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(150 * d / ONE_PLY); }
70 // Futility and reductions lookup tables, initialized at startup
71 int FutilityMoveCounts[2][16]; // [improving][depth]
72 int 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 / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
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]));
117 pos.do_move(newPv[1], st[1], pos.gives_check(newPv[1]));
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_cm_stats(Stack* ss, Piece pc, Square s, Value bonus);
172 void update_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, Value bonus);
178 /// Search::init() is called during startup to initialize various lookup tables
180 void Search::init() {
182 for (int imp = 0; imp <= 1; ++imp)
183 for (int d = 1; d < 64; ++d)
184 for (int mc = 1; mc < 64; ++mc)
186 double r = log(d) * log(mc) / 2;
190 Reductions[NonPV][imp][d][mc] = int(std::round(r));
191 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
193 // Increase reduction for non-PV nodes when eval is not improving
194 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
195 Reductions[NonPV][imp][d][mc]++;
198 for (int d = 0; d < 16; ++d)
200 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
201 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
206 /// Search::clear() resets search state to zero, to obtain reproducible results
208 void Search::clear() {
211 CounterMoveHistory.clear();
213 for (Thread* th : Threads)
216 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;
231 const bool leaf = (depth == 2 * ONE_PLY);
233 for (const auto& m : MoveList<LEGAL>(pos))
235 if (Root && depth <= ONE_PLY)
239 pos.do_move(m, st, pos.gives_check(m));
240 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
245 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
250 template uint64_t Search::perft<true>(Position&, Depth);
253 /// MainThread::search() is called by the main thread when the program receives
254 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
256 void MainThread::search() {
258 Color us = rootPos.side_to_move();
259 Time.init(Limits, us, rootPos.game_ply());
261 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
262 DrawValue[ us] = VALUE_DRAW - Value(contempt);
263 DrawValue[~us] = VALUE_DRAW + Value(contempt);
265 if (rootMoves.empty())
267 rootMoves.push_back(RootMove(MOVE_NONE));
268 sync_cout << "info depth 0 score "
269 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
274 for (Thread* th : Threads)
276 th->start_searching();
278 Thread::search(); // Let's start searching!
281 // When playing in 'nodes as time' mode, subtract the searched nodes from
282 // the available ones before exiting.
284 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
286 // When we reach the maximum depth, we can arrive here without a raise of
287 // Signals.stop. However, if we are pondering or in an infinite search,
288 // the UCI protocol states that we shouldn't print the best move before the
289 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
290 // until the GUI sends one of those commands (which also raises Signals.stop).
291 if (!Signals.stop && (Limits.ponder || Limits.infinite))
293 Signals.stopOnPonderhit = true;
297 // Stop the threads if not already stopped
300 // Wait until all threads have finished
301 for (Thread* th : Threads)
303 th->wait_for_search_finished();
305 // Check if there are threads with a better score than main thread
306 Thread* bestThread = this;
307 if ( !this->easyMovePlayed
308 && Options["MultiPV"] == 1
310 && !Skill(Options["Skill Level"]).enabled()
311 && rootMoves[0].pv[0] != MOVE_NONE)
313 for (Thread* th : Threads)
314 if ( th->completedDepth > bestThread->completedDepth
315 && th->rootMoves[0].score > bestThread->rootMoves[0].score)
319 previousScore = bestThread->rootMoves[0].score;
321 // Send new PV when needed
322 if (bestThread != this)
323 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
325 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
327 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
328 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
330 std::cout << sync_endl;
334 // Thread::search() is the main iterative deepening loop. It calls search()
335 // repeatedly with increasing depth until the allocated thinking time has been
336 // consumed, the user stops the search, or the maximum search depth is reached.
338 void Thread::search() {
340 Stack stack[MAX_PLY+7], *ss = stack+5; // To allow referencing (ss-5) and (ss+2)
341 Value bestValue, alpha, beta, delta;
342 Move easyMove = MOVE_NONE;
343 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
345 std::memset(ss-5, 0, 8 * sizeof(Stack));
347 bestValue = delta = alpha = -VALUE_INFINITE;
348 beta = VALUE_INFINITE;
349 completedDepth = DEPTH_ZERO;
353 easyMove = EasyMove.get(rootPos.key());
355 mainThread->easyMovePlayed = mainThread->failedLow = false;
356 mainThread->bestMoveChanges = 0;
360 size_t multiPV = Options["MultiPV"];
361 Skill skill(Options["Skill Level"]);
363 // When playing with strength handicap enable MultiPV search that we will
364 // use behind the scenes to retrieve a set of possible moves.
366 multiPV = std::max(multiPV, (size_t)4);
368 multiPV = std::min(multiPV, rootMoves.size());
370 // Iterative deepening loop until requested to stop or the target depth is reached
371 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
373 && (!Limits.depth || Threads.main()->rootDepth / ONE_PLY <= Limits.depth))
375 // Set up the new depths for the helper threads skipping on average every
376 // 2nd ply (using a half-density matrix).
379 const Row& row = HalfDensity[(idx - 1) % HalfDensitySize];
380 if (row[(rootDepth / ONE_PLY + rootPos.game_ply()) % row.size()])
384 // Age out PV variability metric
386 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
388 // Save the last iteration's scores before first PV line is searched and
389 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
390 for (RootMove& rm : rootMoves)
391 rm.previousScore = rm.score;
393 // MultiPV loop. We perform a full root search for each PV line
394 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
396 // Reset aspiration window starting size
397 if (rootDepth >= 5 * ONE_PLY)
400 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
401 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
404 // Start with a small aspiration window and, in the case of a fail
405 // high/low, re-search with a bigger window until we're not failing
409 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false);
411 // Bring the best move to the front. It is critical that sorting
412 // is done with a stable algorithm because all the values but the
413 // first and eventually the new best one are set to -VALUE_INFINITE
414 // and we want to keep the same order for all the moves except the
415 // new PV that goes to the front. Note that in case of MultiPV
416 // search the already searched PV lines are preserved.
417 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
419 // If search has been stopped, break immediately. Sorting and
420 // writing PV back to TT is safe because RootMoves is still
421 // valid, although it refers to the previous iteration.
425 // When failing high/low give some update (without cluttering
426 // the UI) before a re-search.
429 && (bestValue <= alpha || bestValue >= beta)
430 && Time.elapsed() > 3000)
431 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
433 // In case of failing low/high increase aspiration window and
434 // re-search, otherwise exit the loop.
435 if (bestValue <= alpha)
437 beta = (alpha + beta) / 2;
438 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
442 mainThread->failedLow = true;
443 Signals.stopOnPonderhit = false;
446 else if (bestValue >= beta)
448 alpha = (alpha + beta) / 2;
449 beta = std::min(bestValue + delta, VALUE_INFINITE);
454 delta += delta / 4 + 5;
456 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
459 // Sort the PV lines searched so far and update the GUI
460 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
466 sync_cout << "info nodes " << Threads.nodes_searched()
467 << " time " << Time.elapsed() << sync_endl;
469 else if (PVIdx + 1 == multiPV || Time.elapsed() > 3000)
470 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
474 completedDepth = rootDepth;
479 // If skill level is enabled and time is up, pick a sub-optimal best move
480 if (skill.enabled() && skill.time_to_pick(rootDepth))
481 skill.pick_best(multiPV);
483 // Have we found a "mate in x"?
485 && bestValue >= VALUE_MATE_IN_MAX_PLY
486 && VALUE_MATE - bestValue <= 2 * Limits.mate)
489 // Do we have time for the next iteration? Can we stop searching now?
490 if (Limits.use_time_management())
492 if (!Signals.stop && !Signals.stopOnPonderhit)
494 // Stop the search if only one legal move is available, or if all
495 // of the available time has been used, or if we matched an easyMove
496 // from the previous search and just did a fast verification.
497 const int F[] = { mainThread->failedLow,
498 bestValue - mainThread->previousScore };
500 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
501 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
503 bool doEasyMove = rootMoves[0].pv[0] == easyMove
504 && mainThread->bestMoveChanges < 0.03
505 && Time.elapsed() > Time.optimum() * 5 / 42;
507 if ( rootMoves.size() == 1
508 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628
509 || (mainThread->easyMovePlayed = doEasyMove))
511 // If we are allowed to ponder do not stop the search now but
512 // keep pondering until the GUI sends "ponderhit" or "stop".
514 Signals.stopOnPonderhit = true;
520 if (rootMoves[0].pv.size() >= 3)
521 EasyMove.update(rootPos, rootMoves[0].pv);
530 // Clear any candidate easy move that wasn't stable for the last search
531 // iterations; the second condition prevents consecutive fast moves.
532 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
535 // If skill level is enabled, swap best PV line with the sub-optimal one
537 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
538 rootMoves.end(), skill.best_move(multiPV)));
544 // search<>() is the main search function for both PV and non-PV nodes
546 template <NodeType NT>
547 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
549 const bool PvNode = NT == PV;
550 const bool rootNode = PvNode && (ss-1)->ply == 0;
552 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
553 assert(PvNode || (alpha == beta - 1));
554 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
555 assert(!(PvNode && cutNode));
556 assert(depth / ONE_PLY * ONE_PLY == depth);
558 Move pv[MAX_PLY+1], quietsSearched[64];
562 Move ttMove, move, excludedMove, bestMove;
563 Depth extension, newDepth, predictedDepth;
564 Value bestValue, value, ttValue, eval, nullValue;
565 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
566 bool captureOrPromotion, doFullDepthSearch, moveCountPruning;
568 int moveCount, quietCount;
570 // Step 1. Initialize node
571 Thread* thisThread = pos.this_thread();
572 inCheck = pos.checkers();
573 moveCount = quietCount = ss->moveCount = 0;
574 bestValue = -VALUE_INFINITE;
575 ss->ply = (ss-1)->ply + 1;
577 // Check for the available remaining time
578 if (thisThread->resetCalls.load(std::memory_order_relaxed))
580 thisThread->resetCalls = false;
581 thisThread->callsCnt = 0;
583 if (++thisThread->callsCnt > 4096)
585 for (Thread* th : Threads)
586 th->resetCalls = true;
591 // Used to send selDepth info to GUI
592 if (PvNode && thisThread->maxPly < ss->ply)
593 thisThread->maxPly = ss->ply;
597 // Step 2. Check for aborted search and immediate draw
598 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw() || ss->ply >= MAX_PLY)
599 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
600 : DrawValue[pos.side_to_move()];
602 // Step 3. Mate distance pruning. Even if we mate at the next move our score
603 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
604 // a shorter mate was found upward in the tree then there is no need to search
605 // because we will never beat the current alpha. Same logic but with reversed
606 // signs applies also in the opposite condition of being mated instead of giving
607 // mate. In this case return a fail-high score.
608 alpha = std::max(mated_in(ss->ply), alpha);
609 beta = std::min(mate_in(ss->ply+1), beta);
614 assert(0 <= ss->ply && ss->ply < MAX_PLY);
616 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
617 ss->counterMoves = nullptr;
618 (ss+1)->skipEarlyPruning = false;
619 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
621 // Step 4. Transposition table lookup. We don't want the score of a partial
622 // search to overwrite a previous full search TT value, so we use a different
623 // position key in case of an excluded move.
624 excludedMove = ss->excludedMove;
625 posKey = pos.key() ^ Key(excludedMove);
626 tte = TT.probe(posKey, ttHit);
627 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
628 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
629 : ttHit ? tte->move() : MOVE_NONE;
631 // At non-PV nodes we check for an early TT cutoff
634 && tte->depth() >= depth
635 && ttValue != VALUE_NONE // Possible in case of TT access race
636 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
637 : (tte->bound() & BOUND_UPPER)))
639 ss->currentMove = ttMove; // Can be MOVE_NONE
641 // If ttMove is quiet, update killers, history, counter move on TT hit
642 if (ttValue >= beta && ttMove)
644 int d = depth / ONE_PLY;
646 if (!pos.capture_or_promotion(ttMove))
648 Value bonus = Value(d * d + 2 * d - 2);
649 update_stats(pos, ss, ttMove, nullptr, 0, bonus);
652 // Extra penalty for a quiet TT move in previous ply when it gets refuted
653 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
655 Value penalty = Value(d * d + 4 * d + 1);
656 Square prevSq = to_sq((ss-1)->currentMove);
657 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -penalty);
663 // Step 4a. Tablebase probe
664 if (!rootNode && TB::Cardinality)
666 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
668 if ( piecesCnt <= TB::Cardinality
669 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
670 && pos.rule50_count() == 0
671 && !pos.can_castle(ANY_CASTLING))
673 int found, v = Tablebases::probe_wdl(pos, &found);
679 int drawScore = TB::UseRule50 ? 1 : 0;
681 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
682 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
683 : VALUE_DRAW + 2 * v * drawScore;
685 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
686 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
687 MOVE_NONE, VALUE_NONE, TT.generation());
694 // Step 5. Evaluate the position statically
697 ss->staticEval = eval = VALUE_NONE;
703 // Never assume anything on values stored in TT
704 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
705 eval = ss->staticEval = evaluate(pos);
707 // Can ttValue be used as a better position evaluation?
708 if (ttValue != VALUE_NONE)
709 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
714 eval = ss->staticEval =
715 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
716 : -(ss-1)->staticEval + 2 * Eval::Tempo;
718 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
719 ss->staticEval, TT.generation());
722 if (ss->skipEarlyPruning)
725 // Step 6. Razoring (skipped when in check)
727 && depth < 4 * ONE_PLY
728 && ttMove == MOVE_NONE
729 && eval + razor_margin[depth / ONE_PLY] <= alpha)
731 if ( depth <= ONE_PLY
732 && eval + razor_margin[3 * ONE_PLY] <= alpha)
733 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
735 Value ralpha = alpha - razor_margin[depth / ONE_PLY];
736 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
741 // Step 7. Futility pruning: child node (skipped when in check)
743 && depth < 7 * ONE_PLY
744 && eval - futility_margin(depth) >= beta
745 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
746 && pos.non_pawn_material(pos.side_to_move()))
747 return eval - futility_margin(depth);
749 // Step 8. Null move search with verification search (is omitted in PV nodes)
752 && (ss->staticEval >= beta - 35 * (depth / ONE_PLY - 6) || depth >= 13 * ONE_PLY)
753 && pos.non_pawn_material(pos.side_to_move()))
755 ss->currentMove = MOVE_NULL;
756 ss->counterMoves = nullptr;
758 assert(eval - beta >= 0);
760 // Null move dynamic reduction based on depth and value
761 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
763 pos.do_null_move(st);
764 (ss+1)->skipEarlyPruning = true;
765 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
766 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
767 (ss+1)->skipEarlyPruning = false;
768 pos.undo_null_move();
770 if (nullValue >= beta)
772 // Do not return unproven mate scores
773 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
776 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
779 // Do verification search at high depths
780 ss->skipEarlyPruning = true;
781 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
782 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
783 ss->skipEarlyPruning = false;
790 // Step 9. ProbCut (skipped when in check)
791 // If we have a good enough capture and a reduced search returns a value
792 // much above beta, we can (almost) safely prune the previous move.
794 && depth >= 5 * ONE_PLY
795 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
797 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
798 Depth rdepth = depth - 4 * ONE_PLY;
800 assert(rdepth >= ONE_PLY);
801 assert((ss-1)->currentMove != MOVE_NONE);
802 assert((ss-1)->currentMove != MOVE_NULL);
804 MovePicker mp(pos, ttMove, rbeta - ss->staticEval);
806 while ((move = mp.next_move()) != MOVE_NONE)
809 ss->currentMove = move;
810 ss->counterMoves = &CounterMoveHistory[pos.moved_piece(move)][to_sq(move)];
811 pos.do_move(move, st, pos.gives_check(move));
812 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
819 // Step 10. Internal iterative deepening (skipped when in check)
820 if ( depth >= 6 * ONE_PLY
822 && (PvNode || ss->staticEval + 256 >= beta))
824 Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
825 ss->skipEarlyPruning = true;
826 search<NT>(pos, ss, alpha, beta, d, cutNode);
827 ss->skipEarlyPruning = false;
829 tte = TT.probe(posKey, ttHit);
830 ttMove = ttHit ? tte->move() : MOVE_NONE;
833 moves_loop: // When in check search starts from here
835 const CounterMoveStats* cmh = (ss-1)->counterMoves;
836 const CounterMoveStats* fmh = (ss-2)->counterMoves;
837 const CounterMoveStats* fmh2 = (ss-4)->counterMoves;
839 MovePicker mp(pos, ttMove, depth, ss);
840 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
841 improving = ss->staticEval >= (ss-2)->staticEval
842 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
843 ||(ss-2)->staticEval == VALUE_NONE;
845 singularExtensionNode = !rootNode
846 && depth >= 8 * ONE_PLY
847 && ttMove != MOVE_NONE
848 /* && ttValue != VALUE_NONE Already implicit in the next condition */
849 && abs(ttValue) < VALUE_KNOWN_WIN
850 && !excludedMove // Recursive singular search is not allowed
851 && (tte->bound() & BOUND_LOWER)
852 && tte->depth() >= depth - 3 * ONE_PLY;
854 // Step 11. Loop through moves
855 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
856 while ((move = mp.next_move()) != MOVE_NONE)
860 if (move == excludedMove)
863 // At root obey the "searchmoves" option and skip moves not listed in Root
864 // Move List. As a consequence any illegal move is also skipped. In MultiPV
865 // mode we also skip PV moves which have been already searched.
866 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
867 thisThread->rootMoves.end(), move))
870 ss->moveCount = ++moveCount;
872 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
873 sync_cout << "info depth " << depth / ONE_PLY
874 << " currmove " << UCI::move(move, pos.is_chess960())
875 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
878 (ss+1)->pv = nullptr;
880 extension = DEPTH_ZERO;
881 captureOrPromotion = pos.capture_or_promotion(move);
882 moved_piece = pos.moved_piece(move);
884 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
885 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
886 : pos.gives_check(move);
888 moveCountPruning = depth < 16 * ONE_PLY
889 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
891 // Step 12. Extend checks
894 && pos.see_sign(move) >= VALUE_ZERO)
897 // Singular extension search. If all moves but one fail low on a search of
898 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
899 // is singular and should be extended. To verify this we do a reduced search
900 // on all the other moves but the ttMove and if the result is lower than
901 // ttValue minus a margin then we extend the ttMove.
902 if ( singularExtensionNode
907 Value rBeta = ttValue - 2 * depth / ONE_PLY;
908 Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
909 ss->excludedMove = move;
910 ss->skipEarlyPruning = true;
911 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode);
912 ss->skipEarlyPruning = false;
913 ss->excludedMove = MOVE_NONE;
919 // Update the current move (this must be done after singular extension search)
920 newDepth = depth - ONE_PLY + extension;
922 // Step 13. Pruning at shallow depth
924 && !captureOrPromotion
927 && bestValue > VALUE_MATED_IN_MAX_PLY
928 && !pos.advanced_pawn_push(move))
930 // Move count based pruning
931 if (moveCountPruning)
934 predictedDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO);
936 // Countermoves based pruning
937 if ( predictedDepth < 3 * ONE_PLY
938 && (!cmh || (*cmh )[moved_piece][to_sq(move)] < VALUE_ZERO)
939 && (!fmh || (*fmh )[moved_piece][to_sq(move)] < VALUE_ZERO)
940 && (!fmh2 || (*fmh2)[moved_piece][to_sq(move)] < VALUE_ZERO || (cmh && fmh)))
943 // Futility pruning: parent node
944 if ( predictedDepth < 7 * ONE_PLY
945 && ss->staticEval + 256 + 200 * predictedDepth / ONE_PLY <= alpha)
948 // Prune moves with negative SEE at low depths and below a decreasing
949 // threshold at higher depths.
950 if (predictedDepth < 8 * ONE_PLY)
952 Value see_v = predictedDepth < 4 * ONE_PLY ? VALUE_ZERO
953 : -PawnValueMg * 2 * int(predictedDepth - 3 * ONE_PLY) / ONE_PLY;
955 if (pos.see_sign(move) < see_v)
960 // Speculative prefetch as early as possible
961 prefetch(TT.first_entry(pos.key_after(move)));
963 // Check for legality just before making the move
964 if (!rootNode && !pos.legal(move))
966 ss->moveCount = --moveCount;
970 ss->currentMove = move;
971 ss->counterMoves = &CounterMoveHistory[moved_piece][to_sq(move)];
973 // Step 14. Make the move
974 pos.do_move(move, st, givesCheck);
976 // Step 15. Reduced depth search (LMR). If the move fails high it will be
977 // re-searched at full depth.
978 if ( depth >= 3 * ONE_PLY
980 && (!captureOrPromotion || moveCountPruning))
982 Depth r = reduction<PvNode>(improving, depth, moveCount);
984 if (captureOrPromotion)
985 r -= r ? ONE_PLY : DEPTH_ZERO;
988 // Increase reduction for cut nodes
992 // Decrease reduction for moves that escape a capture. Filter out
993 // castling moves, because they are coded as "king captures rook" and
994 // hence break make_move(). Also use see() instead of see_sign(),
995 // because the destination square is empty.
996 else if ( type_of(move) == NORMAL
997 && type_of(pos.piece_on(to_sq(move))) != PAWN
998 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
1001 // Decrease/increase reduction for moves with a good/bad history
1002 Value val = thisThread->history[moved_piece][to_sq(move)]
1003 + (cmh ? (*cmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
1004 + (fmh ? (*fmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
1005 + (fmh2 ? (*fmh2)[moved_piece][to_sq(move)] : VALUE_ZERO)
1006 + thisThread->fromTo.get(~pos.side_to_move(), move);
1007 int rHist = (val - 8000) / 20000;
1008 r = std::max(DEPTH_ZERO, (r / ONE_PLY - rHist) * ONE_PLY);
1011 Depth d = std::max(newDepth - r, ONE_PLY);
1013 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1015 doFullDepthSearch = (value > alpha && d != newDepth);
1018 doFullDepthSearch = !PvNode || moveCount > 1;
1020 // Step 16. Full depth search when LMR is skipped or fails high
1021 if (doFullDepthSearch)
1022 value = newDepth < ONE_PLY ?
1023 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1024 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1025 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1027 // For PV nodes only, do a full PV search on the first move or after a fail
1028 // high (in the latter case search only if value < beta), otherwise let the
1029 // parent node fail low with value <= alpha and try another move.
1030 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1033 (ss+1)->pv[0] = MOVE_NONE;
1035 value = newDepth < ONE_PLY ?
1036 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1037 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1038 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1041 // Step 17. Undo move
1042 pos.undo_move(move);
1044 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1046 // Step 18. Check for a new best move
1047 // Finished searching the move. If a stop occurred, the return value of
1048 // the search cannot be trusted, and we return immediately without
1049 // updating best move, PV and TT.
1050 if (Signals.stop.load(std::memory_order_relaxed))
1055 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1056 thisThread->rootMoves.end(), move);
1058 // PV move or new best move ?
1059 if (moveCount == 1 || value > alpha)
1066 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1067 rm.pv.push_back(*m);
1069 // We record how often the best move has been changed in each
1070 // iteration. This information is used for time management: When
1071 // the best move changes frequently, we allocate some more time.
1072 if (moveCount > 1 && thisThread == Threads.main())
1073 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1076 // All other moves but the PV are set to the lowest value: this is
1077 // not a problem when sorting because the sort is stable and the
1078 // move position in the list is preserved - just the PV is pushed up.
1079 rm.score = -VALUE_INFINITE;
1082 if (value > bestValue)
1088 // If there is an easy move for this position, clear it if unstable
1090 && thisThread == Threads.main()
1091 && EasyMove.get(pos.key())
1092 && (move != EasyMove.get(pos.key()) || moveCount > 1))
1097 if (PvNode && !rootNode) // Update pv even in fail-high case
1098 update_pv(ss->pv, move, (ss+1)->pv);
1100 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1104 assert(value >= beta); // Fail high
1110 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1111 quietsSearched[quietCount++] = move;
1114 // The following condition would detect a stop only after move loop has been
1115 // completed. But in this case bestValue is valid because we have fully
1116 // searched our subtree, and we can anyhow save the result in TT.
1122 // Step 20. Check for mate and stalemate
1123 // All legal moves have been searched and if there are no legal moves, it
1124 // must be a mate or a stalemate. If we are in a singular extension search then
1125 // return a fail low score.
1127 bestValue = excludedMove ? alpha
1128 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1131 int d = depth / ONE_PLY;
1133 // Quiet best move: update killers, history and countermoves
1134 if (!pos.capture_or_promotion(bestMove))
1136 Value bonus = Value(d * d + 2 * d - 2);
1137 update_stats(pos, ss, bestMove, quietsSearched, quietCount, bonus);
1140 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1141 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1143 Value penalty = Value(d * d + 4 * d + 1);
1144 Square prevSq = to_sq((ss-1)->currentMove);
1145 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -penalty);
1148 // Bonus for prior countermove that caused the fail low
1149 else if ( depth >= 3 * ONE_PLY
1150 && !pos.captured_piece()
1151 && is_ok((ss-1)->currentMove))
1153 int d = depth / ONE_PLY;
1154 Value bonus = Value(d * d + 2 * d - 2);
1155 Square prevSq = to_sq((ss-1)->currentMove);
1156 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, bonus);
1159 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1160 bestValue >= beta ? BOUND_LOWER :
1161 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1162 depth, bestMove, ss->staticEval, TT.generation());
1164 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1170 // qsearch() is the quiescence search function, which is called by the main
1171 // search function when the remaining depth is zero (or, to be more precise,
1172 // less than ONE_PLY).
1174 template <NodeType NT, bool InCheck>
1175 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1177 const bool PvNode = NT == PV;
1179 assert(InCheck == !!pos.checkers());
1180 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1181 assert(PvNode || (alpha == beta - 1));
1182 assert(depth <= DEPTH_ZERO);
1183 assert(depth / ONE_PLY * ONE_PLY == depth);
1189 Move ttMove, move, bestMove;
1190 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1191 bool ttHit, givesCheck, evasionPrunable;
1196 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1198 ss->pv[0] = MOVE_NONE;
1201 ss->currentMove = bestMove = MOVE_NONE;
1202 ss->ply = (ss-1)->ply + 1;
1204 // Check for an instant draw or if the maximum ply has been reached
1205 if (pos.is_draw() || ss->ply >= MAX_PLY)
1206 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1207 : DrawValue[pos.side_to_move()];
1209 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1211 // Decide whether or not to include checks: this fixes also the type of
1212 // TT entry depth that we are going to use. Note that in qsearch we use
1213 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1214 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1215 : DEPTH_QS_NO_CHECKS;
1217 // Transposition table lookup
1219 tte = TT.probe(posKey, ttHit);
1220 ttMove = ttHit ? tte->move() : MOVE_NONE;
1221 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1225 && tte->depth() >= ttDepth
1226 && ttValue != VALUE_NONE // Only in case of TT access race
1227 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1228 : (tte->bound() & BOUND_UPPER)))
1230 ss->currentMove = ttMove; // Can be MOVE_NONE
1234 // Evaluate the position statically
1237 ss->staticEval = VALUE_NONE;
1238 bestValue = futilityBase = -VALUE_INFINITE;
1244 // Never assume anything on values stored in TT
1245 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1246 ss->staticEval = bestValue = evaluate(pos);
1248 // Can ttValue be used as a better position evaluation?
1249 if (ttValue != VALUE_NONE)
1250 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1251 bestValue = ttValue;
1254 ss->staticEval = bestValue =
1255 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1256 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1258 // Stand pat. Return immediately if static value is at least beta
1259 if (bestValue >= beta)
1262 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1263 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1268 if (PvNode && bestValue > alpha)
1271 futilityBase = bestValue + 128;
1274 // Initialize a MovePicker object for the current position, and prepare
1275 // to search the moves. Because the depth is <= 0 here, only captures,
1276 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1278 MovePicker mp(pos, ttMove, depth, to_sq((ss-1)->currentMove));
1280 // Loop through the moves until no moves remain or a beta cutoff occurs
1281 while ((move = mp.next_move()) != MOVE_NONE)
1283 assert(is_ok(move));
1285 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
1286 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
1287 : pos.gives_check(move);
1292 && futilityBase > -VALUE_KNOWN_WIN
1293 && !pos.advanced_pawn_push(move))
1295 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1297 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1299 if (futilityValue <= alpha)
1301 bestValue = std::max(bestValue, futilityValue);
1305 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1307 bestValue = std::max(bestValue, futilityBase);
1312 // Detect non-capture evasions that are candidates to be pruned
1313 evasionPrunable = InCheck
1314 && bestValue > VALUE_MATED_IN_MAX_PLY
1315 && !pos.capture(move);
1317 // Don't search moves with negative SEE values
1318 if ( (!InCheck || evasionPrunable)
1319 && type_of(move) != PROMOTION
1320 && pos.see_sign(move) < VALUE_ZERO)
1323 // Speculative prefetch as early as possible
1324 prefetch(TT.first_entry(pos.key_after(move)));
1326 // Check for legality just before making the move
1327 if (!pos.legal(move))
1330 ss->currentMove = move;
1332 // Make and search the move
1333 pos.do_move(move, st, givesCheck);
1334 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1335 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1336 pos.undo_move(move);
1338 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1340 // Check for a new best move
1341 if (value > bestValue)
1347 if (PvNode) // Update pv even in fail-high case
1348 update_pv(ss->pv, move, (ss+1)->pv);
1350 if (PvNode && value < beta) // Update alpha here!
1357 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1358 ttDepth, move, ss->staticEval, TT.generation());
1366 // All legal moves have been searched. A special case: If we're in check
1367 // and no legal moves were found, it is checkmate.
1368 if (InCheck && bestValue == -VALUE_INFINITE)
1369 return mated_in(ss->ply); // Plies to mate from the root
1371 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1372 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1373 ttDepth, bestMove, ss->staticEval, TT.generation());
1375 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1381 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1382 // "plies to mate from the current position". Non-mate scores are unchanged.
1383 // The function is called before storing a value in the transposition table.
1385 Value value_to_tt(Value v, int ply) {
1387 assert(v != VALUE_NONE);
1389 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1390 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1394 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1395 // from the transposition table (which refers to the plies to mate/be mated
1396 // from current position) to "plies to mate/be mated from the root".
1398 Value value_from_tt(Value v, int ply) {
1400 return v == VALUE_NONE ? VALUE_NONE
1401 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1402 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1406 // update_pv() adds current move and appends child pv[]
1408 void update_pv(Move* pv, Move move, Move* childPv) {
1410 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1416 // update_cm_stats() updates countermove and follow-up move history
1418 void update_cm_stats(Stack* ss, Piece pc, Square s, Value bonus) {
1420 CounterMoveStats* cmh = (ss-1)->counterMoves;
1421 CounterMoveStats* fmh1 = (ss-2)->counterMoves;
1422 CounterMoveStats* fmh2 = (ss-4)->counterMoves;
1425 cmh->update(pc, s, bonus);
1428 fmh1->update(pc, s, bonus);
1431 fmh2->update(pc, s, bonus);
1435 // update_stats() updates killers, history, countermove and countermove plus
1436 // follow-up move history when a new quiet best move is found.
1438 void update_stats(const Position& pos, Stack* ss, Move move,
1439 Move* quiets, int quietsCnt, Value bonus) {
1441 if (ss->killers[0] != move)
1443 ss->killers[1] = ss->killers[0];
1444 ss->killers[0] = move;
1447 Color c = pos.side_to_move();
1448 Thread* thisThread = pos.this_thread();
1449 thisThread->fromTo.update(c, move, bonus);
1450 thisThread->history.update(pos.moved_piece(move), to_sq(move), bonus);
1451 update_cm_stats(ss, pos.moved_piece(move), to_sq(move), bonus);
1453 if ((ss-1)->counterMoves)
1455 Square prevSq = to_sq((ss-1)->currentMove);
1456 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1459 // Decrease all the other played quiet moves
1460 for (int i = 0; i < quietsCnt; ++i)
1462 thisThread->fromTo.update(c, quiets[i], -bonus);
1463 thisThread->history.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1464 update_cm_stats(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1469 // When playing with strength handicap, choose best move among a set of RootMoves
1470 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1472 Move Skill::pick_best(size_t multiPV) {
1474 const RootMoves& rootMoves = Threads.main()->rootMoves;
1475 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1477 // RootMoves are already sorted by score in descending order
1478 Value topScore = rootMoves[0].score;
1479 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1480 int weakness = 120 - 2 * level;
1481 int maxScore = -VALUE_INFINITE;
1483 // Choose best move. For each move score we add two terms, both dependent on
1484 // weakness. One is deterministic and bigger for weaker levels, and one is
1485 // random. Then we choose the move with the resulting highest score.
1486 for (size_t i = 0; i < multiPV; ++i)
1488 // This is our magic formula
1489 int push = ( weakness * int(topScore - rootMoves[i].score)
1490 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1492 if (rootMoves[i].score + push > maxScore)
1494 maxScore = rootMoves[i].score + push;
1495 best = rootMoves[i].pv[0];
1503 // check_time() is used to print debug info and, more importantly, to detect
1504 // when we are out of available time and thus stop the search.
1508 static TimePoint lastInfoTime = now();
1510 int elapsed = Time.elapsed();
1511 TimePoint tick = Limits.startTime + elapsed;
1513 if (tick - lastInfoTime >= 1000)
1515 lastInfoTime = tick;
1519 // An engine may not stop pondering until told so by the GUI
1523 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1524 || (Limits.movetime && elapsed >= Limits.movetime)
1525 || (Limits.nodes && Threads.nodes_searched() >= Limits.nodes))
1526 Signals.stop = true;
1532 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1533 /// that all (if any) unsearched PV lines are sent using a previous search score.
1535 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1537 std::stringstream ss;
1538 int elapsed = Time.elapsed() + 1;
1539 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1540 size_t PVIdx = pos.this_thread()->PVIdx;
1541 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1542 uint64_t nodes_searched = Threads.nodes_searched();
1544 for (size_t i = 0; i < multiPV; ++i)
1546 bool updated = (i <= PVIdx);
1548 if (depth == ONE_PLY && !updated)
1551 Depth d = updated ? depth : depth - ONE_PLY;
1552 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1554 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1555 v = tb ? TB::Score : v;
1557 if (ss.rdbuf()->in_avail()) // Not at first line
1561 << " depth " << d / ONE_PLY
1562 << " seldepth " << pos.this_thread()->maxPly
1563 << " multipv " << i + 1
1564 << " score " << UCI::value(v);
1566 if (!tb && i == PVIdx)
1567 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1569 ss << " nodes " << nodes_searched
1570 << " nps " << nodes_searched * 1000 / elapsed;
1572 if (elapsed > 1000) // Earlier makes little sense
1573 ss << " hashfull " << TT.hashfull();
1575 ss << " tbhits " << TB::Hits
1576 << " time " << elapsed
1579 for (Move m : rootMoves[i].pv)
1580 ss << " " << UCI::move(m, pos.is_chess960());
1587 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1588 /// before exiting the search, for instance, in case we stop the search during a
1589 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1590 /// otherwise in case of 'ponder on' we have nothing to think on.
1592 bool RootMove::extract_ponder_from_tt(Position& pos)
1597 assert(pv.size() == 1);
1599 pos.do_move(pv[0], st, pos.gives_check(pv[0]));
1600 TTEntry* tte = TT.probe(pos.key(), ttHit);
1604 Move m = tte->move(); // Local copy to be SMP safe
1605 if (MoveList<LEGAL>(pos).contains(m))
1609 pos.undo_move(pv[0]);
1610 return pv.size() > 1;
1613 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1617 UseRule50 = Options["Syzygy50MoveRule"];
1618 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1619 Cardinality = Options["SyzygyProbeLimit"];
1621 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1622 if (Cardinality > MaxCardinality)
1624 Cardinality = MaxCardinality;
1625 ProbeDepth = DEPTH_ZERO;
1628 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1631 // If the current root position is in the tablebases, then RootMoves
1632 // contains only moves that preserve the draw or the win.
1633 RootInTB = root_probe(pos, rootMoves, TB::Score);
1636 Cardinality = 0; // Do not probe tablebases during the search
1638 else // If DTZ tables are missing, use WDL tables as a fallback
1640 // Filter out moves that do not preserve the draw or the win.
1641 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1643 // Only probe during search if winning
1644 if (RootInTB && TB::Score <= VALUE_DRAW)
1650 Hits = rootMoves.size();
1653 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1654 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1