2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2016 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
24 #include <cstring> // For std::memset
37 #include "syzygy/tbprobe.h"
43 StateStackPtr SetupStates;
46 namespace Tablebases {
56 namespace TB = Tablebases;
60 using namespace Search;
64 // Different node types, used as a template parameter
65 enum NodeType { NonPV, PV };
67 // Razoring and futility margin based on depth
68 const int razor_margin[4] = { 483, 570, 603, 554 };
69 Value futility_margin(Depth d) { return Value(200 * d); }
71 // Futility and reductions lookup tables, initialized at startup
72 int FutilityMoveCounts[2][16]; // [improving][depth]
73 Depth Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
75 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
76 return Reductions[PvNode][i][std::min(d, 63 * ONE_PLY)][std::min(mn, 63)];
79 // Skill structure is used to implement strength limit
81 Skill(int l) : level(l) {}
82 bool enabled() const { return level < 20; }
83 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
84 Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
85 Move pick_best(size_t multiPV);
88 Move best = MOVE_NONE;
91 // EasyMoveManager structure is used to detect an 'easy move'. When the PV is
92 // stable across multiple search iterations, we can quickly return the best move.
93 struct EasyMoveManager {
98 pv[0] = pv[1] = pv[2] = MOVE_NONE;
101 Move get(Key key) const {
102 return expectedPosKey == key ? pv[2] : MOVE_NONE;
105 void update(Position& pos, const std::vector<Move>& newPv) {
107 assert(newPv.size() >= 3);
109 // Keep track of how many times in a row the 3rd ply remains stable
110 stableCnt = (newPv[2] == pv[2]) ? stableCnt + 1 : 0;
112 if (!std::equal(newPv.begin(), newPv.begin() + 3, pv))
114 std::copy(newPv.begin(), newPv.begin() + 3, pv);
117 pos.do_move(newPv[0], st[0], pos.gives_check(newPv[0], CheckInfo(pos)));
118 pos.do_move(newPv[1], st[1], pos.gives_check(newPv[1], CheckInfo(pos)));
119 expectedPosKey = pos.key();
120 pos.undo_move(newPv[1]);
121 pos.undo_move(newPv[0]);
130 // Set of rows with half bits set to 1 and half to 0. It is used to allocate
131 // the search depths across the threads.
132 typedef std::vector<int> Row;
134 const Row HalfDensity[] = {
147 {0, 0, 0, 0, 1, 1, 1, 1},
148 {0, 0, 0, 1, 1, 1, 1, 0},
149 {0, 0, 1, 1, 1, 1, 0 ,0},
150 {0, 1, 1, 1, 1, 0, 0 ,0},
151 {1, 1, 1, 1, 0, 0, 0 ,0},
152 {1, 1, 1, 0, 0, 0, 0 ,1},
153 {1, 1, 0, 0, 0, 0, 1 ,1},
154 {1, 0, 0, 0, 0, 1, 1 ,1},
157 const size_t HalfDensitySize = std::extent<decltype(HalfDensity)>::value;
159 EasyMoveManager EasyMove;
160 Value DrawValue[COLOR_NB];
161 CounterMoveHistoryStats CounterMoveHistory;
163 template <NodeType NT>
164 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
166 template <NodeType NT, bool InCheck>
167 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
169 Value value_to_tt(Value v, int ply);
170 Value value_from_tt(Value v, int ply);
171 void update_pv(Move* pv, Move move, Move* childPv);
172 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
178 /// Search::init() is called during startup to initialize various lookup tables
180 void Search::init() {
182 for (int imp = 0; imp <= 1; ++imp)
183 for (int d = 1; d < 64; ++d)
184 for (int mc = 1; mc < 64; ++mc)
186 double r = log(d) * log(mc) / 2;
190 Reductions[NonPV][imp][d][mc] = int(std::round(r)) * ONE_PLY;
191 Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - ONE_PLY, DEPTH_ZERO);
193 // Increase reduction for non-PV nodes when eval is not improving
194 if (!imp && Reductions[NonPV][imp][d][mc] >= 2 * ONE_PLY)
195 Reductions[NonPV][imp][d][mc] += ONE_PLY;
198 for (int d = 0; d < 16; ++d)
200 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
201 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
206 /// Search::clear() resets search state to zero, to obtain reproducible results
208 void Search::clear() {
211 CounterMoveHistory.clear();
213 for (Thread* th : Threads)
216 th->counterMoves.clear();
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;
231 const bool leaf = (depth == 2 * ONE_PLY);
233 for (const auto& m : MoveList<LEGAL>(pos))
235 if (Root && depth <= ONE_PLY)
239 pos.do_move(m, st, pos.gives_check(m, ci));
240 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
245 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
250 template uint64_t Search::perft<true>(Position&, Depth);
253 /// MainThread::search() is called by the main thread when the program receives
254 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
256 void MainThread::search() {
258 Color us = rootPos.side_to_move();
259 Time.init(Limits, us, rootPos.game_ply());
261 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
262 DrawValue[ us] = VALUE_DRAW - Value(contempt);
263 DrawValue[~us] = VALUE_DRAW + Value(contempt);
266 TB::RootInTB = false;
267 TB::UseRule50 = Options["Syzygy50MoveRule"];
268 TB::ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
269 TB::Cardinality = Options["SyzygyProbeLimit"];
271 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
272 if (TB::Cardinality > TB::MaxCardinality)
274 TB::Cardinality = TB::MaxCardinality;
275 TB::ProbeDepth = DEPTH_ZERO;
278 if (rootMoves.empty())
280 rootMoves.push_back(RootMove(MOVE_NONE));
281 sync_cout << "info depth 0 score "
282 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
287 if ( TB::Cardinality >= rootPos.count<ALL_PIECES>(WHITE)
288 + rootPos.count<ALL_PIECES>(BLACK)
289 && !rootPos.can_castle(ANY_CASTLING))
291 // If the current root position is in the tablebases, then RootMoves
292 // contains only moves that preserve the draw or the win.
293 TB::RootInTB = Tablebases::root_probe(rootPos, rootMoves, TB::Score);
296 TB::Cardinality = 0; // Do not probe tablebases during the search
298 else // If DTZ tables are missing, use WDL tables as a fallback
300 // Filter out moves that do not preserve the draw or the win.
301 TB::RootInTB = Tablebases::root_probe_wdl(rootPos, rootMoves, TB::Score);
303 // Only probe during search if winning
304 if (TB::Score <= VALUE_DRAW)
310 TB::Hits = rootMoves.size();
313 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
314 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
319 for (Thread* th : Threads)
322 th->rootDepth = DEPTH_ZERO;
325 th->rootPos = Position(rootPos, th);
326 th->rootMoves = rootMoves;
327 th->start_searching();
331 Thread::search(); // Let's start searching!
334 // When playing in 'nodes as time' mode, subtract the searched nodes from
335 // the available ones before exiting.
337 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
339 // When we reach the maximum depth, we can arrive here without a raise of
340 // Signals.stop. However, if we are pondering or in an infinite search,
341 // the UCI protocol states that we shouldn't print the best move before the
342 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
343 // until the GUI sends one of those commands (which also raises Signals.stop).
344 if (!Signals.stop && (Limits.ponder || Limits.infinite))
346 Signals.stopOnPonderhit = true;
350 // Stop the threads if not already stopped
353 // Wait until all threads have finished
354 for (Thread* th : Threads)
356 th->wait_for_search_finished();
358 // Check if there are threads with a better score than main thread
359 Thread* bestThread = this;
360 if ( !this->easyMovePlayed
361 && Options["MultiPV"] == 1
362 && !Skill(Options["Skill Level"]).enabled())
364 for (Thread* th : Threads)
365 if ( th->completedDepth > bestThread->completedDepth
366 && th->rootMoves[0].score > bestThread->rootMoves[0].score)
370 previousScore = bestThread->rootMoves[0].score;
372 // Send new PV when needed
373 if (bestThread != this)
374 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
376 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
378 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
379 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
381 std::cout << sync_endl;
385 // Thread::search() is the main iterative deepening loop. It calls search()
386 // repeatedly with increasing depth until the allocated thinking time has been
387 // consumed, the user stops the search, or the maximum search depth is reached.
389 void Thread::search() {
391 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
392 Value bestValue, alpha, beta, delta;
393 Move easyMove = MOVE_NONE;
394 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
396 std::memset(ss-2, 0, 5 * sizeof(Stack));
398 bestValue = delta = alpha = -VALUE_INFINITE;
399 beta = VALUE_INFINITE;
400 completedDepth = DEPTH_ZERO;
404 easyMove = EasyMove.get(rootPos.key());
406 mainThread->easyMovePlayed = mainThread->failedLow = false;
407 mainThread->bestMoveChanges = 0;
411 size_t multiPV = Options["MultiPV"];
412 Skill skill(Options["Skill Level"]);
414 // When playing with strength handicap enable MultiPV search that we will
415 // use behind the scenes to retrieve a set of possible moves.
417 multiPV = std::max(multiPV, (size_t)4);
419 multiPV = std::min(multiPV, rootMoves.size());
421 // Iterative deepening loop until requested to stop or the target depth is reached.
422 while (++rootDepth < DEPTH_MAX && !Signals.stop && (!Limits.depth || rootDepth <= Limits.depth))
424 // Set up the new depths for the helper threads skipping on average every
425 // 2nd ply (using a half-density matrix).
428 const Row& row = HalfDensity[(idx - 1) % HalfDensitySize];
429 if (row[(rootDepth + rootPos.game_ply()) % row.size()])
433 // Age out PV variability metric
435 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
437 // Save the last iteration's scores before first PV line is searched and
438 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
439 for (RootMove& rm : rootMoves)
440 rm.previousScore = rm.score;
442 // MultiPV loop. We perform a full root search for each PV line
443 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
445 // Reset aspiration window starting size
446 if (rootDepth >= 5 * ONE_PLY)
449 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
450 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
453 // Start with a small aspiration window and, in the case of a fail
454 // high/low, re-search with a bigger window until we're not failing
458 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false);
460 // Bring the best move to the front. It is critical that sorting
461 // is done with a stable algorithm because all the values but the
462 // first and eventually the new best one are set to -VALUE_INFINITE
463 // and we want to keep the same order for all the moves except the
464 // new PV that goes to the front. Note that in case of MultiPV
465 // search the already searched PV lines are preserved.
466 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
468 // Write PV back to the transposition table in case the relevant
469 // entries have been overwritten during the search.
470 for (size_t i = 0; i <= PVIdx; ++i)
471 rootMoves[i].insert_pv_in_tt(rootPos);
473 // If search has been stopped, break immediately. Sorting and
474 // writing PV back to TT is safe because RootMoves is still
475 // valid, although it refers to the previous iteration.
479 // When failing high/low give some update (without cluttering
480 // the UI) before a re-search.
483 && (bestValue <= alpha || bestValue >= beta)
484 && Time.elapsed() > 3000)
485 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
487 // In case of failing low/high increase aspiration window and
488 // re-search, otherwise exit the loop.
489 if (bestValue <= alpha)
491 beta = (alpha + beta) / 2;
492 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
496 mainThread->failedLow = true;
497 Signals.stopOnPonderhit = false;
500 else if (bestValue >= beta)
502 alpha = (alpha + beta) / 2;
503 beta = std::min(bestValue + delta, VALUE_INFINITE);
508 delta += delta / 4 + 5;
510 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
513 // Sort the PV lines searched so far and update the GUI
514 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
520 sync_cout << "info nodes " << Threads.nodes_searched()
521 << " time " << Time.elapsed() << sync_endl;
523 else if (PVIdx + 1 == multiPV || Time.elapsed() > 3000)
524 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
528 completedDepth = rootDepth;
533 // If skill level is enabled and time is up, pick a sub-optimal best move
534 if (skill.enabled() && skill.time_to_pick(rootDepth))
535 skill.pick_best(multiPV);
537 // Have we found a "mate in x"?
539 && bestValue >= VALUE_MATE_IN_MAX_PLY
540 && VALUE_MATE - bestValue <= 2 * Limits.mate)
543 // Do we have time for the next iteration? Can we stop searching now?
544 if (Limits.use_time_management())
546 if (!Signals.stop && !Signals.stopOnPonderhit)
548 // Stop the search if only one legal move is available, or if all
549 // of the available time has been used, or if we matched an easyMove
550 // from the previous search and just did a fast verification.
551 const bool F[] = { !mainThread->failedLow,
552 bestValue >= mainThread->previousScore };
554 int improvingFactor = 640 - 160*F[0] - 126*F[1] - 124*F[0]*F[1];
555 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
557 bool doEasyMove = rootMoves[0].pv[0] == easyMove
558 && mainThread->bestMoveChanges < 0.03
559 && Time.elapsed() > Time.optimum() * 25 / 204;
561 if ( rootMoves.size() == 1
562 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 634
563 || (mainThread->easyMovePlayed = doEasyMove))
565 // If we are allowed to ponder do not stop the search now but
566 // keep pondering until the GUI sends "ponderhit" or "stop".
568 Signals.stopOnPonderhit = true;
574 if (rootMoves[0].pv.size() >= 3)
575 EasyMove.update(rootPos, rootMoves[0].pv);
584 // Clear any candidate easy move that wasn't stable for the last search
585 // iterations; the second condition prevents consecutive fast moves.
586 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
589 // If skill level is enabled, swap best PV line with the sub-optimal one
591 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
592 rootMoves.end(), skill.best_move(multiPV)));
598 // search<>() is the main search function for both PV and non-PV nodes
600 template <NodeType NT>
601 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
603 const bool PvNode = NT == PV;
604 const bool rootNode = PvNode && (ss-1)->ply == 0;
606 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
607 assert(PvNode || (alpha == beta - 1));
608 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
610 Move pv[MAX_PLY+1], quietsSearched[64];
614 Move ttMove, move, excludedMove, bestMove;
615 Depth extension, newDepth, predictedDepth;
616 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
617 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
618 bool captureOrPromotion, doFullDepthSearch;
619 int moveCount, quietCount;
621 // Step 1. Initialize node
622 Thread* thisThread = pos.this_thread();
623 inCheck = pos.checkers();
624 moveCount = quietCount = ss->moveCount = 0;
625 bestValue = -VALUE_INFINITE;
626 ss->ply = (ss-1)->ply + 1;
628 // Check for the available remaining time
629 if (thisThread->resetCalls.load(std::memory_order_relaxed))
631 thisThread->resetCalls = false;
632 thisThread->callsCnt = 0;
634 if (++thisThread->callsCnt > 4096)
636 for (Thread* th : Threads)
637 th->resetCalls = true;
642 // Used to send selDepth info to GUI
643 if (PvNode && thisThread->maxPly < ss->ply)
644 thisThread->maxPly = ss->ply;
648 // Step 2. Check for aborted search and immediate draw
649 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw() || ss->ply >= MAX_PLY)
650 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
651 : DrawValue[pos.side_to_move()];
653 // Step 3. Mate distance pruning. Even if we mate at the next move our score
654 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
655 // a shorter mate was found upward in the tree then there is no need to search
656 // because we will never beat the current alpha. Same logic but with reversed
657 // signs applies also in the opposite condition of being mated instead of giving
658 // mate. In this case return a fail-high score.
659 alpha = std::max(mated_in(ss->ply), alpha);
660 beta = std::min(mate_in(ss->ply+1), beta);
665 assert(0 <= ss->ply && ss->ply < MAX_PLY);
667 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
668 (ss+1)->skipEarlyPruning = false;
669 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
671 // Step 4. Transposition table lookup. We don't want the score of a partial
672 // search to overwrite a previous full search TT value, so we use a different
673 // position key in case of an excluded move.
674 excludedMove = ss->excludedMove;
675 posKey = excludedMove ? pos.exclusion_key() : pos.key();
676 tte = TT.probe(posKey, ttHit);
677 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
678 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
679 : ttHit ? tte->move() : MOVE_NONE;
681 // At non-PV nodes we check for an early TT cutoff
684 && tte->depth() >= depth
685 && ttValue != VALUE_NONE // Possible in case of TT access race
686 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
687 : (tte->bound() & BOUND_UPPER)))
689 ss->currentMove = ttMove; // Can be MOVE_NONE
691 // If ttMove is quiet, update killers, history, counter move on TT hit
692 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove))
693 update_stats(pos, ss, ttMove, depth, nullptr, 0);
698 // Step 4a. Tablebase probe
699 if (!rootNode && TB::Cardinality)
701 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
703 if ( piecesCnt <= TB::Cardinality
704 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
705 && pos.rule50_count() == 0
706 && !pos.can_castle(ANY_CASTLING))
708 int found, v = Tablebases::probe_wdl(pos, &found);
714 int drawScore = TB::UseRule50 ? 1 : 0;
716 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
717 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
718 : VALUE_DRAW + 2 * v * drawScore;
720 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
721 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
722 MOVE_NONE, VALUE_NONE, TT.generation());
729 // Step 5. Evaluate the position statically
732 ss->staticEval = eval = VALUE_NONE;
738 // Never assume anything on values stored in TT
739 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
740 eval = ss->staticEval = evaluate(pos);
742 // Can ttValue be used as a better position evaluation?
743 if (ttValue != VALUE_NONE)
744 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
749 eval = ss->staticEval =
750 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
751 : -(ss-1)->staticEval + 2 * Eval::Tempo;
753 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
754 ss->staticEval, TT.generation());
757 if (ss->skipEarlyPruning)
760 // Step 6. Razoring (skipped when in check)
762 && depth < 4 * ONE_PLY
763 && eval + razor_margin[depth] <= alpha
764 && ttMove == MOVE_NONE)
766 if ( depth <= ONE_PLY
767 && eval + razor_margin[3 * ONE_PLY] <= alpha)
768 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
770 Value ralpha = alpha - razor_margin[depth];
771 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
776 // Step 7. Futility pruning: child node (skipped when in check)
778 && depth < 7 * ONE_PLY
779 && eval - futility_margin(depth) >= beta
780 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
781 && pos.non_pawn_material(pos.side_to_move()))
782 return eval - futility_margin(depth);
784 // Step 8. Null move search with verification search (is omitted in PV nodes)
786 && depth >= 2 * ONE_PLY
788 && pos.non_pawn_material(pos.side_to_move()))
790 ss->currentMove = MOVE_NULL;
792 assert(eval - beta >= 0);
794 // Null move dynamic reduction based on depth and value
795 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
797 pos.do_null_move(st);
798 (ss+1)->skipEarlyPruning = true;
799 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
800 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
801 (ss+1)->skipEarlyPruning = false;
802 pos.undo_null_move();
804 if (nullValue >= beta)
806 // Do not return unproven mate scores
807 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
810 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
813 // Do verification search at high depths
814 ss->skipEarlyPruning = true;
815 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
816 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
817 ss->skipEarlyPruning = false;
824 // Step 9. ProbCut (skipped when in check)
825 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
826 // and a reduced search returns a value much above beta, we can (almost)
827 // safely prune the previous move.
829 && depth >= 5 * ONE_PLY
830 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
832 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
833 Depth rdepth = depth - 4 * ONE_PLY;
835 assert(rdepth >= ONE_PLY);
836 assert((ss-1)->currentMove != MOVE_NONE);
837 assert((ss-1)->currentMove != MOVE_NULL);
839 MovePicker mp(pos, ttMove, thisThread->history, PieceValue[MG][pos.captured_piece_type()]);
842 while ((move = mp.next_move()) != MOVE_NONE)
843 if (pos.legal(move, ci.pinned))
845 ss->currentMove = move;
846 pos.do_move(move, st, pos.gives_check(move, ci));
847 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
854 // Step 10. Internal iterative deepening (skipped when in check)
855 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
857 && (PvNode || ss->staticEval + 256 >= beta))
859 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
860 ss->skipEarlyPruning = true;
861 search<NT>(pos, ss, alpha, beta, d, true);
862 ss->skipEarlyPruning = false;
864 tte = TT.probe(posKey, ttHit);
865 ttMove = ttHit ? tte->move() : MOVE_NONE;
868 moves_loop: // When in check search starts from here
870 Square prevSq = to_sq((ss-1)->currentMove);
871 Square ownPrevSq = to_sq((ss-2)->currentMove);
872 Move cm = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
873 const CounterMoveStats& cmh = CounterMoveHistory[pos.piece_on(prevSq)][prevSq];
874 const CounterMoveStats& fmh = CounterMoveHistory[pos.piece_on(ownPrevSq)][ownPrevSq];
876 MovePicker mp(pos, ttMove, depth, thisThread->history, cmh, fmh, cm, ss);
878 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
879 improving = ss->staticEval >= (ss-2)->staticEval
880 || ss->staticEval == VALUE_NONE
881 ||(ss-2)->staticEval == VALUE_NONE;
883 singularExtensionNode = !rootNode
884 && depth >= 8 * ONE_PLY
885 && ttMove != MOVE_NONE
886 /* && ttValue != VALUE_NONE Already implicit in the next condition */
887 && abs(ttValue) < VALUE_KNOWN_WIN
888 && !excludedMove // Recursive singular search is not allowed
889 && (tte->bound() & BOUND_LOWER)
890 && tte->depth() >= depth - 3 * ONE_PLY;
892 // Step 11. Loop through moves
893 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
894 while ((move = mp.next_move()) != MOVE_NONE)
898 if (move == excludedMove)
901 // At root obey the "searchmoves" option and skip moves not listed in Root
902 // Move List. As a consequence any illegal move is also skipped. In MultiPV
903 // mode we also skip PV moves which have been already searched.
904 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
905 thisThread->rootMoves.end(), move))
908 ss->moveCount = ++moveCount;
910 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
911 sync_cout << "info depth " << depth / ONE_PLY
912 << " currmove " << UCI::move(move, pos.is_chess960())
913 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
916 (ss+1)->pv = nullptr;
918 extension = DEPTH_ZERO;
919 captureOrPromotion = pos.capture_or_promotion(move);
921 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
922 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
923 : pos.gives_check(move, ci);
925 // Step 12. Extend checks
926 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
929 // Singular extension search. If all moves but one fail low on a search of
930 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
931 // is singular and should be extended. To verify this we do a reduced search
932 // on all the other moves but the ttMove and if the result is lower than
933 // ttValue minus a margin then we extend the ttMove.
934 if ( singularExtensionNode
937 && pos.legal(move, ci.pinned))
939 Value rBeta = ttValue - 2 * depth / ONE_PLY;
940 ss->excludedMove = move;
941 ss->skipEarlyPruning = true;
942 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
943 ss->skipEarlyPruning = false;
944 ss->excludedMove = MOVE_NONE;
950 // Update the current move (this must be done after singular extension search)
951 newDepth = depth - ONE_PLY + extension;
953 // Step 13. Pruning at shallow depth
955 && !captureOrPromotion
958 && !pos.advanced_pawn_push(move)
959 && bestValue > VALUE_MATED_IN_MAX_PLY)
961 // Move count based pruning
962 if ( depth < 16 * ONE_PLY
963 && moveCount >= FutilityMoveCounts[improving][depth])
966 // History based pruning
967 if ( depth <= 4 * ONE_PLY
968 && move != ss->killers[0]
969 && thisThread->history[pos.moved_piece(move)][to_sq(move)] < VALUE_ZERO
970 && cmh[pos.moved_piece(move)][to_sq(move)] < VALUE_ZERO)
973 predictedDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO);
975 // Futility pruning: parent node
976 if (predictedDepth < 7 * ONE_PLY)
978 futilityValue = ss->staticEval + futility_margin(predictedDepth) + 256;
980 if (futilityValue <= alpha)
982 bestValue = std::max(bestValue, futilityValue);
987 // Prune moves with negative SEE at low depths
988 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
992 // Speculative prefetch as early as possible
993 prefetch(TT.first_entry(pos.key_after(move)));
995 // Check for legality just before making the move
996 if (!rootNode && !pos.legal(move, ci.pinned))
998 ss->moveCount = --moveCount;
1002 ss->currentMove = move;
1004 // Step 14. Make the move
1005 pos.do_move(move, st, givesCheck);
1007 // Step 15. Reduced depth search (LMR). If the move fails high it will be
1008 // re-searched at full depth.
1009 if ( depth >= 3 * ONE_PLY
1011 && !captureOrPromotion)
1013 Depth r = reduction<PvNode>(improving, depth, moveCount);
1014 Value hValue = thisThread->history[pos.piece_on(to_sq(move))][to_sq(move)];
1015 Value cmhValue = cmh[pos.piece_on(to_sq(move))][to_sq(move)];
1017 // Increase reduction for cut nodes and moves with a bad history
1018 if ( (!PvNode && cutNode)
1019 || (hValue < VALUE_ZERO && cmhValue <= VALUE_ZERO))
1022 // Decrease/increase reduction for moves with a good/bad history
1023 int rHist = (hValue + cmhValue) / 14980;
1024 r = std::max(DEPTH_ZERO, r - rHist * ONE_PLY);
1026 // Decrease reduction for moves that escape a capture. Filter out
1027 // castling moves, because they are coded as "king captures rook" and
1028 // hence break make_move(). Also use see() instead of see_sign(),
1029 // because the destination square is empty.
1031 && type_of(move) == NORMAL
1032 && type_of(pos.piece_on(to_sq(move))) != PAWN
1033 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
1034 r = std::max(DEPTH_ZERO, r - ONE_PLY);
1036 Depth d = std::max(newDepth - r, ONE_PLY);
1038 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1040 doFullDepthSearch = (value > alpha && r != DEPTH_ZERO);
1043 doFullDepthSearch = !PvNode || moveCount > 1;
1045 // Step 16. Full depth search when LMR is skipped or fails high
1046 if (doFullDepthSearch)
1047 value = newDepth < ONE_PLY ?
1048 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1049 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1050 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1052 // For PV nodes only, do a full PV search on the first move or after a fail
1053 // high (in the latter case search only if value < beta), otherwise let the
1054 // parent node fail low with value <= alpha and try another move.
1055 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1058 (ss+1)->pv[0] = MOVE_NONE;
1060 value = newDepth < ONE_PLY ?
1061 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1062 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1063 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1066 // Step 17. Undo move
1067 pos.undo_move(move);
1069 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1071 // Step 18. Check for a new best move
1072 // Finished searching the move. If a stop occurred, the return value of
1073 // the search cannot be trusted, and we return immediately without
1074 // updating best move, PV and TT.
1075 if (Signals.stop.load(std::memory_order_relaxed))
1080 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1081 thisThread->rootMoves.end(), move);
1083 // PV move or new best move ?
1084 if (moveCount == 1 || value > alpha)
1091 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1092 rm.pv.push_back(*m);
1094 // We record how often the best move has been changed in each
1095 // iteration. This information is used for time management: When
1096 // the best move changes frequently, we allocate some more time.
1097 if (moveCount > 1 && thisThread == Threads.main())
1098 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1101 // All other moves but the PV are set to the lowest value: this is
1102 // not a problem when sorting because the sort is stable and the
1103 // move position in the list is preserved - just the PV is pushed up.
1104 rm.score = -VALUE_INFINITE;
1107 if (value > bestValue)
1113 // If there is an easy move for this position, clear it if unstable
1115 && thisThread == Threads.main()
1116 && EasyMove.get(pos.key())
1117 && (move != EasyMove.get(pos.key()) || moveCount > 1))
1122 if (PvNode && !rootNode) // Update pv even in fail-high case
1123 update_pv(ss->pv, move, (ss+1)->pv);
1125 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1129 assert(value >= beta); // Fail high
1135 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1136 quietsSearched[quietCount++] = move;
1139 // The following condition would detect a stop only after move loop has been
1140 // completed. But in this case bestValue is valid because we have fully
1141 // searched our subtree, and we can anyhow save the result in TT.
1147 // Step 20. Check for mate and stalemate
1148 // All legal moves have been searched and if there are no legal moves, it
1149 // must be a mate or a stalemate. If we are in a singular extension search then
1150 // return a fail low score.
1152 bestValue = excludedMove ? alpha
1153 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1155 // Quiet best move: update killers, history and countermoves
1156 else if (bestMove && !pos.capture_or_promotion(bestMove))
1157 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount);
1159 // Bonus for prior countermove that caused the fail low
1160 else if ( depth >= 3 * ONE_PLY
1163 && !pos.captured_piece_type()
1164 && is_ok((ss - 1)->currentMove)
1165 && is_ok((ss - 2)->currentMove))
1167 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY - 1);
1168 Square prevPrevSq = to_sq((ss - 2)->currentMove);
1169 CounterMoveStats& prevCmh = CounterMoveHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
1170 prevCmh.update(pos.piece_on(prevSq), prevSq, bonus);
1173 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1174 bestValue >= beta ? BOUND_LOWER :
1175 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1176 depth, bestMove, ss->staticEval, TT.generation());
1178 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1184 // qsearch() is the quiescence search function, which is called by the main
1185 // search function when the remaining depth is zero (or, to be more precise,
1186 // less than ONE_PLY).
1188 template <NodeType NT, bool InCheck>
1189 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1191 const bool PvNode = NT == PV;
1193 assert(InCheck == !!pos.checkers());
1194 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1195 assert(PvNode || (alpha == beta - 1));
1196 assert(depth <= DEPTH_ZERO);
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)))
1243 ss->currentMove = ttMove; // Can be MOVE_NONE
1247 // Evaluate the position statically
1250 ss->staticEval = VALUE_NONE;
1251 bestValue = futilityBase = -VALUE_INFINITE;
1257 // Never assume anything on values stored in TT
1258 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1259 ss->staticEval = bestValue = evaluate(pos);
1261 // Can ttValue be used as a better position evaluation?
1262 if (ttValue != VALUE_NONE)
1263 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1264 bestValue = ttValue;
1267 ss->staticEval = bestValue =
1268 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1269 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1271 // Stand pat. Return immediately if static value is at least beta
1272 if (bestValue >= beta)
1275 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1276 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1281 if (PvNode && bestValue > alpha)
1284 futilityBase = bestValue + 128;
1287 // Initialize a MovePicker object for the current position, and prepare
1288 // to search the moves. Because the depth is <= 0 here, only captures,
1289 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1291 MovePicker mp(pos, ttMove, depth, pos.this_thread()->history, to_sq((ss-1)->currentMove));
1294 // Loop through the moves until no moves remain or a beta cutoff occurs
1295 while ((move = mp.next_move()) != MOVE_NONE)
1297 assert(is_ok(move));
1299 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1300 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1301 : pos.gives_check(move, ci);
1306 && futilityBase > -VALUE_KNOWN_WIN
1307 && !pos.advanced_pawn_push(move))
1309 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1311 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1313 if (futilityValue <= alpha)
1315 bestValue = std::max(bestValue, futilityValue);
1319 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1321 bestValue = std::max(bestValue, futilityBase);
1326 // Detect non-capture evasions that are candidates to be pruned
1327 evasionPrunable = InCheck
1328 && bestValue > VALUE_MATED_IN_MAX_PLY
1329 && !pos.capture(move);
1331 // Don't search moves with negative SEE values
1332 if ( (!InCheck || evasionPrunable)
1333 && type_of(move) != PROMOTION
1334 && pos.see_sign(move) < VALUE_ZERO)
1337 // Speculative prefetch as early as possible
1338 prefetch(TT.first_entry(pos.key_after(move)));
1340 // Check for legality just before making the move
1341 if (!pos.legal(move, ci.pinned))
1344 ss->currentMove = move;
1346 // Make and search the move
1347 pos.do_move(move, st, givesCheck);
1348 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1349 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1350 pos.undo_move(move);
1352 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1354 // Check for a new best move
1355 if (value > bestValue)
1361 if (PvNode) // Update pv even in fail-high case
1362 update_pv(ss->pv, move, (ss+1)->pv);
1364 if (PvNode && value < beta) // Update alpha here!
1371 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1372 ttDepth, move, ss->staticEval, TT.generation());
1380 // All legal moves have been searched. A special case: If we're in check
1381 // and no legal moves were found, it is checkmate.
1382 if (InCheck && bestValue == -VALUE_INFINITE)
1383 return mated_in(ss->ply); // Plies to mate from the root
1385 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1386 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1387 ttDepth, bestMove, ss->staticEval, TT.generation());
1389 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1395 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1396 // "plies to mate from the current position". Non-mate scores are unchanged.
1397 // The function is called before storing a value in the transposition table.
1399 Value value_to_tt(Value v, int ply) {
1401 assert(v != VALUE_NONE);
1403 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1404 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1408 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1409 // from the transposition table (which refers to the plies to mate/be mated
1410 // from current position) to "plies to mate/be mated from the root".
1412 Value value_from_tt(Value v, int ply) {
1414 return v == VALUE_NONE ? VALUE_NONE
1415 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1416 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1420 // update_pv() adds current move and appends child pv[]
1422 void update_pv(Move* pv, Move move, Move* childPv) {
1424 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1430 // update_stats() updates killers, history, countermove and countermove plus
1431 // follow-up move history when a new quiet best move is found.
1433 void update_stats(const Position& pos, Stack* ss, Move move,
1434 Depth depth, Move* quiets, int quietsCnt) {
1436 if (ss->killers[0] != move)
1438 ss->killers[1] = ss->killers[0];
1439 ss->killers[0] = move;
1442 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY - 1);
1444 Square prevSq = to_sq((ss-1)->currentMove);
1445 Square ownPrevSq = to_sq((ss-2)->currentMove);
1446 CounterMoveStats& cmh = CounterMoveHistory[pos.piece_on(prevSq)][prevSq];
1447 CounterMoveStats& fmh = CounterMoveHistory[pos.piece_on(ownPrevSq)][ownPrevSq];
1448 Thread* thisThread = pos.this_thread();
1450 thisThread->history.update(pos.moved_piece(move), to_sq(move), bonus);
1452 if (is_ok((ss-1)->currentMove))
1454 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1455 cmh.update(pos.moved_piece(move), to_sq(move), bonus);
1458 if (is_ok((ss-2)->currentMove))
1459 fmh.update(pos.moved_piece(move), to_sq(move), bonus);
1461 // Decrease all the other played quiet moves
1462 for (int i = 0; i < quietsCnt; ++i)
1464 thisThread->history.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1466 if (is_ok((ss-1)->currentMove))
1467 cmh.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1469 if (is_ok((ss-2)->currentMove))
1470 fmh.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1473 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1474 if ( (ss-1)->moveCount == 1
1475 && !pos.captured_piece_type()
1476 && is_ok((ss-2)->currentMove))
1478 Square prevPrevSq = to_sq((ss-2)->currentMove);
1479 CounterMoveStats& prevCmh = CounterMoveHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
1480 prevCmh.update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY);
1485 // When playing with strength handicap, choose best move among a set of RootMoves
1486 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1488 Move Skill::pick_best(size_t multiPV) {
1490 const Search::RootMoveVector& rootMoves = Threads.main()->rootMoves;
1491 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1493 // RootMoves are already sorted by score in descending order
1494 Value topScore = rootMoves[0].score;
1495 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1496 int weakness = 120 - 2 * level;
1497 int maxScore = -VALUE_INFINITE;
1499 // Choose best move. For each move score we add two terms, both dependent on
1500 // weakness. One is deterministic and bigger for weaker levels, and one is
1501 // random. Then we choose the move with the resulting highest score.
1502 for (size_t i = 0; i < multiPV; ++i)
1504 // This is our magic formula
1505 int push = ( weakness * int(topScore - rootMoves[i].score)
1506 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1508 if (rootMoves[i].score + push > maxScore)
1510 maxScore = rootMoves[i].score + push;
1511 best = rootMoves[i].pv[0];
1519 // check_time() is used to print debug info and, more importantly, to detect
1520 // when we are out of available time and thus stop the search.
1524 static TimePoint lastInfoTime = now();
1526 int elapsed = Time.elapsed();
1527 TimePoint tick = Limits.startTime + elapsed;
1529 if (tick - lastInfoTime >= 1000)
1531 lastInfoTime = tick;
1535 // An engine may not stop pondering until told so by the GUI
1539 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1540 || (Limits.movetime && elapsed >= Limits.movetime)
1541 || (Limits.nodes && Threads.nodes_searched() >= Limits.nodes))
1542 Signals.stop = true;
1548 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1549 /// that all (if any) unsearched PV lines are sent using a previous search score.
1551 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1553 std::stringstream ss;
1554 int elapsed = Time.elapsed() + 1;
1555 const Search::RootMoveVector& rootMoves = pos.this_thread()->rootMoves;
1556 size_t PVIdx = pos.this_thread()->PVIdx;
1557 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1558 uint64_t nodes_searched = Threads.nodes_searched();
1560 for (size_t i = 0; i < multiPV; ++i)
1562 bool updated = (i <= PVIdx);
1564 if (depth == ONE_PLY && !updated)
1567 Depth d = updated ? depth : depth - ONE_PLY;
1568 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1570 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1571 v = tb ? TB::Score : v;
1573 if (ss.rdbuf()->in_avail()) // Not at first line
1577 << " depth " << d / ONE_PLY
1578 << " seldepth " << pos.this_thread()->maxPly
1579 << " multipv " << i + 1
1580 << " score " << UCI::value(v);
1582 if (!tb && i == PVIdx)
1583 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1585 ss << " nodes " << nodes_searched
1586 << " nps " << nodes_searched * 1000 / elapsed;
1588 if (elapsed > 1000) // Earlier makes little sense
1589 ss << " hashfull " << TT.hashfull();
1591 ss << " tbhits " << TB::Hits
1592 << " time " << elapsed
1595 for (Move m : rootMoves[i].pv)
1596 ss << " " << UCI::move(m, pos.is_chess960());
1603 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1604 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1605 /// first, even if the old TT entries have been overwritten.
1607 void RootMove::insert_pv_in_tt(Position& pos) {
1609 StateInfo state[MAX_PLY], *st = state;
1614 assert(MoveList<LEGAL>(pos).contains(m));
1616 TTEntry* tte = TT.probe(pos.key(), ttHit);
1618 if (!ttHit || tte->move() != m) // Don't overwrite correct entries
1619 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE,
1620 m, VALUE_NONE, TT.generation());
1622 pos.do_move(m, *st++, pos.gives_check(m, CheckInfo(pos)));
1625 for (size_t i = pv.size(); i > 0; )
1626 pos.undo_move(pv[--i]);
1630 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1631 /// before exiting the search, for instance, in case we stop the search during a
1632 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1633 /// otherwise in case of 'ponder on' we have nothing to think on.
1635 bool RootMove::extract_ponder_from_tt(Position& pos)
1640 assert(pv.size() == 1);
1642 pos.do_move(pv[0], st, pos.gives_check(pv[0], CheckInfo(pos)));
1643 TTEntry* tte = TT.probe(pos.key(), ttHit);
1644 pos.undo_move(pv[0]);
1648 Move m = tte->move(); // Local copy to be SMP safe
1649 if (MoveList<LEGAL>(pos).contains(m))
1650 return pv.push_back(m), true;