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], 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];
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;
189 Reductions[NonPV][imp][d][mc] = int(std::round(r));
190 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
192 // Increase reduction for non-PV nodes when eval is not improving
193 if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
194 Reductions[NonPV][imp][d][mc]++;
197 for (int d = 0; d < 16; ++d)
199 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
200 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
205 /// Search::clear() resets search state to zero, to obtain reproducible results
207 void Search::clear() {
211 for (Thread* th : Threads)
214 th->counterMoves.clear();
216 th->counterMoveHistory.clear();
219 Threads.main()->previousScore = VALUE_INFINITE;
223 /// Search::perft() is our utility to verify move generation. All the leaf nodes
224 /// up to the given depth are generated and counted, and the sum is returned.
226 uint64_t Search::perft(Position& pos, Depth depth) {
229 uint64_t cnt, nodes = 0;
230 const bool leaf = (depth == 2 * ONE_PLY);
232 for (const auto& m : MoveList<LEGAL>(pos))
234 if (Root && depth <= ONE_PLY)
238 pos.do_move(m, st, pos.gives_check(m));
239 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
244 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
249 template uint64_t Search::perft<true>(Position&, Depth);
252 /// MainThread::search() is called by the main thread when the program receives
253 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
255 void MainThread::search() {
257 Color us = rootPos.side_to_move();
258 Time.init(Limits, us, rootPos.game_ply());
260 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
261 DrawValue[ us] = VALUE_DRAW - Value(contempt);
262 DrawValue[~us] = VALUE_DRAW + Value(contempt);
264 if (rootMoves.empty())
266 rootMoves.push_back(RootMove(MOVE_NONE));
267 sync_cout << "info depth 0 score "
268 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
273 for (Thread* th : Threads)
275 th->start_searching();
277 Thread::search(); // Let's start searching!
280 // When playing in 'nodes as time' mode, subtract the searched nodes from
281 // the available ones before exiting.
283 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
285 // When we reach the maximum depth, we can arrive here without a raise of
286 // Signals.stop. However, if we are pondering or in an infinite search,
287 // the UCI protocol states that we shouldn't print the best move before the
288 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
289 // until the GUI sends one of those commands (which also raises Signals.stop).
290 if (!Signals.stop && (Limits.ponder || Limits.infinite))
292 Signals.stopOnPonderhit = true;
296 // Stop the threads if not already stopped
299 // Wait until all threads have finished
300 for (Thread* th : Threads)
302 th->wait_for_search_finished();
304 // Check if there are threads with a better score than main thread
305 Thread* bestThread = this;
306 if ( !this->easyMovePlayed
307 && Options["MultiPV"] == 1
309 && !Skill(Options["Skill Level"]).enabled()
310 && rootMoves[0].pv[0] != MOVE_NONE)
312 for (Thread* th : Threads)
313 if ( th->completedDepth > bestThread->completedDepth
314 && th->rootMoves[0].score > bestThread->rootMoves[0].score)
318 previousScore = bestThread->rootMoves[0].score;
320 // Send new PV when needed
321 if (bestThread != this)
322 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
324 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
326 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
327 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
329 std::cout << sync_endl;
333 // Thread::search() is the main iterative deepening loop. It calls search()
334 // repeatedly with increasing depth until the allocated thinking time has been
335 // consumed, the user stops the search, or the maximum search depth is reached.
337 void Thread::search() {
339 Stack stack[MAX_PLY+7], *ss = stack+5; // To allow referencing (ss-5) and (ss+2)
340 Value bestValue, alpha, beta, delta;
341 Move easyMove = MOVE_NONE;
342 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
344 std::memset(ss-5, 0, 8 * sizeof(Stack));
346 bestValue = delta = alpha = -VALUE_INFINITE;
347 beta = VALUE_INFINITE;
348 completedDepth = DEPTH_ZERO;
352 easyMove = EasyMove.get(rootPos.key());
354 mainThread->easyMovePlayed = mainThread->failedLow = false;
355 mainThread->bestMoveChanges = 0;
359 size_t multiPV = Options["MultiPV"];
360 Skill skill(Options["Skill Level"]);
362 // When playing with strength handicap enable MultiPV search that we will
363 // use behind the scenes to retrieve a set of possible moves.
365 multiPV = std::max(multiPV, (size_t)4);
367 multiPV = std::min(multiPV, rootMoves.size());
369 // Iterative deepening loop until requested to stop or the target depth is reached
370 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
372 && (!Limits.depth || Threads.main()->rootDepth / ONE_PLY <= Limits.depth))
374 // Set up the new depths for the helper threads skipping on average every
375 // 2nd ply (using a half-density matrix).
378 const Row& row = HalfDensity[(idx - 1) % HalfDensitySize];
379 if (row[(rootDepth / ONE_PLY + rootPos.game_ply()) % row.size()])
383 // Age out PV variability metric
385 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
387 // Save the last iteration's scores before first PV line is searched and
388 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
389 for (RootMove& rm : rootMoves)
390 rm.previousScore = rm.score;
392 // MultiPV loop. We perform a full root search for each PV line
393 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
395 // Reset aspiration window starting size
396 if (rootDepth >= 5 * ONE_PLY)
399 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
400 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
403 // Start with a small aspiration window and, in the case of a fail
404 // high/low, re-search with a bigger window until we're not failing
408 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false);
410 // Bring the best move to the front. It is critical that sorting
411 // is done with a stable algorithm because all the values but the
412 // first and eventually the new best one are set to -VALUE_INFINITE
413 // and we want to keep the same order for all the moves except the
414 // new PV that goes to the front. Note that in case of MultiPV
415 // search the already searched PV lines are preserved.
416 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
418 // If search has been stopped, break immediately. Sorting and
419 // writing PV back to TT is safe because RootMoves is still
420 // valid, although it refers to the previous iteration.
424 // When failing high/low give some update (without cluttering
425 // the UI) before a re-search.
428 && (bestValue <= alpha || bestValue >= beta)
429 && Time.elapsed() > 3000)
430 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
432 // In case of failing low/high increase aspiration window and
433 // re-search, otherwise exit the loop.
434 if (bestValue <= alpha)
436 beta = (alpha + beta) / 2;
437 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
441 mainThread->failedLow = true;
442 Signals.stopOnPonderhit = false;
445 else if (bestValue >= beta)
447 alpha = (alpha + beta) / 2;
448 beta = std::min(bestValue + delta, VALUE_INFINITE);
453 delta += delta / 4 + 5;
455 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
458 // Sort the PV lines searched so far and update the GUI
459 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
464 if (Signals.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000)
465 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
469 completedDepth = rootDepth;
474 // If skill level is enabled and time is up, pick a sub-optimal best move
475 if (skill.enabled() && skill.time_to_pick(rootDepth))
476 skill.pick_best(multiPV);
478 // Have we found a "mate in x"?
480 && bestValue >= VALUE_MATE_IN_MAX_PLY
481 && VALUE_MATE - bestValue <= 2 * Limits.mate)
484 // Do we have time for the next iteration? Can we stop searching now?
485 if (Limits.use_time_management())
487 if (!Signals.stop && !Signals.stopOnPonderhit)
489 // Stop the search if only one legal move is available, or if all
490 // of the available time has been used, or if we matched an easyMove
491 // from the previous search and just did a fast verification.
492 const int F[] = { mainThread->failedLow,
493 bestValue - mainThread->previousScore };
495 int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
496 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
498 bool doEasyMove = rootMoves[0].pv[0] == easyMove
499 && mainThread->bestMoveChanges < 0.03
500 && Time.elapsed() > Time.optimum() * 5 / 42;
502 if ( rootMoves.size() == 1
503 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628
504 || (mainThread->easyMovePlayed = doEasyMove, doEasyMove))
506 // If we are allowed to ponder do not stop the search now but
507 // keep pondering until the GUI sends "ponderhit" or "stop".
509 Signals.stopOnPonderhit = true;
515 if (rootMoves[0].pv.size() >= 3)
516 EasyMove.update(rootPos, rootMoves[0].pv);
525 // Clear any candidate easy move that wasn't stable for the last search
526 // iterations; the second condition prevents consecutive fast moves.
527 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
530 // If skill level is enabled, swap best PV line with the sub-optimal one
532 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
533 rootMoves.end(), skill.best_move(multiPV)));
539 // search<>() is the main search function for both PV and non-PV nodes
541 template <NodeType NT>
542 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
544 const bool PvNode = NT == PV;
545 const bool rootNode = PvNode && (ss-1)->ply == 0;
547 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
548 assert(PvNode || (alpha == beta - 1));
549 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
550 assert(!(PvNode && cutNode));
551 assert(depth / ONE_PLY * ONE_PLY == depth);
553 Move pv[MAX_PLY+1], quietsSearched[64];
557 Move ttMove, move, excludedMove, bestMove;
558 Depth extension, newDepth;
559 Value bestValue, value, ttValue, eval, nullValue;
560 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
561 bool captureOrPromotion, doFullDepthSearch, moveCountPruning;
563 int moveCount, quietCount;
565 // Step 1. Initialize node
566 Thread* thisThread = pos.this_thread();
567 inCheck = pos.checkers();
568 moveCount = quietCount = ss->moveCount = 0;
569 ss->history = VALUE_ZERO;
570 bestValue = -VALUE_INFINITE;
571 ss->ply = (ss-1)->ply + 1;
573 // Check for the available remaining time
574 if (thisThread->resetCalls.load(std::memory_order_relaxed))
576 thisThread->resetCalls = false;
577 thisThread->callsCnt = 0;
579 if (++thisThread->callsCnt > 4096)
581 for (Thread* th : Threads)
582 th->resetCalls = true;
587 // Used to send selDepth info to GUI
588 if (PvNode && thisThread->maxPly < ss->ply)
589 thisThread->maxPly = ss->ply;
593 // Step 2. Check for aborted search and immediate draw
594 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw() || ss->ply >= MAX_PLY)
595 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
596 : DrawValue[pos.side_to_move()];
598 // Step 3. Mate distance pruning. Even if we mate at the next move our score
599 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
600 // a shorter mate was found upward in the tree then there is no need to search
601 // because we will never beat the current alpha. Same logic but with reversed
602 // signs applies also in the opposite condition of being mated instead of giving
603 // mate. In this case return a fail-high score.
604 alpha = std::max(mated_in(ss->ply), alpha);
605 beta = std::min(mate_in(ss->ply+1), beta);
610 assert(0 <= ss->ply && ss->ply < MAX_PLY);
612 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
613 ss->counterMoves = nullptr;
614 (ss+1)->skipEarlyPruning = false;
615 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
617 // Step 4. Transposition table lookup. We don't want the score of a partial
618 // search to overwrite a previous full search TT value, so we use a different
619 // position key in case of an excluded move.
620 excludedMove = ss->excludedMove;
621 posKey = pos.key() ^ Key(excludedMove);
622 tte = TT.probe(posKey, ttHit);
623 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
624 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
625 : ttHit ? tte->move() : MOVE_NONE;
627 // At non-PV nodes we check for an early TT cutoff
630 && tte->depth() >= depth
631 && ttValue != VALUE_NONE // Possible in case of TT access race
632 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
633 : (tte->bound() & BOUND_UPPER)))
635 // If ttMove is quiet, update killers, history, counter move on TT hit
636 if (ttValue >= beta && ttMove)
638 int d = depth / ONE_PLY;
640 if (!pos.capture_or_promotion(ttMove))
642 Value bonus = Value(d * d + 2 * d - 2);
643 update_stats(pos, ss, ttMove, nullptr, 0, bonus);
646 // Extra penalty for a quiet TT move in previous ply when it gets refuted
647 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
649 Value penalty = Value(d * d + 4 * d + 1);
650 Square prevSq = to_sq((ss-1)->currentMove);
651 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -penalty);
657 // Step 4a. Tablebase probe
658 if (!rootNode && TB::Cardinality)
660 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
662 if ( piecesCnt <= TB::Cardinality
663 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
664 && pos.rule50_count() == 0
665 && !pos.can_castle(ANY_CASTLING))
668 TB::WDLScore v = Tablebases::probe_wdl(pos, &err);
670 if (err != TB::ProbeState::FAIL)
672 thisThread->tbHits++;
674 int drawScore = TB::UseRule50 ? 1 : 0;
676 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
677 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
678 : VALUE_DRAW + 2 * v * drawScore;
680 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
681 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
682 MOVE_NONE, VALUE_NONE, TT.generation());
689 // Step 5. Evaluate the position statically
692 ss->staticEval = eval = VALUE_NONE;
698 // Never assume anything on values stored in TT
699 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
700 eval = ss->staticEval = evaluate(pos);
702 // Can ttValue be used as a better position evaluation?
703 if (ttValue != VALUE_NONE)
704 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
709 eval = ss->staticEval =
710 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
711 : -(ss-1)->staticEval + 2 * Eval::Tempo;
713 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
714 ss->staticEval, TT.generation());
717 if (ss->skipEarlyPruning)
720 // Step 6. Razoring (skipped when in check)
722 && depth < 4 * ONE_PLY
723 && ttMove == MOVE_NONE
724 && eval + razor_margin[depth / ONE_PLY] <= alpha)
726 if (depth <= ONE_PLY)
727 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
729 Value ralpha = alpha - razor_margin[depth / ONE_PLY];
730 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
735 // Step 7. Futility pruning: child node (skipped when in check)
737 && depth < 7 * ONE_PLY
738 && eval - futility_margin(depth) >= beta
739 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
740 && pos.non_pawn_material(pos.side_to_move()))
743 // Step 8. Null move search with verification search (is omitted in PV nodes)
746 && (ss->staticEval >= beta - 35 * (depth / ONE_PLY - 6) || depth >= 13 * ONE_PLY)
747 && pos.non_pawn_material(pos.side_to_move()))
749 ss->currentMove = MOVE_NULL;
750 ss->counterMoves = nullptr;
752 assert(eval - beta >= 0);
754 // Null move dynamic reduction based on depth and value
755 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
757 pos.do_null_move(st);
758 (ss+1)->skipEarlyPruning = true;
759 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
760 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
761 (ss+1)->skipEarlyPruning = false;
762 pos.undo_null_move();
764 if (nullValue >= beta)
766 // Do not return unproven mate scores
767 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
770 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
773 // Do verification search at high depths
774 ss->skipEarlyPruning = true;
775 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
776 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
777 ss->skipEarlyPruning = false;
784 // Step 9. ProbCut (skipped when in check)
785 // If we have a good enough capture and a reduced search returns a value
786 // much above beta, we can (almost) safely prune the previous move.
788 && depth >= 5 * ONE_PLY
789 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
791 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
792 Depth rdepth = depth - 4 * ONE_PLY;
794 assert(rdepth >= ONE_PLY);
795 assert((ss-1)->currentMove != MOVE_NONE);
796 assert((ss-1)->currentMove != MOVE_NULL);
798 MovePicker mp(pos, ttMove, rbeta - ss->staticEval);
800 while ((move = mp.next_move()) != MOVE_NONE)
803 ss->currentMove = move;
804 ss->counterMoves = &thisThread->counterMoveHistory[pos.moved_piece(move)][to_sq(move)];
805 pos.do_move(move, st, pos.gives_check(move));
806 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
813 // Step 10. Internal iterative deepening (skipped when in check)
814 if ( depth >= 6 * ONE_PLY
816 && (PvNode || ss->staticEval + 256 >= beta))
818 Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
819 ss->skipEarlyPruning = true;
820 search<NT>(pos, ss, alpha, beta, d, cutNode);
821 ss->skipEarlyPruning = false;
823 tte = TT.probe(posKey, ttHit);
824 ttMove = ttHit ? tte->move() : MOVE_NONE;
827 moves_loop: // When in check search starts from here
829 const CounterMoveStats* cmh = (ss-1)->counterMoves;
830 const CounterMoveStats* fmh = (ss-2)->counterMoves;
831 const CounterMoveStats* fmh2 = (ss-4)->counterMoves;
833 MovePicker mp(pos, ttMove, depth, ss);
834 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
835 improving = ss->staticEval >= (ss-2)->staticEval
836 /* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
837 ||(ss-2)->staticEval == VALUE_NONE;
839 singularExtensionNode = !rootNode
840 && depth >= 8 * ONE_PLY
841 && ttMove != MOVE_NONE
842 && ttValue != VALUE_NONE
843 && !excludedMove // Recursive singular search is not allowed
844 && (tte->bound() & BOUND_LOWER)
845 && tte->depth() >= depth - 3 * ONE_PLY;
847 // Step 11. Loop through moves
848 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
849 while ((move = mp.next_move()) != MOVE_NONE)
853 if (move == excludedMove)
856 // At root obey the "searchmoves" option and skip moves not listed in Root
857 // Move List. As a consequence any illegal move is also skipped. In MultiPV
858 // mode we also skip PV moves which have been already searched.
859 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
860 thisThread->rootMoves.end(), move))
863 ss->moveCount = ++moveCount;
865 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
866 sync_cout << "info depth " << depth / ONE_PLY
867 << " currmove " << UCI::move(move, pos.is_chess960())
868 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
871 (ss+1)->pv = nullptr;
873 extension = DEPTH_ZERO;
874 captureOrPromotion = pos.capture_or_promotion(move);
875 moved_piece = pos.moved_piece(move);
877 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
878 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
879 : pos.gives_check(move);
881 moveCountPruning = depth < 16 * ONE_PLY
882 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
884 // Step 12. Extend checks
887 && pos.see_ge(move, VALUE_ZERO))
890 // Singular extension search. If all moves but one fail low on a search of
891 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
892 // is singular and should be extended. To verify this we do a reduced search
893 // on all the other moves but the ttMove and if the result is lower than
894 // ttValue minus a margin then we extend the ttMove.
895 if ( singularExtensionNode
900 Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
901 Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
902 ss->excludedMove = move;
903 ss->skipEarlyPruning = true;
904 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode);
905 ss->skipEarlyPruning = false;
906 ss->excludedMove = MOVE_NONE;
912 // Update the current move (this must be done after singular extension search)
913 newDepth = depth - ONE_PLY + extension;
915 // Step 13. Pruning at shallow depth
917 && bestValue > VALUE_MATED_IN_MAX_PLY)
919 if ( !captureOrPromotion
921 && !pos.advanced_pawn_push(move))
923 // Move count based pruning
924 if (moveCountPruning)
927 // Reduced depth of the next LMR search
928 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
930 // Countermoves based pruning
932 && (!cmh || (*cmh )[moved_piece][to_sq(move)] < VALUE_ZERO)
933 && (!fmh || (*fmh )[moved_piece][to_sq(move)] < VALUE_ZERO)
934 && (!fmh2 || (*fmh2)[moved_piece][to_sq(move)] < VALUE_ZERO || (cmh && fmh)))
937 // Futility pruning: parent node
940 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
943 // Prune moves with negative SEE
945 && !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
948 else if ( depth < 7 * ONE_PLY
950 && !pos.see_ge(move, Value(-35 * depth / ONE_PLY * depth / ONE_PLY)))
954 // Speculative prefetch as early as possible
955 prefetch(TT.first_entry(pos.key_after(move)));
957 // Check for legality just before making the move
958 if (!rootNode && !pos.legal(move))
960 ss->moveCount = --moveCount;
964 ss->currentMove = move;
965 ss->counterMoves = &thisThread->counterMoveHistory[moved_piece][to_sq(move)];
967 // Step 14. Make the move
968 pos.do_move(move, st, givesCheck);
970 // Step 15. Reduced depth search (LMR). If the move fails high it will be
971 // re-searched at full depth.
972 if ( depth >= 3 * ONE_PLY
974 && (!captureOrPromotion || moveCountPruning))
976 Depth r = reduction<PvNode>(improving, depth, moveCount);
978 if (captureOrPromotion)
979 r -= r ? ONE_PLY : DEPTH_ZERO;
982 // Increase reduction for cut nodes
986 // Decrease reduction for moves that escape a capture. Filter out
987 // castling moves, because they are coded as "king captures rook" and
988 // hence break make_move(). Also use see() instead of see_sign(),
989 // because the destination square is empty.
990 else if ( type_of(move) == NORMAL
991 && type_of(pos.piece_on(to_sq(move))) != PAWN
992 && !pos.see_ge(make_move(to_sq(move), from_sq(move)), VALUE_ZERO))
995 ss->history = thisThread->history[moved_piece][to_sq(move)]
996 + (cmh ? (*cmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
997 + (fmh ? (*fmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
998 + (fmh2 ? (*fmh2)[moved_piece][to_sq(move)] : VALUE_ZERO)
999 + thisThread->fromTo.get(~pos.side_to_move(), move)
1000 - 8000; // Correction factor
1002 // Decrease/increase reduction by comparing opponent's stat score
1003 if (ss->history > VALUE_ZERO && (ss-1)->history < VALUE_ZERO)
1006 else if (ss->history < VALUE_ZERO && (ss-1)->history > VALUE_ZERO)
1009 // Decrease/increase reduction for moves with a good/bad history
1010 r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->history / 20000) * ONE_PLY);
1013 Depth d = std::max(newDepth - r, ONE_PLY);
1015 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1017 doFullDepthSearch = (value > alpha && d != newDepth);
1020 doFullDepthSearch = !PvNode || moveCount > 1;
1022 // Step 16. Full depth search when LMR is skipped or fails high
1023 if (doFullDepthSearch)
1024 value = newDepth < ONE_PLY ?
1025 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1026 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1027 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1029 // For PV nodes only, do a full PV search on the first move or after a fail
1030 // high (in the latter case search only if value < beta), otherwise let the
1031 // parent node fail low with value <= alpha and try another move.
1032 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1035 (ss+1)->pv[0] = MOVE_NONE;
1037 value = newDepth < ONE_PLY ?
1038 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1039 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1040 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1043 // Step 17. Undo move
1044 pos.undo_move(move);
1046 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1048 // Step 18. Check for a new best move
1049 // Finished searching the move. If a stop occurred, the return value of
1050 // the search cannot be trusted, and we return immediately without
1051 // updating best move, PV and TT.
1052 if (Signals.stop.load(std::memory_order_relaxed))
1057 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1058 thisThread->rootMoves.end(), move);
1060 // PV move or new best move ?
1061 if (moveCount == 1 || value > alpha)
1068 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1069 rm.pv.push_back(*m);
1071 // We record how often the best move has been changed in each
1072 // iteration. This information is used for time management: When
1073 // the best move changes frequently, we allocate some more time.
1074 if (moveCount > 1 && thisThread == Threads.main())
1075 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1078 // All other moves but the PV are set to the lowest value: this is
1079 // not a problem when sorting because the sort is stable and the
1080 // move position in the list is preserved - just the PV is pushed up.
1081 rm.score = -VALUE_INFINITE;
1084 if (value > bestValue)
1090 // If there is an easy move for this position, clear it if unstable
1092 && thisThread == Threads.main()
1093 && EasyMove.get(pos.key())
1094 && (move != EasyMove.get(pos.key()) || moveCount > 1))
1099 if (PvNode && !rootNode) // Update pv even in fail-high case
1100 update_pv(ss->pv, move, (ss+1)->pv);
1102 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1106 assert(value >= beta); // Fail high
1112 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1113 quietsSearched[quietCount++] = move;
1116 // The following condition would detect a stop only after move loop has been
1117 // completed. But in this case bestValue is valid because we have fully
1118 // searched our subtree, and we can anyhow save the result in TT.
1124 // Step 20. Check for mate and stalemate
1125 // All legal moves have been searched and if there are no legal moves, it
1126 // must be a mate or a stalemate. If we are in a singular extension search then
1127 // return a fail low score.
1129 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1132 bestValue = excludedMove ? alpha
1133 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1136 int d = depth / ONE_PLY;
1138 // Quiet best move: update killers, history and countermoves
1139 if (!pos.capture_or_promotion(bestMove))
1141 Value bonus = Value(d * d + 2 * d - 2);
1142 update_stats(pos, ss, bestMove, quietsSearched, quietCount, bonus);
1145 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1146 if ((ss-1)->moveCount == 1 && !pos.captured_piece())
1148 Value penalty = Value(d * d + 4 * d + 1);
1149 Square prevSq = to_sq((ss-1)->currentMove);
1150 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, -penalty);
1153 // Bonus for prior countermove that caused the fail low
1154 else if ( depth >= 3 * ONE_PLY
1155 && !pos.captured_piece()
1156 && is_ok((ss-1)->currentMove))
1158 int d = depth / ONE_PLY;
1159 Value bonus = Value(d * d + 2 * d - 2);
1160 Square prevSq = to_sq((ss-1)->currentMove);
1161 update_cm_stats(ss-1, pos.piece_on(prevSq), prevSq, bonus);
1164 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1165 bestValue >= beta ? BOUND_LOWER :
1166 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1167 depth, bestMove, ss->staticEval, TT.generation());
1169 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1175 // qsearch() is the quiescence search function, which is called by the main
1176 // search function when the remaining depth is zero (or, to be more precise,
1177 // less than ONE_PLY).
1179 template <NodeType NT, bool InCheck>
1180 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1182 const bool PvNode = NT == PV;
1184 assert(InCheck == !!pos.checkers());
1185 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1186 assert(PvNode || (alpha == beta - 1));
1187 assert(depth <= DEPTH_ZERO);
1188 assert(depth / ONE_PLY * ONE_PLY == depth);
1194 Move ttMove, move, bestMove;
1195 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1196 bool ttHit, givesCheck, evasionPrunable;
1201 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1203 ss->pv[0] = MOVE_NONE;
1206 ss->currentMove = bestMove = MOVE_NONE;
1207 ss->ply = (ss-1)->ply + 1;
1209 // Check for an instant draw or if the maximum ply has been reached
1210 if (pos.is_draw() || ss->ply >= MAX_PLY)
1211 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1212 : DrawValue[pos.side_to_move()];
1214 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1216 // Decide whether or not to include checks: this fixes also the type of
1217 // TT entry depth that we are going to use. Note that in qsearch we use
1218 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1219 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1220 : DEPTH_QS_NO_CHECKS;
1222 // Transposition table lookup
1224 tte = TT.probe(posKey, ttHit);
1225 ttMove = ttHit ? tte->move() : MOVE_NONE;
1226 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1230 && tte->depth() >= ttDepth
1231 && ttValue != VALUE_NONE // Only in case of TT access race
1232 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1233 : (tte->bound() & BOUND_UPPER)))
1236 // Evaluate the position statically
1239 ss->staticEval = VALUE_NONE;
1240 bestValue = futilityBase = -VALUE_INFINITE;
1246 // Never assume anything on values stored in TT
1247 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1248 ss->staticEval = bestValue = evaluate(pos);
1250 // Can ttValue be used as a better position evaluation?
1251 if (ttValue != VALUE_NONE)
1252 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1253 bestValue = ttValue;
1256 ss->staticEval = bestValue =
1257 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1258 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1260 // Stand pat. Return immediately if static value is at least beta
1261 if (bestValue >= beta)
1264 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1265 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1270 if (PvNode && bestValue > alpha)
1273 futilityBase = bestValue + 128;
1276 // Initialize a MovePicker object for the current position, and prepare
1277 // to search the moves. Because the depth is <= 0 here, only captures,
1278 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1280 MovePicker mp(pos, ttMove, depth, to_sq((ss-1)->currentMove));
1282 // Loop through the moves until no moves remain or a beta cutoff occurs
1283 while ((move = mp.next_move()) != MOVE_NONE)
1285 assert(is_ok(move));
1287 givesCheck = type_of(move) == NORMAL && !pos.discovered_check_candidates()
1288 ? pos.check_squares(type_of(pos.piece_on(from_sq(move)))) & to_sq(move)
1289 : pos.gives_check(move);
1294 && futilityBase > -VALUE_KNOWN_WIN
1295 && !pos.advanced_pawn_push(move))
1297 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1299 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1301 if (futilityValue <= alpha)
1303 bestValue = std::max(bestValue, futilityValue);
1307 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1309 bestValue = std::max(bestValue, futilityBase);
1314 // Detect non-capture evasions that are candidates to be pruned
1315 evasionPrunable = InCheck
1316 && bestValue > VALUE_MATED_IN_MAX_PLY
1317 && !pos.capture(move);
1319 // Don't search moves with negative SEE values
1320 if ( (!InCheck || evasionPrunable)
1321 && type_of(move) != PROMOTION
1322 && !pos.see_ge(move, VALUE_ZERO))
1325 // Speculative prefetch as early as possible
1326 prefetch(TT.first_entry(pos.key_after(move)));
1328 // Check for legality just before making the move
1329 if (!pos.legal(move))
1332 ss->currentMove = move;
1334 // Make and search the move
1335 pos.do_move(move, st, givesCheck);
1336 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1337 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1338 pos.undo_move(move);
1340 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1342 // Check for a new best move
1343 if (value > bestValue)
1349 if (PvNode) // Update pv even in fail-high case
1350 update_pv(ss->pv, move, (ss+1)->pv);
1352 if (PvNode && value < beta) // Update alpha here!
1359 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1360 ttDepth, move, ss->staticEval, TT.generation());
1368 // All legal moves have been searched. A special case: If we're in check
1369 // and no legal moves were found, it is checkmate.
1370 if (InCheck && bestValue == -VALUE_INFINITE)
1371 return mated_in(ss->ply); // Plies to mate from the root
1373 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1374 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1375 ttDepth, bestMove, ss->staticEval, TT.generation());
1377 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1383 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1384 // "plies to mate from the current position". Non-mate scores are unchanged.
1385 // The function is called before storing a value in the transposition table.
1387 Value value_to_tt(Value v, int ply) {
1389 assert(v != VALUE_NONE);
1391 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1392 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1396 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1397 // from the transposition table (which refers to the plies to mate/be mated
1398 // from current position) to "plies to mate/be mated from the root".
1400 Value value_from_tt(Value v, int ply) {
1402 return v == VALUE_NONE ? VALUE_NONE
1403 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1404 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1408 // update_pv() adds current move and appends child pv[]
1410 void update_pv(Move* pv, Move move, Move* childPv) {
1412 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1418 // update_cm_stats() updates countermove and follow-up move history
1420 void update_cm_stats(Stack* ss, Piece pc, Square s, Value bonus) {
1422 CounterMoveStats* cmh = (ss-1)->counterMoves;
1423 CounterMoveStats* fmh1 = (ss-2)->counterMoves;
1424 CounterMoveStats* fmh2 = (ss-4)->counterMoves;
1427 cmh->update(pc, s, bonus);
1430 fmh1->update(pc, s, bonus);
1433 fmh2->update(pc, s, bonus);
1437 // update_stats() updates killers, history, countermove and countermove plus
1438 // follow-up move history when a new quiet best move is found.
1440 void update_stats(const Position& pos, Stack* ss, Move move,
1441 Move* quiets, int quietsCnt, Value bonus) {
1443 if (ss->killers[0] != move)
1445 ss->killers[1] = ss->killers[0];
1446 ss->killers[0] = move;
1449 Color c = pos.side_to_move();
1450 Thread* thisThread = pos.this_thread();
1451 thisThread->fromTo.update(c, move, bonus);
1452 thisThread->history.update(pos.moved_piece(move), to_sq(move), bonus);
1453 update_cm_stats(ss, pos.moved_piece(move), to_sq(move), bonus);
1455 if ((ss-1)->counterMoves)
1457 Square prevSq = to_sq((ss-1)->currentMove);
1458 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1461 // Decrease all the other played quiet moves
1462 for (int i = 0; i < quietsCnt; ++i)
1464 thisThread->fromTo.update(c, quiets[i], -bonus);
1465 thisThread->history.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1466 update_cm_stats(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1471 // When playing with strength handicap, choose best move among a set of RootMoves
1472 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1474 Move Skill::pick_best(size_t multiPV) {
1476 const RootMoves& rootMoves = Threads.main()->rootMoves;
1477 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1479 // RootMoves are already sorted by score in descending order
1480 Value topScore = rootMoves[0].score;
1481 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1482 int weakness = 120 - 2 * level;
1483 int maxScore = -VALUE_INFINITE;
1485 // Choose best move. For each move score we add two terms, both dependent on
1486 // weakness. One is deterministic and bigger for weaker levels, and one is
1487 // random. Then we choose the move with the resulting highest score.
1488 for (size_t i = 0; i < multiPV; ++i)
1490 // This is our magic formula
1491 int push = ( weakness * int(topScore - rootMoves[i].score)
1492 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1494 if (rootMoves[i].score + push > maxScore)
1496 maxScore = rootMoves[i].score + push;
1497 best = rootMoves[i].pv[0];
1505 // check_time() is used to print debug info and, more importantly, to detect
1506 // when we are out of available time and thus stop the search.
1510 static TimePoint lastInfoTime = now();
1512 int elapsed = Time.elapsed();
1513 TimePoint tick = Limits.startTime + elapsed;
1515 if (tick - lastInfoTime >= 1000)
1517 lastInfoTime = tick;
1521 // An engine may not stop pondering until told so by the GUI
1525 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1526 || (Limits.movetime && elapsed >= Limits.movetime)
1527 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1528 Signals.stop = true;
1534 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1535 /// that all (if any) unsearched PV lines are sent using a previous search score.
1537 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1539 std::stringstream ss;
1540 int elapsed = Time.elapsed() + 1;
1541 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1542 size_t PVIdx = pos.this_thread()->PVIdx;
1543 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1544 uint64_t nodesSearched = Threads.nodes_searched();
1545 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1547 for (size_t i = 0; i < multiPV; ++i)
1549 bool updated = (i <= PVIdx);
1551 if (depth == ONE_PLY && !updated)
1554 Depth d = updated ? depth : depth - ONE_PLY;
1555 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1557 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1558 v = tb ? TB::Score : v;
1560 if (ss.rdbuf()->in_avail()) // Not at first line
1564 << " depth " << d / ONE_PLY
1565 << " seldepth " << pos.this_thread()->maxPly
1566 << " multipv " << i + 1
1567 << " score " << UCI::value(v);
1569 if (!tb && i == PVIdx)
1570 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1572 ss << " nodes " << nodesSearched
1573 << " nps " << nodesSearched * 1000 / elapsed;
1575 if (elapsed > 1000) // Earlier makes little sense
1576 ss << " hashfull " << TT.hashfull();
1578 ss << " tbhits " << tbHits
1579 << " time " << elapsed
1582 for (Move m : rootMoves[i].pv)
1583 ss << " " << UCI::move(m, pos.is_chess960());
1590 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1591 /// before exiting the search, for instance, in case we stop the search during a
1592 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1593 /// otherwise in case of 'ponder on' we have nothing to think on.
1595 bool RootMove::extract_ponder_from_tt(Position& pos) {
1600 assert(pv.size() == 1);
1605 pos.do_move(pv[0], st, pos.gives_check(pv[0]));
1606 TTEntry* tte = TT.probe(pos.key(), ttHit);
1610 Move m = tte->move(); // Local copy to be SMP safe
1611 if (MoveList<LEGAL>(pos).contains(m))
1615 pos.undo_move(pv[0]);
1616 return pv.size() > 1;
1619 void Tablebases::filter_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1622 UseRule50 = Options["Syzygy50MoveRule"];
1623 ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
1624 Cardinality = Options["SyzygyProbeLimit"];
1626 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
1627 if (Cardinality > MaxCardinality)
1629 Cardinality = MaxCardinality;
1630 ProbeDepth = DEPTH_ZERO;
1633 if (Cardinality < popcount(pos.pieces()) || pos.can_castle(ANY_CASTLING))
1636 // If the current root position is in the tablebases, then RootMoves
1637 // contains only moves that preserve the draw or the win.
1638 RootInTB = root_probe(pos, rootMoves, TB::Score);
1641 Cardinality = 0; // Do not probe tablebases during the search
1643 else // If DTZ tables are missing, use WDL tables as a fallback
1645 // Filter out moves that do not preserve the draw or the win.
1646 RootInTB = root_probe_wdl(pos, rootMoves, TB::Score);
1648 // Only probe during search if winning
1649 if (RootInTB && TB::Score <= VALUE_DRAW)
1653 if (RootInTB && !UseRule50)
1654 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
1655 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1