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1555 /// that all (if any) unsearched PV lines are sent using a previous search score.
1557 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1559 std::stringstream ss;
1560 int elapsed = Time.elapsed() + 1;
1561 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1562 size_t PVIdx = pos.this_thread()->PVIdx;
1563 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1564 uint64_t nodes_searched = Threads.nodes_searched();
1566 for (size_t i = 0; i < multiPV; ++i)
1568 bool updated = (i <= PVIdx);
1570 if (depth == ONE_PLY && !updated)
1573 Depth d = updated ? depth : depth - ONE_PLY;
1574 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1576 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1577 v = tb ? TB::Score : v;
1579 if (ss.rdbuf()->in_avail()) // Not at first line
1583 << " depth " << d / ONE_PLY
1584 << " seldepth " << pos.this_thread()->maxPly
1585 << " multipv " << i + 1
1586 << " score " << UCI::value(v);
1588 if (!tb && i == PVIdx)
1589 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1591 ss << " nodes " << nodes_searched
1592 << " nps " << nodes_searched * 1000 / elapsed;
1594 if (elapsed > 1000) // Earlier makes little sense
1595 ss << " hashfull " << TT.hashfull();
1597 ss << " tbhits " << TB::Hits
1598 << " time " << elapsed
1601 for (Move m : rootMoves[i].pv)
1602 ss << " " << UCI::move(m, pos.is_chess960());
1609 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1610 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1611 /// first, even if the old TT entries have been overwritten.
1613 void RootMove::insert_pv_in_tt(Position& pos) {
1615 StateInfo state[MAX_PLY], *st = state;
1620 assert(MoveList<LEGAL>(pos).contains(m));
1622 TTEntry* tte = TT.probe(pos.key(), ttHit);
1624 if (!ttHit || tte->move() != m) // Don't overwrite correct entries
1625 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE,
1626 m, VALUE_NONE, TT.generation());
1628 pos.do_move(m, *st++, pos.gives_check(m, CheckInfo(pos)));
1631 for (size_t i = pv.size(); i > 0; )
1632 pos.undo_move(pv[--i]);
1636 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1637 /// before exiting the search, for instance, in case we stop the search during a
1638 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1639 /// otherwise in case of 'ponder on' we have nothing to think on.
1641 bool RootMove::extract_ponder_from_tt(Position& pos)
1646 assert(pv.size() == 1);
1648 pos.do_move(pv[0], st, pos.gives_check(pv[0], CheckInfo(pos)));
1649 TTEntry* tte = TT.probe(pos.key(), ttHit);
1650 pos.undo_move(pv[0]);
1654 Move m = tte->move(); // Local copy to be SMP safe
1655 if (MoveList<LEGAL>(pos).contains(m))
1656 return pv.push_back(m), true;