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"
43 StateStackPtr SetupStates;
46 namespace Tablebases {
56 namespace TB = Tablebases;
60 using namespace Search;
64 // Different node types, used as a template parameter
65 enum NodeType { NonPV, PV };
67 // Razoring and futility margin based on depth
68 const int razor_margin[4] = { 483, 570, 603, 554 };
69 Value futility_margin(Depth d) { return Value(200 * d); }
71 // Futility and reductions lookup tables, initialized at startup
72 int FutilityMoveCounts[2][16]; // [improving][depth]
73 Depth Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
75 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
76 return Reductions[PvNode][i][std::min(d, 63 * ONE_PLY)][std::min(mn, 63)];
79 // Skill structure is used to implement strength limit
81 Skill(int l) : level(l) {}
82 bool enabled() const { return level < 20; }
83 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
84 Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
85 Move pick_best(size_t multiPV);
88 Move best = MOVE_NONE;
91 // EasyMoveManager structure is used to detect an 'easy move'. When the PV is
92 // stable across multiple search iterations, we can quickly return the best move.
93 struct EasyMoveManager {
98 pv[0] = pv[1] = pv[2] = MOVE_NONE;
101 Move get(Key key) const {
102 return expectedPosKey == key ? pv[2] : MOVE_NONE;
105 void update(Position& pos, const std::vector<Move>& newPv) {
107 assert(newPv.size() >= 3);
109 // Keep track of how many times in a row the 3rd ply remains stable
110 stableCnt = (newPv[2] == pv[2]) ? stableCnt + 1 : 0;
112 if (!std::equal(newPv.begin(), newPv.begin() + 3, pv))
114 std::copy(newPv.begin(), newPv.begin() + 3, pv);
117 pos.do_move(newPv[0], st[0], pos.gives_check(newPv[0], CheckInfo(pos)));
118 pos.do_move(newPv[1], st[1], pos.gives_check(newPv[1], CheckInfo(pos)));
119 expectedPosKey = pos.key();
120 pos.undo_move(newPv[1]);
121 pos.undo_move(newPv[0]);
130 // Set of rows with half bits set to 1 and half to 0. It is used to allocate
131 // the search depths across the threads.
132 typedef std::vector<int> Row;
134 const Row HalfDensity[] = {
147 {0, 0, 0, 0, 1, 1, 1, 1},
148 {0, 0, 0, 1, 1, 1, 1, 0},
149 {0, 0, 1, 1, 1, 1, 0 ,0},
150 {0, 1, 1, 1, 1, 0, 0 ,0},
151 {1, 1, 1, 1, 0, 0, 0 ,0},
152 {1, 1, 1, 0, 0, 0, 0 ,1},
153 {1, 1, 0, 0, 0, 0, 1 ,1},
154 {1, 0, 0, 0, 0, 1, 1 ,1},
157 const size_t HalfDensitySize = std::extent<decltype(HalfDensity)>::value;
159 EasyMoveManager EasyMove;
160 Value DrawValue[COLOR_NB];
161 CounterMoveHistoryStats CounterMoveHistory;
163 template <NodeType NT>
164 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
166 template <NodeType NT, bool InCheck>
167 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
169 Value value_to_tt(Value v, int ply);
170 Value value_from_tt(Value v, int ply);
171 void update_pv(Move* pv, Move move, Move* childPv);
172 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
178 /// Search::init() is called during startup to initialize various lookup tables
180 void Search::init() {
182 const double K[][2] = {{ 0.799, 2.281 }, { 0.484, 3.023 }};
184 for (int pv = 0; pv <= 1; ++pv)
185 for (int imp = 0; imp <= 1; ++imp)
186 for (int d = 1; d < 64; ++d)
187 for (int mc = 1; mc < 64; ++mc)
189 double r = K[pv][0] + log(d) * log(mc) / K[pv][1];
192 Reductions[pv][imp][d][mc] = int(r) * ONE_PLY;
194 // Increase reduction when eval is not improving
195 if (!pv && !imp && Reductions[pv][imp][d][mc] >= 2 * ONE_PLY)
196 Reductions[pv][imp][d][mc] += ONE_PLY;
199 for (int d = 0; d < 16; ++d)
201 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
202 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
207 /// Search::clear() resets search state to zero, to obtain reproducible results
209 void Search::clear() {
212 CounterMoveHistory.clear();
214 for (Thread* th : Threads)
217 th->counterMoves.clear();
220 Threads.main()->previousScore = VALUE_INFINITE;
224 /// Search::perft() is our utility to verify move generation. All the leaf nodes
225 /// up to the given depth are generated and counted, and the sum is returned.
227 uint64_t Search::perft(Position& pos, Depth depth) {
230 uint64_t cnt, nodes = 0;
232 const bool leaf = (depth == 2 * ONE_PLY);
234 for (const auto& m : MoveList<LEGAL>(pos))
236 if (Root && depth <= ONE_PLY)
240 pos.do_move(m, st, pos.gives_check(m, ci));
241 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
246 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
251 template uint64_t Search::perft<true>(Position&, Depth);
254 /// MainThread::search() is called by the main thread when the program receives
255 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
257 void MainThread::search() {
259 Color us = rootPos.side_to_move();
260 Time.init(Limits, us, rootPos.game_ply());
262 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
263 DrawValue[ us] = VALUE_DRAW - Value(contempt);
264 DrawValue[~us] = VALUE_DRAW + Value(contempt);
267 TB::RootInTB = false;
268 TB::UseRule50 = Options["Syzygy50MoveRule"];
269 TB::ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
270 TB::Cardinality = Options["SyzygyProbeLimit"];
272 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
273 if (TB::Cardinality > TB::MaxCardinality)
275 TB::Cardinality = TB::MaxCardinality;
276 TB::ProbeDepth = DEPTH_ZERO;
279 if (rootMoves.empty())
281 rootMoves.push_back(RootMove(MOVE_NONE));
282 sync_cout << "info depth 0 score "
283 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
288 if ( TB::Cardinality >= rootPos.count<ALL_PIECES>(WHITE)
289 + rootPos.count<ALL_PIECES>(BLACK)
290 && !rootPos.can_castle(ANY_CASTLING))
292 // If the current root position is in the tablebases, then RootMoves
293 // contains only moves that preserve the draw or the win.
294 TB::RootInTB = Tablebases::root_probe(rootPos, rootMoves, TB::Score);
297 TB::Cardinality = 0; // Do not probe tablebases during the search
299 else // If DTZ tables are missing, use WDL tables as a fallback
301 // Filter out moves that do not preserve the draw or the win.
302 TB::RootInTB = Tablebases::root_probe_wdl(rootPos, rootMoves, TB::Score);
304 // Only probe during search if winning
305 if (TB::Score <= VALUE_DRAW)
311 TB::Hits = rootMoves.size();
314 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
315 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
320 for (Thread* th : Threads)
323 th->rootDepth = DEPTH_ZERO;
326 th->rootPos = Position(rootPos, th);
327 th->rootMoves = rootMoves;
328 th->start_searching();
332 Thread::search(); // Let's start searching!
335 // When playing in 'nodes as time' mode, subtract the searched nodes from
336 // the available ones before exiting.
338 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
340 // When we reach the maximum depth, we can arrive here without a raise of
341 // Signals.stop. However, if we are pondering or in an infinite search,
342 // the UCI protocol states that we shouldn't print the best move before the
343 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
344 // until the GUI sends one of those commands (which also raises Signals.stop).
345 if (!Signals.stop && (Limits.ponder || Limits.infinite))
347 Signals.stopOnPonderhit = true;
351 // Stop the threads if not already stopped
354 // Wait until all threads have finished
355 for (Thread* th : Threads)
357 th->wait_for_search_finished();
359 // Check if there are threads with a better score than main thread
360 Thread* bestThread = this;
361 if ( !this->easyMovePlayed
362 && Options["MultiPV"] == 1
363 && !Skill(Options["Skill Level"]).enabled())
365 for (Thread* th : Threads)
366 if ( th->completedDepth > bestThread->completedDepth
367 && th->rootMoves[0].score > bestThread->rootMoves[0].score)
371 previousScore = bestThread->rootMoves[0].score;
373 // Send new PV when needed
374 if (bestThread != this)
375 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
377 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
379 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
380 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
382 std::cout << sync_endl;
386 // Thread::search() is the main iterative deepening loop. It calls search()
387 // repeatedly with increasing depth until the allocated thinking time has been
388 // consumed, the user stops the search, or the maximum search depth is reached.
390 void Thread::search() {
392 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
393 Value bestValue, alpha, beta, delta;
394 Move easyMove = MOVE_NONE;
395 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
397 std::memset(ss-2, 0, 5 * sizeof(Stack));
399 bestValue = delta = alpha = -VALUE_INFINITE;
400 beta = VALUE_INFINITE;
401 completedDepth = DEPTH_ZERO;
405 easyMove = EasyMove.get(rootPos.key());
407 mainThread->easyMovePlayed = mainThread->failedLow = false;
408 mainThread->bestMoveChanges = 0;
412 size_t multiPV = Options["MultiPV"];
413 Skill skill(Options["Skill Level"]);
415 // When playing with strength handicap enable MultiPV search that we will
416 // use behind the scenes to retrieve a set of possible moves.
418 multiPV = std::max(multiPV, (size_t)4);
420 multiPV = std::min(multiPV, rootMoves.size());
422 // Iterative deepening loop until requested to stop or the target depth is reached.
423 while (++rootDepth < DEPTH_MAX && !Signals.stop && (!Limits.depth || rootDepth <= Limits.depth))
425 // Set up the new depths for the helper threads skipping on average every
426 // 2nd ply (using a half-density matrix).
429 const Row& row = HalfDensity[(idx - 1) % HalfDensitySize];
430 if (row[(rootDepth + rootPos.game_ply()) % row.size()])
434 // Age out PV variability metric
436 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
438 // Save the last iteration's scores before first PV line is searched and
439 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
440 for (RootMove& rm : rootMoves)
441 rm.previousScore = rm.score;
443 // MultiPV loop. We perform a full root search for each PV line
444 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
446 // Reset aspiration window starting size
447 if (rootDepth >= 5 * ONE_PLY)
450 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
451 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
454 // Start with a small aspiration window and, in the case of a fail
455 // high/low, re-search with a bigger window until we're not failing
459 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false);
461 // Bring the best move to the front. It is critical that sorting
462 // is done with a stable algorithm because all the values but the
463 // first and eventually the new best one are set to -VALUE_INFINITE
464 // and we want to keep the same order for all the moves except the
465 // new PV that goes to the front. Note that in case of MultiPV
466 // search the already searched PV lines are preserved.
467 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
469 // Write PV back to the transposition table in case the relevant
470 // entries have been overwritten during the search.
471 for (size_t i = 0; i <= PVIdx; ++i)
472 rootMoves[i].insert_pv_in_tt(rootPos);
474 // If search has been stopped, break immediately. Sorting and
475 // writing PV back to TT is safe because RootMoves is still
476 // valid, although it refers to the previous iteration.
480 // When failing high/low give some update (without cluttering
481 // the UI) before a re-search.
484 && (bestValue <= alpha || bestValue >= beta)
485 && Time.elapsed() > 3000)
486 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
488 // In case of failing low/high increase aspiration window and
489 // re-search, otherwise exit the loop.
490 if (bestValue <= alpha)
492 beta = (alpha + beta) / 2;
493 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
497 mainThread->failedLow = true;
498 Signals.stopOnPonderhit = false;
501 else if (bestValue >= beta)
503 alpha = (alpha + beta) / 2;
504 beta = std::min(bestValue + delta, VALUE_INFINITE);
509 delta += delta / 4 + 5;
511 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
514 // Sort the PV lines searched so far and update the GUI
515 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
521 sync_cout << "info nodes " << Threads.nodes_searched()
522 << " time " << Time.elapsed() << sync_endl;
524 else if (PVIdx + 1 == multiPV || Time.elapsed() > 3000)
525 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
529 completedDepth = rootDepth;
534 // If skill level is enabled and time is up, pick a sub-optimal best move
535 if (skill.enabled() && skill.time_to_pick(rootDepth))
536 skill.pick_best(multiPV);
538 // Have we found a "mate in x"?
540 && bestValue >= VALUE_MATE_IN_MAX_PLY
541 && VALUE_MATE - bestValue <= 2 * Limits.mate)
544 // Do we have time for the next iteration? Can we stop searching now?
545 if (Limits.use_time_management())
547 if (!Signals.stop && !Signals.stopOnPonderhit)
549 // Stop the search if only one legal move is available, or if all
550 // of the available time has been used, or if we matched an easyMove
551 // from the previous search and just did a fast verification.
552 const bool F[] = { !mainThread->failedLow,
553 bestValue >= mainThread->previousScore };
555 int improvingFactor = 640 - 160*F[0] - 126*F[1] - 124*F[0]*F[1];
556 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
558 bool doEasyMove = rootMoves[0].pv[0] == easyMove
559 && mainThread->bestMoveChanges < 0.03
560 && Time.elapsed() > Time.optimum() * 25 / 204;
562 if ( rootMoves.size() == 1
563 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 634
564 || (mainThread->easyMovePlayed = doEasyMove))
566 // If we are allowed to ponder do not stop the search now but
567 // keep pondering until the GUI sends "ponderhit" or "stop".
569 Signals.stopOnPonderhit = true;
575 if (rootMoves[0].pv.size() >= 3)
576 EasyMove.update(rootPos, rootMoves[0].pv);
585 // Clear any candidate easy move that wasn't stable for the last search
586 // iterations; the second condition prevents consecutive fast moves.
587 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
590 // If skill level is enabled, swap best PV line with the sub-optimal one
592 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
593 rootMoves.end(), skill.best_move(multiPV)));
599 // search<>() is the main search function for both PV and non-PV nodes
601 template <NodeType NT>
602 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
604 const bool PvNode = NT == PV;
605 const bool rootNode = PvNode && (ss-1)->ply == 0;
607 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
608 assert(PvNode || (alpha == beta - 1));
609 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
611 Move pv[MAX_PLY+1], quietsSearched[64];
615 Move ttMove, move, excludedMove, bestMove;
616 Depth extension, newDepth, predictedDepth;
617 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
618 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
619 bool captureOrPromotion, doFullDepthSearch;
620 int moveCount, quietCount;
622 // Step 1. Initialize node
623 Thread* thisThread = pos.this_thread();
624 inCheck = pos.checkers();
625 moveCount = quietCount = ss->moveCount = 0;
626 bestValue = -VALUE_INFINITE;
627 ss->ply = (ss-1)->ply + 1;
629 // Check for the available remaining time
630 if (thisThread->resetCalls.load(std::memory_order_relaxed))
632 thisThread->resetCalls = false;
633 thisThread->callsCnt = 0;
635 if (++thisThread->callsCnt > 4096)
637 for (Thread* th : Threads)
638 th->resetCalls = true;
643 // Used to send selDepth info to GUI
644 if (PvNode && thisThread->maxPly < ss->ply)
645 thisThread->maxPly = ss->ply;
649 // Step 2. Check for aborted search and immediate draw
650 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw() || ss->ply >= MAX_PLY)
651 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
652 : DrawValue[pos.side_to_move()];
654 // Step 3. Mate distance pruning. Even if we mate at the next move our score
655 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
656 // a shorter mate was found upward in the tree then there is no need to search
657 // because we will never beat the current alpha. Same logic but with reversed
658 // signs applies also in the opposite condition of being mated instead of giving
659 // mate. In this case return a fail-high score.
660 alpha = std::max(mated_in(ss->ply), alpha);
661 beta = std::min(mate_in(ss->ply+1), beta);
666 assert(0 <= ss->ply && ss->ply < MAX_PLY);
668 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
669 (ss+1)->skipEarlyPruning = false;
670 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
672 // Step 4. Transposition table lookup. We don't want the score of a partial
673 // search to overwrite a previous full search TT value, so we use a different
674 // position key in case of an excluded move.
675 excludedMove = ss->excludedMove;
676 posKey = excludedMove ? pos.exclusion_key() : pos.key();
677 tte = TT.probe(posKey, ttHit);
678 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
679 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
680 : ttHit ? tte->move() : MOVE_NONE;
682 // At non-PV nodes we check for an early TT cutoff
685 && tte->depth() >= depth
686 && ttValue != VALUE_NONE // Possible in case of TT access race
687 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
688 : (tte->bound() & BOUND_UPPER)))
690 ss->currentMove = ttMove; // Can be MOVE_NONE
692 // If ttMove is quiet, update killers, history, counter move on TT hit
693 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove))
694 update_stats(pos, ss, ttMove, depth, nullptr, 0);
699 // Step 4a. Tablebase probe
700 if (!rootNode && TB::Cardinality)
702 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
704 if ( piecesCnt <= TB::Cardinality
705 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
706 && pos.rule50_count() == 0
707 && !pos.can_castle(ANY_CASTLING))
709 int found, v = Tablebases::probe_wdl(pos, &found);
715 int drawScore = TB::UseRule50 ? 1 : 0;
717 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
718 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
719 : VALUE_DRAW + 2 * v * drawScore;
721 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
722 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
723 MOVE_NONE, VALUE_NONE, TT.generation());
730 // Step 5. Evaluate the position statically
733 ss->staticEval = eval = VALUE_NONE;
739 // Never assume anything on values stored in TT
740 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
741 eval = ss->staticEval = evaluate(pos);
743 // Can ttValue be used as a better position evaluation?
744 if (ttValue != VALUE_NONE)
745 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
750 eval = ss->staticEval =
751 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
752 : -(ss-1)->staticEval + 2 * Eval::Tempo;
754 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
755 ss->staticEval, TT.generation());
758 if (ss->skipEarlyPruning)
761 // Step 6. Razoring (skipped when in check)
763 && depth < 4 * ONE_PLY
764 && eval + razor_margin[depth] <= alpha
765 && ttMove == MOVE_NONE)
767 if ( depth <= ONE_PLY
768 && eval + razor_margin[3 * ONE_PLY] <= alpha)
769 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
771 Value ralpha = alpha - razor_margin[depth];
772 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
777 // Step 7. Futility pruning: child node (skipped when in check)
779 && depth < 7 * ONE_PLY
780 && eval - futility_margin(depth) >= beta
781 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
782 && pos.non_pawn_material(pos.side_to_move()))
783 return eval - futility_margin(depth);
785 // Step 8. Null move search with verification search (is omitted in PV nodes)
787 && depth >= 2 * ONE_PLY
789 && pos.non_pawn_material(pos.side_to_move()))
791 ss->currentMove = MOVE_NULL;
793 assert(eval - beta >= 0);
795 // Null move dynamic reduction based on depth and value
796 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
798 pos.do_null_move(st);
799 (ss+1)->skipEarlyPruning = true;
800 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
801 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
802 (ss+1)->skipEarlyPruning = false;
803 pos.undo_null_move();
805 if (nullValue >= beta)
807 // Do not return unproven mate scores
808 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
811 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
814 // Do verification search at high depths
815 ss->skipEarlyPruning = true;
816 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
817 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
818 ss->skipEarlyPruning = false;
825 // Step 9. ProbCut (skipped when in check)
826 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
827 // and a reduced search returns a value much above beta, we can (almost)
828 // safely prune the previous move.
830 && depth >= 5 * ONE_PLY
831 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
833 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
834 Depth rdepth = depth - 4 * ONE_PLY;
836 assert(rdepth >= ONE_PLY);
837 assert((ss-1)->currentMove != MOVE_NONE);
838 assert((ss-1)->currentMove != MOVE_NULL);
840 MovePicker mp(pos, ttMove, thisThread->history, PieceValue[MG][pos.captured_piece_type()]);
843 while ((move = mp.next_move()) != MOVE_NONE)
844 if (pos.legal(move, ci.pinned))
846 ss->currentMove = move;
847 pos.do_move(move, st, pos.gives_check(move, ci));
848 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
855 // Step 10. Internal iterative deepening (skipped when in check)
856 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
858 && (PvNode || ss->staticEval + 256 >= beta))
860 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
861 ss->skipEarlyPruning = true;
862 search<NT>(pos, ss, alpha, beta, d, true);
863 ss->skipEarlyPruning = false;
865 tte = TT.probe(posKey, ttHit);
866 ttMove = ttHit ? tte->move() : MOVE_NONE;
869 moves_loop: // When in check search starts from here
871 Square prevSq = to_sq((ss-1)->currentMove);
872 Move cm = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
873 const CounterMoveStats& cmh = CounterMoveHistory[pos.piece_on(prevSq)][prevSq];
875 MovePicker mp(pos, ttMove, depth, thisThread->history, cmh, cm, ss);
877 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
878 improving = ss->staticEval >= (ss-2)->staticEval
879 || ss->staticEval == VALUE_NONE
880 ||(ss-2)->staticEval == VALUE_NONE;
882 singularExtensionNode = !rootNode
883 && depth >= 8 * ONE_PLY
884 && ttMove != MOVE_NONE
885 /* && ttValue != VALUE_NONE Already implicit in the next condition */
886 && abs(ttValue) < VALUE_KNOWN_WIN
887 && !excludedMove // Recursive singular search is not allowed
888 && (tte->bound() & BOUND_LOWER)
889 && tte->depth() >= depth - 3 * ONE_PLY;
891 // Step 11. Loop through moves
892 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
893 while ((move = mp.next_move()) != MOVE_NONE)
897 if (move == excludedMove)
900 // At root obey the "searchmoves" option and skip moves not listed in Root
901 // Move List. As a consequence any illegal move is also skipped. In MultiPV
902 // mode we also skip PV moves which have been already searched.
903 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
904 thisThread->rootMoves.end(), move))
907 ss->moveCount = ++moveCount;
909 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
910 sync_cout << "info depth " << depth / ONE_PLY
911 << " currmove " << UCI::move(move, pos.is_chess960())
912 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
915 (ss+1)->pv = nullptr;
917 extension = DEPTH_ZERO;
918 captureOrPromotion = pos.capture_or_promotion(move);
920 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
921 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
922 : pos.gives_check(move, ci);
924 // Step 12. Extend checks
925 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
928 // Singular extension search. If all moves but one fail low on a search of
929 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
930 // is singular and should be extended. To verify this we do a reduced search
931 // on all the other moves but the ttMove and if the result is lower than
932 // ttValue minus a margin then we extend the ttMove.
933 if ( singularExtensionNode
936 && pos.legal(move, ci.pinned))
938 Value rBeta = ttValue - 2 * depth / ONE_PLY;
939 ss->excludedMove = move;
940 ss->skipEarlyPruning = true;
941 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
942 ss->skipEarlyPruning = false;
943 ss->excludedMove = MOVE_NONE;
949 // Update the current move (this must be done after singular extension search)
950 newDepth = depth - ONE_PLY + extension;
952 // Step 13. Pruning at shallow depth
954 && !captureOrPromotion
957 && !pos.advanced_pawn_push(move)
958 && bestValue > VALUE_MATED_IN_MAX_PLY)
960 // Move count based pruning
961 if ( depth < 16 * ONE_PLY
962 && moveCount >= FutilityMoveCounts[improving][depth])
965 // History based pruning
966 if ( depth <= 4 * ONE_PLY
967 && move != ss->killers[0]
968 && thisThread->history[pos.moved_piece(move)][to_sq(move)] < VALUE_ZERO
969 && cmh[pos.moved_piece(move)][to_sq(move)] < VALUE_ZERO)
972 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
974 // Futility pruning: parent node
975 if (predictedDepth < 7 * ONE_PLY)
977 futilityValue = ss->staticEval + futility_margin(predictedDepth) + 256;
979 if (futilityValue <= alpha)
981 bestValue = std::max(bestValue, futilityValue);
986 // Prune moves with negative SEE at low depths
987 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
991 // Speculative prefetch as early as possible
992 prefetch(TT.first_entry(pos.key_after(move)));
994 // Check for legality just before making the move
995 if (!rootNode && !pos.legal(move, ci.pinned))
997 ss->moveCount = --moveCount;
1001 ss->currentMove = move;
1003 // Step 14. Make the move
1004 pos.do_move(move, st, givesCheck);
1006 // Step 15. Reduced depth search (LMR). If the move fails high it will be
1007 // re-searched at full depth.
1008 if ( depth >= 3 * ONE_PLY
1010 && !captureOrPromotion)
1012 Depth r = reduction<PvNode>(improving, depth, moveCount);
1014 // Increase reduction for cut nodes and moves with a bad history
1015 if ( (!PvNode && cutNode)
1016 || ( thisThread->history[pos.piece_on(to_sq(move))][to_sq(move)] < VALUE_ZERO
1017 && cmh[pos.piece_on(to_sq(move))][to_sq(move)] <= VALUE_ZERO))
1020 // Decrease/increase reduction for moves with a good/bad history
1021 int rHist = ( thisThread->history[pos.piece_on(to_sq(move))][to_sq(move)]
1022 + cmh[pos.piece_on(to_sq(move))][to_sq(move)]) / 14980;
1023 r = std::max(DEPTH_ZERO, r - rHist * ONE_PLY);
1025 // Decrease reduction for moves that escape a capture. Filter out
1026 // castling moves, because they are coded as "king captures rook" and
1027 // hence break make_move(). Also use see() instead of see_sign(),
1028 // because the destination square is empty.
1030 && type_of(move) == NORMAL
1031 && type_of(pos.piece_on(to_sq(move))) != PAWN
1032 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
1033 r = std::max(DEPTH_ZERO, r - ONE_PLY);
1035 Depth d = std::max(newDepth - r, ONE_PLY);
1037 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1039 doFullDepthSearch = (value > alpha && r != DEPTH_ZERO);
1042 doFullDepthSearch = !PvNode || moveCount > 1;
1044 // Step 16. Full depth search when LMR is skipped or fails high
1045 if (doFullDepthSearch)
1046 value = newDepth < ONE_PLY ?
1047 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1048 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1049 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1051 // For PV nodes only, do a full PV search on the first move or after a fail
1052 // high (in the latter case search only if value < beta), otherwise let the
1053 // parent node fail low with value <= alpha and try another move.
1054 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1057 (ss+1)->pv[0] = MOVE_NONE;
1059 value = newDepth < ONE_PLY ?
1060 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1061 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1062 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1065 // Step 17. Undo move
1066 pos.undo_move(move);
1068 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1070 // Step 18. Check for a new best move
1071 // Finished searching the move. If a stop occurred, the return value of
1072 // the search cannot be trusted, and we return immediately without
1073 // updating best move, PV and TT.
1074 if (Signals.stop.load(std::memory_order_relaxed))
1079 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1080 thisThread->rootMoves.end(), move);
1082 // PV move or new best move ?
1083 if (moveCount == 1 || value > alpha)
1090 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1091 rm.pv.push_back(*m);
1093 // We record how often the best move has been changed in each
1094 // iteration. This information is used for time management: When
1095 // the best move changes frequently, we allocate some more time.
1096 if (moveCount > 1 && thisThread == Threads.main())
1097 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1100 // All other moves but the PV are set to the lowest value: this is
1101 // not a problem when sorting because the sort is stable and the
1102 // move position in the list is preserved - just the PV is pushed up.
1103 rm.score = -VALUE_INFINITE;
1106 if (value > bestValue)
1112 // If there is an easy move for this position, clear it if unstable
1114 && thisThread == Threads.main()
1115 && EasyMove.get(pos.key())
1116 && (move != EasyMove.get(pos.key()) || moveCount > 1))
1121 if (PvNode && !rootNode) // Update pv even in fail-high case
1122 update_pv(ss->pv, move, (ss+1)->pv);
1124 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1128 assert(value >= beta); // Fail high
1134 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1135 quietsSearched[quietCount++] = move;
1138 // The following condition would detect a stop only after move loop has been
1139 // completed. But in this case bestValue is valid because we have fully
1140 // searched our subtree, and we can anyhow save the result in TT.
1146 // Step 20. Check for mate and stalemate
1147 // All legal moves have been searched and if there are no legal moves, it
1148 // must be a mate or a stalemate. If we are in a singular extension search then
1149 // return a fail low score.
1151 bestValue = excludedMove ? alpha
1152 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1154 // Quiet best move: update killers, history and countermoves
1155 else if (bestMove && !pos.capture_or_promotion(bestMove))
1156 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount);
1158 // Bonus for prior countermove that caused the fail low
1159 else if ( depth >= 3 * ONE_PLY
1162 && !pos.captured_piece_type()
1163 && is_ok((ss - 1)->currentMove)
1164 && is_ok((ss - 2)->currentMove))
1166 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY - 1);
1167 Square prevPrevSq = to_sq((ss - 2)->currentMove);
1168 CounterMoveStats& prevCmh = CounterMoveHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
1169 prevCmh.update(pos.piece_on(prevSq), prevSq, bonus);
1172 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1173 bestValue >= beta ? BOUND_LOWER :
1174 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1175 depth, bestMove, ss->staticEval, TT.generation());
1177 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1183 // qsearch() is the quiescence search function, which is called by the main
1184 // search function when the remaining depth is zero (or, to be more precise,
1185 // less than ONE_PLY).
1187 template <NodeType NT, bool InCheck>
1188 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1190 const bool PvNode = NT == PV;
1192 assert(InCheck == !!pos.checkers());
1193 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1194 assert(PvNode || (alpha == beta - 1));
1195 assert(depth <= DEPTH_ZERO);
1201 Move ttMove, move, bestMove;
1202 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1203 bool ttHit, givesCheck, evasionPrunable;
1208 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1210 ss->pv[0] = MOVE_NONE;
1213 ss->currentMove = bestMove = MOVE_NONE;
1214 ss->ply = (ss-1)->ply + 1;
1216 // Check for an instant draw or if the maximum ply has been reached
1217 if (pos.is_draw() || ss->ply >= MAX_PLY)
1218 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1219 : DrawValue[pos.side_to_move()];
1221 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1223 // Decide whether or not to include checks: this fixes also the type of
1224 // TT entry depth that we are going to use. Note that in qsearch we use
1225 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1226 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1227 : DEPTH_QS_NO_CHECKS;
1229 // Transposition table lookup
1231 tte = TT.probe(posKey, ttHit);
1232 ttMove = ttHit ? tte->move() : MOVE_NONE;
1233 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1237 && tte->depth() >= ttDepth
1238 && ttValue != VALUE_NONE // Only in case of TT access race
1239 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1240 : (tte->bound() & BOUND_UPPER)))
1242 ss->currentMove = ttMove; // Can be MOVE_NONE
1246 // Evaluate the position statically
1249 ss->staticEval = VALUE_NONE;
1250 bestValue = futilityBase = -VALUE_INFINITE;
1256 // Never assume anything on values stored in TT
1257 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1258 ss->staticEval = bestValue = evaluate(pos);
1260 // Can ttValue be used as a better position evaluation?
1261 if (ttValue != VALUE_NONE)
1262 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1263 bestValue = ttValue;
1266 ss->staticEval = bestValue =
1267 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1268 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1270 // Stand pat. Return immediately if static value is at least beta
1271 if (bestValue >= beta)
1274 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1275 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1280 if (PvNode && bestValue > alpha)
1283 futilityBase = bestValue + 128;
1286 // Initialize a MovePicker object for the current position, and prepare
1287 // to search the moves. Because the depth is <= 0 here, only captures,
1288 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1290 MovePicker mp(pos, ttMove, depth, pos.this_thread()->history, to_sq((ss-1)->currentMove));
1293 // Loop through the moves until no moves remain or a beta cutoff occurs
1294 while ((move = mp.next_move()) != MOVE_NONE)
1296 assert(is_ok(move));
1298 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1299 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1300 : pos.gives_check(move, ci);
1305 && futilityBase > -VALUE_KNOWN_WIN
1306 && !pos.advanced_pawn_push(move))
1308 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1310 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1312 if (futilityValue <= alpha)
1314 bestValue = std::max(bestValue, futilityValue);
1318 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1320 bestValue = std::max(bestValue, futilityBase);
1325 // Detect non-capture evasions that are candidates to be pruned
1326 evasionPrunable = InCheck
1327 && bestValue > VALUE_MATED_IN_MAX_PLY
1328 && !pos.capture(move);
1330 // Don't search moves with negative SEE values
1331 if ( (!InCheck || evasionPrunable)
1332 && type_of(move) != PROMOTION
1333 && pos.see_sign(move) < VALUE_ZERO)
1336 // Speculative prefetch as early as possible
1337 prefetch(TT.first_entry(pos.key_after(move)));
1339 // Check for legality just before making the move
1340 if (!pos.legal(move, ci.pinned))
1343 ss->currentMove = move;
1345 // Make and search the move
1346 pos.do_move(move, st, givesCheck);
1347 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1348 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1349 pos.undo_move(move);
1351 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1353 // Check for a new best move
1354 if (value > bestValue)
1360 if (PvNode) // Update pv even in fail-high case
1361 update_pv(ss->pv, move, (ss+1)->pv);
1363 if (PvNode && value < beta) // Update alpha here!
1370 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1371 ttDepth, move, ss->staticEval, TT.generation());
1379 // All legal moves have been searched. A special case: If we're in check
1380 // and no legal moves were found, it is checkmate.
1381 if (InCheck && bestValue == -VALUE_INFINITE)
1382 return mated_in(ss->ply); // Plies to mate from the root
1384 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1385 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1386 ttDepth, bestMove, ss->staticEval, TT.generation());
1388 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1394 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1395 // "plies to mate from the current position". Non-mate scores are unchanged.
1396 // The function is called before storing a value in the transposition table.
1398 Value value_to_tt(Value v, int ply) {
1400 assert(v != VALUE_NONE);
1402 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1403 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1407 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1408 // from the transposition table (which refers to the plies to mate/be mated
1409 // from current position) to "plies to mate/be mated from the root".
1411 Value value_from_tt(Value v, int ply) {
1413 return v == VALUE_NONE ? VALUE_NONE
1414 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1415 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1419 // update_pv() adds current move and appends child pv[]
1421 void update_pv(Move* pv, Move move, Move* childPv) {
1423 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1429 // update_stats() updates killers, history, countermove and countermove
1430 // history when a new quiet best move is found.
1432 void update_stats(const Position& pos, Stack* ss, Move move,
1433 Depth depth, Move* quiets, int quietsCnt) {
1435 if (ss->killers[0] != move)
1437 ss->killers[1] = ss->killers[0];
1438 ss->killers[0] = move;
1441 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY - 1);
1443 Square prevSq = to_sq((ss-1)->currentMove);
1444 CounterMoveStats& cmh = CounterMoveHistory[pos.piece_on(prevSq)][prevSq];
1445 Thread* thisThread = pos.this_thread();
1447 thisThread->history.update(pos.moved_piece(move), to_sq(move), bonus);
1449 if (is_ok((ss-1)->currentMove))
1451 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1452 cmh.update(pos.moved_piece(move), to_sq(move), bonus);
1455 // Decrease all the other played quiet moves
1456 for (int i = 0; i < quietsCnt; ++i)
1458 thisThread->history.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1460 if (is_ok((ss-1)->currentMove))
1461 cmh.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1464 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1465 if ( (ss-1)->moveCount == 1
1466 && !pos.captured_piece_type()
1467 && is_ok((ss-2)->currentMove))
1469 Square prevPrevSq = to_sq((ss-2)->currentMove);
1470 CounterMoveStats& prevCmh = CounterMoveHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
1471 prevCmh.update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY);
1476 // When playing with strength handicap, choose best move among a set of RootMoves
1477 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1479 Move Skill::pick_best(size_t multiPV) {
1481 const Search::RootMoveVector& rootMoves = Threads.main()->rootMoves;
1482 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1484 // RootMoves are already sorted by score in descending order
1485 Value topScore = rootMoves[0].score;
1486 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1487 int weakness = 120 - 2 * level;
1488 int maxScore = -VALUE_INFINITE;
1490 // Choose best move. For each move score we add two terms, both dependent on
1491 // weakness. One is deterministic and bigger for weaker levels, and one is
1492 // random. Then we choose the move with the resulting highest score.
1493 for (size_t i = 0; i < multiPV; ++i)
1495 // This is our magic formula
1496 int push = ( weakness * int(topScore - rootMoves[i].score)
1497 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1499 if (rootMoves[i].score + push > maxScore)
1501 maxScore = rootMoves[i].score + push;
1502 best = rootMoves[i].pv[0];
1510 // check_time() is used to print debug info and, more importantly, to detect
1511 // when we are out of available time and thus stop the search.
1515 static TimePoint lastInfoTime = now();
1517 int elapsed = Time.elapsed();
1518 TimePoint tick = Limits.startTime + elapsed;
1520 if (tick - lastInfoTime >= 1000)
1522 lastInfoTime = tick;
1526 // An engine may not stop pondering until told so by the GUI
1530 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1531 || (Limits.movetime && elapsed >= Limits.movetime)
1532 || (Limits.nodes && Threads.nodes_searched() >= Limits.nodes))
1533 Signals.stop = true;
1539 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1540 /// that all (if any) unsearched PV lines are sent using a previous search score.
1542 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1544 std::stringstream ss;
1545 int elapsed = Time.elapsed() + 1;
1546 const Search::RootMoveVector& rootMoves = pos.this_thread()->rootMoves;
1547 size_t PVIdx = pos.this_thread()->PVIdx;
1548 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1549 uint64_t nodes_searched = Threads.nodes_searched();
1551 for (size_t i = 0; i < multiPV; ++i)
1553 bool updated = (i <= PVIdx);
1555 if (depth == ONE_PLY && !updated)
1558 Depth d = updated ? depth : depth - ONE_PLY;
1559 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1561 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1562 v = tb ? TB::Score : v;
1564 if (ss.rdbuf()->in_avail()) // Not at first line
1568 << " depth " << d / ONE_PLY
1569 << " seldepth " << pos.this_thread()->maxPly
1570 << " multipv " << i + 1
1571 << " score " << UCI::value(v);
1573 if (!tb && i == PVIdx)
1574 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1576 ss << " nodes " << nodes_searched
1577 << " nps " << nodes_searched * 1000 / elapsed;
1579 if (elapsed > 1000) // Earlier makes little sense
1580 ss << " hashfull " << TT.hashfull();
1582 ss << " tbhits " << TB::Hits
1583 << " time " << elapsed
1586 for (Move m : rootMoves[i].pv)
1587 ss << " " << UCI::move(m, pos.is_chess960());
1594 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1595 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1596 /// first, even if the old TT entries have been overwritten.
1598 void RootMove::insert_pv_in_tt(Position& pos) {
1600 StateInfo state[MAX_PLY], *st = state;
1605 assert(MoveList<LEGAL>(pos).contains(m));
1607 TTEntry* tte = TT.probe(pos.key(), ttHit);
1609 if (!ttHit || tte->move() != m) // Don't overwrite correct entries
1610 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE,
1611 m, VALUE_NONE, TT.generation());
1613 pos.do_move(m, *st++, pos.gives_check(m, CheckInfo(pos)));
1616 for (size_t i = pv.size(); i > 0; )
1617 pos.undo_move(pv[--i]);
1621 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1622 /// before exiting the search, for instance, in case we stop the search during a
1623 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1624 /// otherwise in case of 'ponder on' we have nothing to think on.
1626 bool RootMove::extract_ponder_from_tt(Position& pos)
1631 assert(pv.size() == 1);
1633 pos.do_move(pv[0], st, pos.gives_check(pv[0], CheckInfo(pos)));
1634 TTEntry* tte = TT.probe(pos.key(), ttHit);
1635 pos.undo_move(pv[0]);
1639 Move m = tte->move(); // Local copy to be SMP safe
1640 if (MoveList<LEGAL>(pos).contains(m))
1641 return pv.push_back(m), true;