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
38 #include "syzygy/tbprobe.h"
46 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]);
117 pos.do_move(newPv[1], st[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];
161 template <NodeType NT>
162 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
164 template <NodeType NT, bool InCheck>
165 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
167 Value value_to_tt(Value v, int ply);
168 Value value_from_tt(Value v, int ply);
169 void update_pv(Move* pv, Move move, Move* childPv);
170 void update_cm_stats(Stack* ss, Piece pc, Square s, Value bonus);
171 void update_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, Value bonus);
177 /// Search::init() is called during startup to initialize various lookup tables
179 void Search::init() {
181 for (int imp = 0; imp <= 1; ++imp)
182 for (int d = 1; d < 64; ++d)
183 for (int mc = 1; mc < 64; ++mc)
185 double r = log(d) * log(mc) / 2;
187 Reductions[NonPV][imp][d][mc] = int(std::round(r));
188 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
190 // Increase reduction for non-PV nodes when eval is not improving
191 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
192 Reductions[NonPV][imp][d][mc]++;
195 for (int d = 0; d < 16; ++d)
197 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
198 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
203 /// Search::clear() resets search state to zero, to obtain reproducible results
205 void Search::clear() {
209 for (Thread* th : Threads)
212 th->counterMoves.clear();
214 th->counterMoveHistory.clear();
215 th->resetCalls = true;
218 Threads.main()->previousScore = VALUE_INFINITE;
222 /// Search::perft() is our utility to verify move generation. All the leaf nodes
223 /// up to the given depth are generated and counted, and the sum is returned.
225 uint64_t Search::perft(Position& pos, Depth depth) {
228 uint64_t cnt, nodes = 0;
229 const bool leaf = (depth == 2 * ONE_PLY);
231 for (const auto& m : MoveList<LEGAL>(pos))
233 if (Root && depth <= ONE_PLY)
238 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
243 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
248 template uint64_t Search::perft<true>(Position&, Depth);
251 /// MainThread::search() is called by the main thread when the program receives
252 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
254 void MainThread::search() {
256 Color us = rootPos.side_to_move();
257 Time.init(Limits, us, rootPos.game_ply());
259 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
260 DrawValue[ us] = VALUE_DRAW - Value(contempt);
261 DrawValue[~us] = VALUE_DRAW + Value(contempt);
263 if (rootMoves.empty())
265 rootMoves.push_back(RootMove(MOVE_NONE));
266 sync_cout << "info depth 0 score "
267 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
272 for (Thread* th : Threads)
274 th->start_searching();
276 Thread::search(); // Let's start searching!
279 // When playing in 'nodes as time' mode, subtract the searched nodes from
280 // the available ones before exiting.
282 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
284 // When we reach the maximum depth, we can arrive here without a raise of
285 // Signals.stop. However, if we are pondering or in an infinite search,
286 // the UCI protocol states that we shouldn't print the best move before the
287 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
288 // until the GUI sends one of those commands (which also raises Signals.stop).
289 if (!Signals.stop && (Limits.ponder || Limits.infinite))
291 Signals.stopOnPonderhit = true;
295 // Stop the threads if not already stopped
298 // Wait until all threads have finished
299 for (Thread* th : Threads)
301 th->wait_for_search_finished();
303 // Check if there are threads with a better score than main thread
304 Thread* bestThread = this;
305 if ( !this->easyMovePlayed
306 && Options["MultiPV"] == 1
308 && !Skill(Options["Skill Level"]).enabled()
309 && rootMoves[0].pv[0] != MOVE_NONE)
311 for (Thread* th : Threads)
312 if ( th->completedDepth > bestThread->completedDepth
313 && th->rootMoves[0].score > bestThread->rootMoves[0].score)
317 previousScore = bestThread->rootMoves[0].score;
319 // Send new PV when needed
320 if (bestThread != this)
321 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
323 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
325 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
326 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
328 std::cout << sync_endl;
332 // Thread::search() is the main iterative deepening loop. It calls search()
333 // repeatedly with increasing depth until the allocated thinking time has been
334 // consumed, the user stops the search, or the maximum search depth is reached.
336 void Thread::search() {
338 Stack stack[MAX_PLY+7], *ss = stack+4; // To allow referencing (ss-4) and (ss+2)
339 Value bestValue, alpha, beta, delta;
340 Move easyMove = MOVE_NONE;
341 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
343 std::memset(ss-4, 0, 7 * sizeof(Stack));
345 bestValue = delta = alpha = -VALUE_INFINITE;
346 beta = VALUE_INFINITE;
347 completedDepth = DEPTH_ZERO;
351 easyMove = EasyMove.get(rootPos.key());
353 mainThread->easyMovePlayed = mainThread->failedLow = false;
354 mainThread->bestMoveChanges = 0;
358 size_t multiPV = Options["MultiPV"];
359 Skill skill(Options["Skill Level"]);
361 // When playing with strength handicap enable MultiPV search that we will
362 // use behind the scenes to retrieve a set of possible moves.
364 multiPV = std::max(multiPV, (size_t)4);
366 multiPV = std::min(multiPV, rootMoves.size());
368 // Iterative deepening loop until requested to stop or the target depth is reached
369 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
371 && (!Limits.depth || Threads.main()->rootDepth / ONE_PLY <= Limits.depth))
373 // Set up the new depths for the helper threads skipping on average every
374 // 2nd ply (using a half-density matrix).
377 const Row& row = HalfDensity[(idx - 1) % HalfDensitySize];
378 if (row[(rootDepth / ONE_PLY + rootPos.game_ply()) % row.size()])
382 // Age out PV variability metric
384 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
386 // Save the last iteration's scores before first PV line is searched and
387 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
388 for (RootMove& rm : rootMoves)
389 rm.previousScore = rm.score;
391 // MultiPV loop. We perform a full root search for each PV line
392 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
394 // Reset aspiration window starting size
395 if (rootDepth >= 5 * ONE_PLY)
398 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
399 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
402 // Start with a small aspiration window and, in the case of a fail
403 // high/low, re-search with a bigger window until we're not failing
407 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false);
409 // Bring the best move to the front. It is critical that sorting
410 // is done with a stable algorithm because all the values but the
411 // first and eventually the new best one are set to -VALUE_INFINITE
412 // and we want to keep the same order for all the moves except the
413 // new PV that goes to the front. Note that in case of MultiPV
414 // search the already searched PV lines are preserved.
415 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
417 // If search has been stopped, break immediately. Sorting and
418 // writing PV back to TT is safe because RootMoves is still
419 // valid, although it refers to the previous iteration.
423 // When failing high/low give some update (without cluttering
424 // the UI) before a re-search.
427 && (bestValue <= alpha || bestValue >= beta)
428 && Time.elapsed() > 3000)
429 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
431 // In case of failing low/high increase aspiration window and
432 // re-search, otherwise exit the loop.
433 if (bestValue <= alpha)
435 beta = (alpha + beta) / 2;
436 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
440 mainThread->failedLow = true;
441 Signals.stopOnPonderhit = false;
444 else if (bestValue >= beta)
446 alpha = (alpha + beta) / 2;
447 beta = std::min(bestValue + delta, VALUE_INFINITE);
452 delta += delta / 4 + 5;
454 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
457 // Sort the PV lines searched so far and update the GUI
458 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
463 if (Signals.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000)
464 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
468 completedDepth = rootDepth;
473 // If skill level is enabled and time is up, pick a sub-optimal best move
474 if (skill.enabled() && skill.time_to_pick(rootDepth))
475 skill.pick_best(multiPV);
477 // Have we found a "mate in x"?
479 && bestValue >= VALUE_MATE_IN_MAX_PLY
480 && VALUE_MATE - bestValue <= 2 * Limits.mate)
483 // Do we have time for the next iteration? Can we stop searching now?
484 if (Limits.use_time_management())
486 if (!Signals.stop && !Signals.stopOnPonderhit)
488 // Stop the search if only one legal move is available, or if all
489 // of the available time has been used, or if we matched an easyMove
490 // from the previous search and just did a fast verification.
491 const int F[] = { mainThread->failedLow,
492 bestValue - mainThread->previousScore };
494 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
495 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
497 bool doEasyMove = rootMoves[0].pv[0] == easyMove
498 && mainThread->bestMoveChanges < 0.03
499 && Time.elapsed() > Time.optimum() * 5 / 42;
501 if ( rootMoves.size() == 1
502 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628
503 || (mainThread->easyMovePlayed = doEasyMove, doEasyMove))
505 // If we are allowed to ponder do not stop the search now but
506 // keep pondering until the GUI sends "ponderhit" or "stop".
508 Signals.stopOnPonderhit = true;
514 if (rootMoves[0].pv.size() >= 3)
515 EasyMove.update(rootPos, rootMoves[0].pv);
524 // Clear any candidate easy move that wasn't stable for the last search
525 // iterations; the second condition prevents consecutive fast moves.
526 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
529 // If skill level is enabled, swap best PV line with the sub-optimal one
531 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
532 rootMoves.end(), skill.best_move(multiPV)));
538 // search<>() is the main search function for both PV and non-PV nodes
540 template <NodeType NT>
541 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
543 const bool PvNode = NT == PV;
544 const bool rootNode = PvNode && (ss-1)->ply == 0;
546 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
547 assert(PvNode || (alpha == beta - 1));
548 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
549 assert(!(PvNode && cutNode));
550 assert(depth / ONE_PLY * ONE_PLY == depth);
552 Move pv[MAX_PLY+1], quietsSearched[64];
556 Move ttMove, move, excludedMove, bestMove;
557 Depth extension, newDepth;
558 Value bestValue, value, ttValue, eval, nullValue;
559 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
560 bool captureOrPromotion, doFullDepthSearch, moveCountPruning;
562 int moveCount, quietCount;
564 // Step 1. Initialize node
565 Thread* thisThread = pos.this_thread();
566 inCheck = pos.checkers();
567 moveCount = quietCount = ss->moveCount = 0;
568 ss->history = VALUE_ZERO;
569 bestValue = -VALUE_INFINITE;
570 ss->ply = (ss-1)->ply + 1;
572 // Check for the available remaining time
573 if (thisThread->resetCalls.load(std::memory_order_relaxed))
575 thisThread->resetCalls = false;
576 // At low node count increase the checking rate to about 0.1% of nodes
577 // otherwise use a default value.
578 thisThread->callsCnt = Limits.nodes ? std::min((int64_t)4096, Limits.nodes / 1024)
582 if (--thisThread->callsCnt <= 0)
584 for (Thread* th : Threads)
585 th->resetCalls = true;
590 // Used to send selDepth info to GUI
591 if (PvNode && thisThread->maxPly < ss->ply)
592 thisThread->maxPly = ss->ply;
596 // Step 2. Check for aborted search and immediate draw
597 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw() || ss->ply >= MAX_PLY)
598 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
599 : DrawValue[pos.side_to_move()];
601 // Step 3. Mate distance pruning. Even if we mate at the next move our score
602 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
603 // a shorter mate was found upward in the tree then there is no need to search
604 // because we will never beat the current alpha. Same logic but with reversed
605 // signs applies also in the opposite condition of being mated instead of giving
606 // mate. In this case return a fail-high score.
607 alpha = std::max(mated_in(ss->ply), alpha);
608 beta = std::min(mate_in(ss->ply+1), beta);
613 assert(0 <= ss->ply && ss->ply < MAX_PLY);
615 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
616 ss->counterMoves = nullptr;
617 (ss+1)->skipEarlyPruning = false;
618 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
620 // Step 4. Transposition table lookup. We don't want the score of a partial
621 // search to overwrite a previous full search TT value, so we use a different
622 // position key in case of an excluded move.
623 excludedMove = ss->excludedMove;
624 posKey = pos.key() ^ Key(excludedMove);
625 tte = TT.probe(posKey, ttHit);
626 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
627 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
628 : ttHit ? tte->move() : MOVE_NONE;
630 // At non-PV nodes we check for an early TT cutoff
633 && tte->depth() >= depth
634 && ttValue != VALUE_NONE // Possible in case of TT access race
635 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
636 : (tte->bound() & BOUND_UPPER)))
638 // If ttMove is quiet, update killers, history, counter move on TT hit
639 if (ttValue >= beta && ttMove)
641 int d = depth / ONE_PLY;
643 if (!pos.capture_or_promotion(ttMove))
645 Value bonus = Value(d * d + 2 * d - 2);
646 update_stats(pos, ss, ttMove, nullptr, 0, bonus);
649 // Extra penalty for a quiet TT move in previous ply when it gets refuted
650 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
652 Value penalty = Value(d * d + 4 * d + 1);
653 Square prevSq = to_sq((ss-1)->currentMove);
654 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -penalty);
660 // Step 4a. Tablebase probe
661 if (!rootNode && TB::Cardinality)
663 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
665 if ( piecesCnt <= TB::Cardinality
666 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
667 && pos.rule50_count() == 0
668 && !pos.can_castle(ANY_CASTLING))
671 TB::WDLScore v = Tablebases::probe_wdl(pos, &err);
673 if (err != TB::ProbeState::FAIL)
675 thisThread->tbHits++;
677 int drawScore = TB::UseRule50 ? 1 : 0;
679 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
680 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
681 : VALUE_DRAW + 2 * v * drawScore;
683 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
684 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
685 MOVE_NONE, VALUE_NONE, TT.generation());
692 // Step 5. Evaluate the position statically
695 ss->staticEval = eval = VALUE_NONE;
701 // Never assume anything on values stored in TT
702 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
703 eval = ss->staticEval = evaluate(pos);
705 // Can ttValue be used as a better position evaluation?
706 if (ttValue != VALUE_NONE)
707 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
712 eval = ss->staticEval =
713 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
714 : -(ss-1)->staticEval + 2 * Eval::Tempo;
716 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
717 ss->staticEval, TT.generation());
720 if (ss->skipEarlyPruning)
723 // Step 6. Razoring (skipped when in check)
725 && depth < 4 * ONE_PLY
726 && ttMove == MOVE_NONE
727 && eval + razor_margin[depth / ONE_PLY] <= alpha)
729 if (depth <= ONE_PLY)
730 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
732 Value ralpha = alpha - razor_margin[depth / ONE_PLY];
733 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
738 // Step 7. Futility pruning: child node (skipped when in check)
740 && depth < 7 * ONE_PLY
741 && eval - futility_margin(depth) >= beta
742 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
743 && pos.non_pawn_material(pos.side_to_move()))
746 // Step 8. Null move search with verification search (is omitted in PV nodes)
749 && (ss->staticEval >= beta - 35 * (depth / ONE_PLY - 6) || depth >= 13 * ONE_PLY)
750 && pos.non_pawn_material(pos.side_to_move()))
752 ss->currentMove = MOVE_NULL;
753 ss->counterMoves = nullptr;
755 assert(eval - beta >= 0);
757 // Null move dynamic reduction based on depth and value
758 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
760 pos.do_null_move(st);
761 (ss+1)->skipEarlyPruning = true;
762 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
763 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
764 (ss+1)->skipEarlyPruning = false;
765 pos.undo_null_move();
767 if (nullValue >= beta)
769 // Do not return unproven mate scores
770 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
773 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
776 // Do verification search at high depths
777 ss->skipEarlyPruning = true;
778 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
779 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
780 ss->skipEarlyPruning = false;
787 // Step 9. ProbCut (skipped when in check)
788 // If we have a good enough capture and a reduced search returns a value
789 // much above beta, we can (almost) safely prune the previous move.
791 && depth >= 5 * ONE_PLY
792 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
794 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
795 Depth rdepth = depth - 4 * ONE_PLY;
797 assert(rdepth >= ONE_PLY);
798 assert((ss-1)->currentMove != MOVE_NONE);
799 assert((ss-1)->currentMove != MOVE_NULL);
801 MovePicker mp(pos, ttMove, rbeta - ss->staticEval);
803 while ((move = mp.next_move()) != MOVE_NONE)
806 ss->currentMove = move;
807 ss->counterMoves = &thisThread->counterMoveHistory[pos.moved_piece(move)][to_sq(move)];
808 pos.do_move(move, st);
809 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
816 // Step 10. Internal iterative deepening (skipped when in check)
817 if ( depth >= 6 * ONE_PLY
819 && (PvNode || ss->staticEval + 256 >= beta))
821 Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
822 ss->skipEarlyPruning = true;
823 search<NT>(pos, ss, alpha, beta, d, cutNode);
824 ss->skipEarlyPruning = false;
826 tte = TT.probe(posKey, ttHit);
827 ttMove = ttHit ? tte->move() : MOVE_NONE;
830 moves_loop: // When in check search starts from here
832 const CounterMoveStats* cmh = (ss-1)->counterMoves;
833 const CounterMoveStats* fmh = (ss-2)->counterMoves;
834 const CounterMoveStats* fmh2 = (ss-4)->counterMoves;
836 MovePicker mp(pos, ttMove, depth, ss);
837 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
838 improving = ss->staticEval >= (ss-2)->staticEval
839 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
840 ||(ss-2)->staticEval == VALUE_NONE;
842 singularExtensionNode = !rootNode
843 && depth >= 8 * ONE_PLY
844 && ttMove != MOVE_NONE
845 && ttValue != VALUE_NONE
846 && !excludedMove // Recursive singular search is not allowed
847 && (tte->bound() & BOUND_LOWER)
848 && tte->depth() >= depth - 3 * ONE_PLY;
850 // Step 11. Loop through moves
851 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
852 while ((move = mp.next_move()) != MOVE_NONE)
856 if (move == excludedMove)
859 // At root obey the "searchmoves" option and skip moves not listed in Root
860 // Move List. As a consequence any illegal move is also skipped. In MultiPV
861 // mode we also skip PV moves which have been already searched.
862 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
863 thisThread->rootMoves.end(), move))
866 ss->moveCount = ++moveCount;
868 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
869 sync_cout << "info depth " << depth / ONE_PLY
870 << " currmove " << UCI::move(move, pos.is_chess960())
871 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
874 (ss+1)->pv = nullptr;
876 extension = DEPTH_ZERO;
877 captureOrPromotion = pos.capture_or_promotion(move);
878 moved_piece = pos.moved_piece(move);
880 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
881 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
882 : pos.gives_check(move);
884 moveCountPruning = depth < 16 * ONE_PLY
885 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
887 // Step 12. Extend checks
890 && pos.see_ge(move, VALUE_ZERO))
893 // Singular extension search. If all moves but one fail low on a search of
894 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
895 // is singular and should be extended. To verify this we do a reduced search
896 // on all the other moves but the ttMove and if the result is lower than
897 // ttValue minus a margin then we extend the ttMove.
898 if ( singularExtensionNode
903 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
904 Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
905 ss->excludedMove = move;
906 ss->skipEarlyPruning = true;
907 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode);
908 ss->skipEarlyPruning = false;
909 ss->excludedMove = MOVE_NONE;
915 // Update the current move (this must be done after singular extension search)
916 newDepth = depth - ONE_PLY + extension;
918 // Step 13. Pruning at shallow depth
920 && bestValue > VALUE_MATED_IN_MAX_PLY)
922 if ( !captureOrPromotion
924 && !pos.advanced_pawn_push(move))
926 // Move count based pruning
927 if (moveCountPruning)
930 // Reduced depth of the next LMR search
931 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
933 // Countermoves based pruning
935 && (!cmh || (*cmh )[moved_piece][to_sq(move)] < VALUE_ZERO)
936 && (!fmh || (*fmh )[moved_piece][to_sq(move)] < VALUE_ZERO)
937 && (!fmh2 || (*fmh2)[moved_piece][to_sq(move)] < VALUE_ZERO || (cmh && fmh)))
940 // Futility pruning: parent node
943 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
946 // Prune moves with negative SEE
948 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
951 else if (depth < 7 * ONE_PLY && !extension)
953 Value v = -Value(399 + 35 * depth / ONE_PLY * depth / ONE_PLY);
956 v += beta - alpha - 1;
958 if (!pos.see_ge(move, v))
963 // Speculative prefetch as early as possible
964 prefetch(TT.first_entry(pos.key_after(move)));
966 // Check for legality just before making the move
967 if (!rootNode && !pos.legal(move))
969 ss->moveCount = --moveCount;
973 ss->currentMove = move;
974 ss->counterMoves = &thisThread->counterMoveHistory[moved_piece][to_sq(move)];
976 // Step 14. Make the move
977 pos.do_move(move, st, givesCheck);
979 // Step 15. Reduced depth search (LMR). If the move fails high it will be
980 // re-searched at full depth.
981 if ( depth >= 3 * ONE_PLY
983 && (!captureOrPromotion || moveCountPruning))
985 Depth r = reduction<PvNode>(improving, depth, moveCount);
987 if (captureOrPromotion)
988 r -= r ? ONE_PLY : DEPTH_ZERO;
991 // Increase reduction for cut nodes
995 // Decrease reduction for moves that escape a capture. Filter out
996 // castling moves, because they are coded as "king captures rook" and
997 // hence break make_move().
998 else if ( type_of(move) == NORMAL
999 && type_of(pos.piece_on(to_sq(move))) != PAWN
1000 && !pos.see_ge(make_move(to_sq(move), from_sq(move)), VALUE_ZERO))
1003 ss->history = thisThread->history[moved_piece][to_sq(move)]
1004 + (cmh ? (*cmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
1005 + (fmh ? (*fmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
1006 + (fmh2 ? (*fmh2)[moved_piece][to_sq(move)] : VALUE_ZERO)
1007 + thisThread->fromTo.get(~pos.side_to_move(), move)
1008 - 8000; // Correction factor
1010 // Decrease/increase reduction by comparing opponent's stat score
1011 if (ss->history > VALUE_ZERO && (ss-1)->history < VALUE_ZERO)
1014 else if (ss->history < VALUE_ZERO && (ss-1)->history > VALUE_ZERO)
1017 // Decrease/increase reduction for moves with a good/bad history
1018 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->history / 20000) * ONE_PLY);
1021 Depth d = std::max(newDepth - r, ONE_PLY);
1023 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1025 doFullDepthSearch = (value > alpha && d != newDepth);
1028 doFullDepthSearch = !PvNode || moveCount > 1;
1030 // Step 16. Full depth search when LMR is skipped or fails high
1031 if (doFullDepthSearch)
1032 value = newDepth < ONE_PLY ?
1033 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1034 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1035 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1037 // For PV nodes only, do a full PV search on the first move or after a fail
1038 // high (in the latter case search only if value < beta), otherwise let the
1039 // parent node fail low with value <= alpha and try another move.
1040 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1043 (ss+1)->pv[0] = MOVE_NONE;
1045 value = newDepth < ONE_PLY ?
1046 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1047 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1048 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1051 // Step 17. Undo move
1052 pos.undo_move(move);
1054 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1056 // Step 18. Check for a new best move
1057 // Finished searching the move. If a stop occurred, the return value of
1058 // the search cannot be trusted, and we return immediately without
1059 // updating best move, PV and TT.
1060 if (Signals.stop.load(std::memory_order_relaxed))
1065 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1066 thisThread->rootMoves.end(), move);
1068 // PV move or new best move ?
1069 if (moveCount == 1 || value > alpha)
1076 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1077 rm.pv.push_back(*m);
1079 // We record how often the best move has been changed in each
1080 // iteration. This information is used for time management: When
1081 // the best move changes frequently, we allocate some more time.
1082 if (moveCount > 1 && thisThread == Threads.main())
1083 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1086 // All other moves but the PV are set to the lowest value: this is
1087 // not a problem when sorting because the sort is stable and the
1088 // move position in the list is preserved - just the PV is pushed up.
1089 rm.score = -VALUE_INFINITE;
1092 if (value > bestValue)
1098 // If there is an easy move for this position, clear it if unstable
1100 && thisThread == Threads.main()
1101 && EasyMove.get(pos.key())
1102 && (move != EasyMove.get(pos.key()) || moveCount > 1))
1107 if (PvNode && !rootNode) // Update pv even in fail-high case
1108 update_pv(ss->pv, move, (ss+1)->pv);
1110 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1114 assert(value >= beta); // Fail high
1120 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1121 quietsSearched[quietCount++] = move;
1124 // The following condition would detect a stop only after move loop has been
1125 // completed. But in this case bestValue is valid because we have fully
1126 // searched our subtree, and we can anyhow save the result in TT.
1132 // Step 20. Check for mate and stalemate
1133 // All legal moves have been searched and if there are no legal moves, it
1134 // must be a mate or a stalemate. If we are in a singular extension search then
1135 // return a fail low score.
1137 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1140 bestValue = excludedMove ? alpha
1141 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1144 int d = depth / ONE_PLY;
1146 // Quiet best move: update killers, history and countermoves
1147 if (!pos.capture_or_promotion(bestMove))
1149 Value bonus = Value(d * d + 2 * d - 2);
1150 update_stats(pos, ss, bestMove, quietsSearched, quietCount, bonus);
1153 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1154 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1156 Value penalty = Value(d * d + 4 * d + 1);
1157 Square prevSq = to_sq((ss-1)->currentMove);
1158 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -penalty);
1161 // Bonus for prior countermove that caused the fail low
1162 else if ( depth >= 3 * ONE_PLY
1163 && !pos.captured_piece()
1164 && is_ok((ss-1)->currentMove))
1166 int d = depth / ONE_PLY;
1167 Value bonus = Value(d * d + 2 * d - 2);
1168 Square prevSq = to_sq((ss-1)->currentMove);
1169 update_cm_stats(ss-1, 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);
1196 assert(depth / ONE_PLY * ONE_PLY == depth);
1202 Move ttMove, move, bestMove;
1203 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1204 bool ttHit, givesCheck, evasionPrunable;
1209 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1211 ss->pv[0] = MOVE_NONE;
1214 ss->currentMove = bestMove = MOVE_NONE;
1215 ss->ply = (ss-1)->ply + 1;
1217 // Check for an instant draw or if the maximum ply has been reached
1218 if (pos.is_draw() || ss->ply >= MAX_PLY)
1219 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1220 : DrawValue[pos.side_to_move()];
1222 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1224 // Decide whether or not to include checks: this fixes also the type of
1225 // TT entry depth that we are going to use. Note that in qsearch we use
1226 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1227 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1228 : DEPTH_QS_NO_CHECKS;
1230 // Transposition table lookup
1232 tte = TT.probe(posKey, ttHit);
1233 ttMove = ttHit ? tte->move() : MOVE_NONE;
1234 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1238 && tte->depth() >= ttDepth
1239 && ttValue != VALUE_NONE // Only in case of TT access race
1240 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1241 : (tte->bound() & BOUND_UPPER)))
1244 // Evaluate the position statically
1247 ss->staticEval = VALUE_NONE;
1248 bestValue = futilityBase = -VALUE_INFINITE;
1254 // Never assume anything on values stored in TT
1255 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1256 ss->staticEval = bestValue = evaluate(pos);
1258 // Can ttValue be used as a better position evaluation?
1259 if (ttValue != VALUE_NONE)
1260 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1261 bestValue = ttValue;
1264 ss->staticEval = bestValue =
1265 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1266 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1268 // Stand pat. Return immediately if static value is at least beta
1269 if (bestValue >= beta)
1272 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1273 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1278 if (PvNode && bestValue > alpha)
1281 futilityBase = bestValue + 128;
1284 // Initialize a MovePicker object for the current position, and prepare
1285 // to search the moves. Because the depth is <= 0 here, only captures,
1286 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1288 MovePicker mp(pos, ttMove, depth, to_sq((ss-1)->currentMove));
1290 // Loop through the moves until no moves remain or a beta cutoff occurs
1291 while ((move = mp.next_move()) != MOVE_NONE)
1293 assert(is_ok(move));
1295 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
1296 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
1297 : pos.gives_check(move);
1302 && futilityBase > -VALUE_KNOWN_WIN
1303 && !pos.advanced_pawn_push(move))
1305 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1307 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1309 if (futilityValue <= alpha)
1311 bestValue = std::max(bestValue, futilityValue);
1315 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1317 bestValue = std::max(bestValue, futilityBase);
1322 // Detect non-capture evasions that are candidates to be pruned
1323 evasionPrunable = InCheck
1324 && bestValue > VALUE_MATED_IN_MAX_PLY
1325 && !pos.capture(move);
1327 // Don't search moves with negative SEE values
1328 if ( (!InCheck || evasionPrunable)
1329 && type_of(move) != PROMOTION
1330 && !pos.see_ge(move, VALUE_ZERO))
1333 // Speculative prefetch as early as possible
1334 prefetch(TT.first_entry(pos.key_after(move)));
1336 // Check for legality just before making the move
1337 if (!pos.legal(move))
1340 ss->currentMove = move;
1342 // Make and search the move
1343 pos.do_move(move, st, givesCheck);
1344 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1345 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1346 pos.undo_move(move);
1348 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1350 // Check for a new best move
1351 if (value > bestValue)
1357 if (PvNode) // Update pv even in fail-high case
1358 update_pv(ss->pv, move, (ss+1)->pv);
1360 if (PvNode && value < beta) // Update alpha here!
1367 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1368 ttDepth, move, ss->staticEval, TT.generation());
1376 // All legal moves have been searched. A special case: If we're in check
1377 // and no legal moves were found, it is checkmate.
1378 if (InCheck && bestValue == -VALUE_INFINITE)
1379 return mated_in(ss->ply); // Plies to mate from the root
1381 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1382 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1383 ttDepth, bestMove, ss->staticEval, TT.generation());
1385 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1391 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1392 // "plies to mate from the current position". Non-mate scores are unchanged.
1393 // The function is called before storing a value in the transposition table.
1395 Value value_to_tt(Value v, int ply) {
1397 assert(v != VALUE_NONE);
1399 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1400 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1404 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1405 // from the transposition table (which refers to the plies to mate/be mated
1406 // from current position) to "plies to mate/be mated from the root".
1408 Value value_from_tt(Value v, int ply) {
1410 return v == VALUE_NONE ? VALUE_NONE
1411 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1412 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1416 // update_pv() adds current move and appends child pv[]
1418 void update_pv(Move* pv, Move move, Move* childPv) {
1420 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1426 // update_cm_stats() updates countermove and follow-up move history
1428 void update_cm_stats(Stack* ss, Piece pc, Square s, Value bonus) {
1430 CounterMoveStats* cmh = (ss-1)->counterMoves;
1431 CounterMoveStats* fmh1 = (ss-2)->counterMoves;
1432 CounterMoveStats* fmh2 = (ss-4)->counterMoves;
1435 cmh->update(pc, s, bonus);
1438 fmh1->update(pc, s, bonus);
1441 fmh2->update(pc, s, bonus);
1445 // update_stats() updates killers, history, countermove and countermove plus
1446 // follow-up move history when a new quiet best move is found.
1448 void update_stats(const Position& pos, Stack* ss, Move move,
1449 Move* quiets, int quietsCnt, Value bonus) {
1451 if (ss->killers[0] != move)
1453 ss->killers[1] = ss->killers[0];
1454 ss->killers[0] = move;
1457 Color c = pos.side_to_move();
1458 Thread* thisThread = pos.this_thread();
1459 thisThread->fromTo.update(c, move, bonus);
1460 thisThread->history.update(pos.moved_piece(move), to_sq(move), bonus);
1461 update_cm_stats(ss, pos.moved_piece(move), to_sq(move), bonus);
1463 if ((ss-1)->counterMoves)
1465 Square prevSq = to_sq((ss-1)->currentMove);
1466 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1469 // Decrease all the other played quiet moves
1470 for (int i = 0; i < quietsCnt; ++i)
1472 thisThread->fromTo.update(c, quiets[i], -bonus);
1473 thisThread->history.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1474 update_cm_stats(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1479 // When playing with strength handicap, choose best move among a set of RootMoves
1480 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1482 Move Skill::pick_best(size_t multiPV) {
1484 const RootMoves& rootMoves = Threads.main()->rootMoves;
1485 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1487 // RootMoves are already sorted by score in descending order
1488 Value topScore = rootMoves[0].score;
1489 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1490 int weakness = 120 - 2 * level;
1491 int maxScore = -VALUE_INFINITE;
1493 // Choose best move. For each move score we add two terms, both dependent on
1494 // weakness. One is deterministic and bigger for weaker levels, and one is
1495 // random. Then we choose the move with the resulting highest score.
1496 for (size_t i = 0; i < multiPV; ++i)
1498 // This is our magic formula
1499 int push = ( weakness * int(topScore - rootMoves[i].score)
1500 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1502 if (rootMoves[i].score + push > maxScore)
1504 maxScore = rootMoves[i].score + push;
1505 best = rootMoves[i].pv[0];
1513 // check_time() is used to print debug info and, more importantly, to detect
1514 // when we are out of available time and thus stop the search.
1518 static TimePoint lastInfoTime = now();
1520 int elapsed = Time.elapsed();
1521 TimePoint tick = Limits.startTime + elapsed;
1523 if (tick - lastInfoTime >= 1000)
1525 lastInfoTime = tick;
1529 // An engine may not stop pondering until told so by the GUI
1533 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1534 || (Limits.movetime && elapsed >= Limits.movetime)
1535 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1536 Signals.stop = true;
1542 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1543 /// that all (if any) unsearched PV lines are sent using a previous search score.
1545 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1547 std::stringstream ss;
1548 int elapsed = Time.elapsed() + 1;
1549 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1550 size_t PVIdx = pos.this_thread()->PVIdx;
1551 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1552 uint64_t nodesSearched = Threads.nodes_searched();
1553 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1555 for (size_t i = 0; i < multiPV; ++i)
1557 bool updated = (i <= PVIdx);
1559 if (depth == ONE_PLY && !updated)
1562 Depth d = updated ? depth : depth - ONE_PLY;
1563 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1565 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1566 v = tb ? TB::Score : v;
1568 if (ss.rdbuf()->in_avail()) // Not at first line
1572 << " depth " << d / ONE_PLY
1573 << " seldepth " << pos.this_thread()->maxPly
1574 << " multipv " << i + 1
1575 << " score " << UCI::value(v);
1577 if (!tb && i == PVIdx)
1578 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1580 ss << " nodes " << nodesSearched
1581 << " nps " << nodesSearched * 1000 / elapsed;
1583 if (elapsed > 1000) // Earlier makes little sense
1584 ss << " hashfull " << TT.hashfull();
1586 ss << " tbhits " << tbHits
1587 << " time " << elapsed
1590 for (Move m : rootMoves[i].pv)
1591 ss << " " << UCI::move(m, pos.is_chess960());
1598 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1599 /// before exiting the search, for instance, in case we stop the search during a
1600 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1601 /// otherwise in case of 'ponder on' we have nothing to think on.
1603 bool RootMove::extract_ponder_from_tt(Position& pos) {
1608 assert(pv.size() == 1);
1613 pos.do_move(pv[0], st);
1614 TTEntry* tte = TT.probe(pos.key(), ttHit);
1618 Move m = tte->move(); // Local copy to be SMP safe
1619 if (MoveList<LEGAL>(pos).contains(m))
1623 pos.undo_move(pv[0]);
1624 return pv.size() > 1;
1627 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1630 UseRule50 = Options["Syzygy50MoveRule"];
1631 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1632 Cardinality = Options["SyzygyProbeLimit"];
1634 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1635 if (Cardinality > MaxCardinality)
1637 Cardinality = MaxCardinality;
1638 ProbeDepth = DEPTH_ZERO;
1641 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1644 // If the current root position is in the tablebases, then RootMoves
1645 // contains only moves that preserve the draw or the win.
1646 RootInTB = root_probe(pos, rootMoves, TB::Score);
1649 Cardinality = 0; // Do not probe tablebases during the search
1651 else // If DTZ tables are missing, use WDL tables as a fallback
1653 // Filter out moves that do not preserve the draw or the win.
1654 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1656 // Only probe during search if winning
1657 if (RootInTB && TB::Score <= VALUE_DRAW)
1661 if (RootInTB && !UseRule50)
1662 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1663 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1