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 EasyMoveManager EasyMove;
131 Value DrawValue[COLOR_NB];
132 CounterMoveHistoryStats CounterMoveHistory;
134 template <NodeType NT>
135 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
137 template <NodeType NT, bool InCheck>
138 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
140 Value value_to_tt(Value v, int ply);
141 Value value_from_tt(Value v, int ply);
142 void update_pv(Move* pv, Move move, Move* childPv);
143 void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
149 /// Search::init() is called during startup to initialize various lookup tables
151 void Search::init() {
153 const double K[][2] = {{ 0.799, 2.281 }, { 0.484, 3.023 }};
155 for (int pv = 0; pv <= 1; ++pv)
156 for (int imp = 0; imp <= 1; ++imp)
157 for (int d = 1; d < 64; ++d)
158 for (int mc = 1; mc < 64; ++mc)
160 double r = K[pv][0] + log(d) * log(mc) / K[pv][1];
163 Reductions[pv][imp][d][mc] = int(r) * ONE_PLY;
165 // Increase reduction when eval is not improving
166 if (!pv && !imp && Reductions[pv][imp][d][mc] >= 2 * ONE_PLY)
167 Reductions[pv][imp][d][mc] += ONE_PLY;
170 for (int d = 0; d < 16; ++d)
172 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
173 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
178 /// Search::clear() resets search state to zero, to obtain reproducible results
180 void Search::clear() {
183 CounterMoveHistory.clear();
185 for (Thread* th : Threads)
188 th->counterMoves.clear();
191 Threads.main()->previousScore = VALUE_INFINITE;
195 /// Search::perft() is our utility to verify move generation. All the leaf nodes
196 /// up to the given depth are generated and counted, and the sum is returned.
198 uint64_t Search::perft(Position& pos, Depth depth) {
201 uint64_t cnt, nodes = 0;
203 const bool leaf = (depth == 2 * ONE_PLY);
205 for (const auto& m : MoveList<LEGAL>(pos))
207 if (Root && depth <= ONE_PLY)
211 pos.do_move(m, st, pos.gives_check(m, ci));
212 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
217 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
222 template uint64_t Search::perft<true>(Position&, Depth);
225 /// MainThread::search() is called by the main thread when the program receives
226 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
228 void MainThread::search() {
230 Color us = rootPos.side_to_move();
231 Time.init(Limits, us, rootPos.game_ply());
233 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
234 DrawValue[ us] = VALUE_DRAW - Value(contempt);
235 DrawValue[~us] = VALUE_DRAW + Value(contempt);
238 TB::RootInTB = false;
239 TB::UseRule50 = Options["Syzygy50MoveRule"];
240 TB::ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
241 TB::Cardinality = Options["SyzygyProbeLimit"];
243 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
244 if (TB::Cardinality > TB::MaxCardinality)
246 TB::Cardinality = TB::MaxCardinality;
247 TB::ProbeDepth = DEPTH_ZERO;
250 if (rootMoves.empty())
252 rootMoves.push_back(RootMove(MOVE_NONE));
253 sync_cout << "info depth 0 score "
254 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
259 if ( TB::Cardinality >= rootPos.count<ALL_PIECES>(WHITE)
260 + rootPos.count<ALL_PIECES>(BLACK)
261 && !rootPos.can_castle(ANY_CASTLING))
263 // If the current root position is in the tablebases, then RootMoves
264 // contains only moves that preserve the draw or the win.
265 TB::RootInTB = Tablebases::root_probe(rootPos, rootMoves, TB::Score);
268 TB::Cardinality = 0; // Do not probe tablebases during the search
270 else // If DTZ tables are missing, use WDL tables as a fallback
272 // Filter out moves that do not preserve the draw or the win.
273 TB::RootInTB = Tablebases::root_probe_wdl(rootPos, rootMoves, TB::Score);
275 // Only probe during search if winning
276 if (TB::Score <= VALUE_DRAW)
282 TB::Hits = rootMoves.size();
285 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
286 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
291 for (Thread* th : Threads)
294 th->rootDepth = DEPTH_ZERO;
297 th->rootPos = Position(rootPos, th);
298 th->rootMoves = rootMoves;
299 th->start_searching();
303 Thread::search(); // Let's start searching!
306 // When playing in 'nodes as time' mode, subtract the searched nodes from
307 // the available ones before exiting.
309 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
311 // When we reach the maximum depth, we can arrive here without a raise of
312 // Signals.stop. However, if we are pondering or in an infinite search,
313 // the UCI protocol states that we shouldn't print the best move before the
314 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
315 // until the GUI sends one of those commands (which also raises Signals.stop).
316 if (!Signals.stop && (Limits.ponder || Limits.infinite))
318 Signals.stopOnPonderhit = true;
322 // Stop the threads if not already stopped
325 // Wait until all threads have finished
326 for (Thread* th : Threads)
328 th->wait_for_search_finished();
330 // Check if there are threads with a better score than main thread
331 Thread* bestThread = this;
332 if ( !this->easyMovePlayed
333 && Options["MultiPV"] == 1
334 && !Skill(Options["Skill Level"]).enabled())
336 for (Thread* th : Threads)
337 if ( th->completedDepth > bestThread->completedDepth
338 && th->rootMoves[0].score > bestThread->rootMoves[0].score)
342 previousScore = bestThread->rootMoves[0].score;
344 // Send new PV when needed
345 if (bestThread != this)
346 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
348 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
350 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
351 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
353 std::cout << sync_endl;
356 const int halfDensityMap[][9] =
366 {6, 0, 0, 0, 1, 1, 1},
367 {6, 0, 0, 1, 1, 1, 0},
368 {6, 0, 1, 1, 1, 0, 0},
369 {6, 1, 1, 1, 0, 0, 0},
370 {6, 1, 1, 0, 0, 0, 1},
371 {6, 1, 0, 0, 0, 1, 1},
373 {8, 0, 0, 0, 0, 1, 1, 1, 1},
374 {8, 0, 0, 0, 1, 1, 1, 1, 0},
375 {8, 0, 0, 1, 1, 1, 1, 0 ,0},
376 {8, 0, 1, 1, 1, 1, 0, 0 ,0},
377 {8, 1, 1, 1, 1, 0, 0, 0 ,0},
378 {8, 1, 1, 1, 0, 0, 0, 0 ,1},
379 {8, 1, 1, 0, 0, 0, 0, 1 ,1},
380 {8, 1, 0, 0, 0, 0, 1, 1 ,1},
384 // Thread::search() is the main iterative deepening loop. It calls search()
385 // repeatedly with increasing depth until the allocated thinking time has been
386 // consumed, the user stops the search, or the maximum search depth is reached.
388 void Thread::search() {
390 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
391 Value bestValue, alpha, beta, delta;
392 Move easyMove = MOVE_NONE;
393 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
395 std::memset(ss-2, 0, 5 * sizeof(Stack));
397 bestValue = delta = alpha = -VALUE_INFINITE;
398 beta = VALUE_INFINITE;
399 completedDepth = DEPTH_ZERO;
403 easyMove = EasyMove.get(rootPos.key());
405 mainThread->easyMovePlayed = mainThread->failedLow = false;
406 mainThread->bestMoveChanges = 0;
410 size_t multiPV = Options["MultiPV"];
411 Skill skill(Options["Skill Level"]);
413 // When playing with strength handicap enable MultiPV search that we will
414 // use behind the scenes to retrieve a set of possible moves.
416 multiPV = std::max(multiPV, (size_t)4);
418 multiPV = std::min(multiPV, rootMoves.size());
420 // Iterative deepening loop until requested to stop or the target depth is reached.
421 while (++rootDepth < DEPTH_MAX && !Signals.stop && (!Limits.depth || rootDepth <= Limits.depth))
423 // Set up the new depths for the helper threads skipping on average every
424 // 2nd ply (using a half-density matrix).
427 int row = (idx - 1) % 20;
428 if (halfDensityMap[row][(rootDepth + rootPos.game_ply()) % halfDensityMap[row][0] + 1])
432 // Age out PV variability metric
434 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
436 // Save the last iteration's scores before first PV line is searched and
437 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
438 for (RootMove& rm : rootMoves)
439 rm.previousScore = rm.score;
441 // MultiPV loop. We perform a full root search for each PV line
442 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
444 // Reset aspiration window starting size
445 if (rootDepth >= 5 * ONE_PLY)
448 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
449 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
452 // Start with a small aspiration window and, in the case of a fail
453 // high/low, re-search with a bigger window until we're not failing
457 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false);
459 // Bring the best move to the front. It is critical that sorting
460 // is done with a stable algorithm because all the values but the
461 // first and eventually the new best one are set to -VALUE_INFINITE
462 // and we want to keep the same order for all the moves except the
463 // new PV that goes to the front. Note that in case of MultiPV
464 // search the already searched PV lines are preserved.
465 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
467 // Write PV back to the transposition table in case the relevant
468 // entries have been overwritten during the search.
469 for (size_t i = 0; i <= PVIdx; ++i)
470 rootMoves[i].insert_pv_in_tt(rootPos);
472 // If search has been stopped, break immediately. Sorting and
473 // writing PV back to TT is safe because RootMoves is still
474 // valid, although it refers to the previous iteration.
478 // When failing high/low give some update (without cluttering
479 // the UI) before a re-search.
482 && (bestValue <= alpha || bestValue >= beta)
483 && Time.elapsed() > 3000)
484 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
486 // In case of failing low/high increase aspiration window and
487 // re-search, otherwise exit the loop.
488 if (bestValue <= alpha)
490 beta = (alpha + beta) / 2;
491 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
495 mainThread->failedLow = true;
496 Signals.stopOnPonderhit = false;
499 else if (bestValue >= beta)
501 alpha = (alpha + beta) / 2;
502 beta = std::min(bestValue + delta, VALUE_INFINITE);
507 delta += delta / 4 + 5;
509 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
512 // Sort the PV lines searched so far and update the GUI
513 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
519 sync_cout << "info nodes " << Threads.nodes_searched()
520 << " time " << Time.elapsed() << sync_endl;
522 else if (PVIdx + 1 == multiPV || Time.elapsed() > 3000)
523 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
527 completedDepth = rootDepth;
532 // If skill level is enabled and time is up, pick a sub-optimal best move
533 if (skill.enabled() && skill.time_to_pick(rootDepth))
534 skill.pick_best(multiPV);
536 // Have we found a "mate in x"?
538 && bestValue >= VALUE_MATE_IN_MAX_PLY
539 && VALUE_MATE - bestValue <= 2 * Limits.mate)
542 // Do we have time for the next iteration? Can we stop searching now?
543 if (Limits.use_time_management())
545 if (!Signals.stop && !Signals.stopOnPonderhit)
547 // Stop the search if only one legal move is available, or if all
548 // of the available time has been used, or if we matched an easyMove
549 // from the previous search and just did a fast verification.
550 const bool F[] = { !mainThread->failedLow,
551 bestValue >= mainThread->previousScore };
553 int improvingFactor = 640 - 160*F[0] - 126*F[1] - 124*F[0]*F[1];
554 double unstablePvFactor = 1 + mainThread->bestMoveChanges;
556 bool doEasyMove = rootMoves[0].pv[0] == easyMove
557 && mainThread->bestMoveChanges < 0.03
558 && Time.elapsed() > Time.optimum() * 25 / 204;
560 if ( rootMoves.size() == 1
561 || Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 634
562 || (mainThread->easyMovePlayed = doEasyMove))
564 // If we are allowed to ponder do not stop the search now but
565 // keep pondering until the GUI sends "ponderhit" or "stop".
567 Signals.stopOnPonderhit = true;
573 if (rootMoves[0].pv.size() >= 3)
574 EasyMove.update(rootPos, rootMoves[0].pv);
583 // Clear any candidate easy move that wasn't stable for the last search
584 // iterations; the second condition prevents consecutive fast moves.
585 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
588 // If skill level is enabled, swap best PV line with the sub-optimal one
590 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
591 rootMoves.end(), skill.best_move(multiPV)));
597 // search<>() is the main search function for both PV and non-PV nodes
599 template <NodeType NT>
600 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
602 const bool PvNode = NT == PV;
603 const bool rootNode = PvNode && (ss-1)->ply == 0;
605 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
606 assert(PvNode || (alpha == beta - 1));
607 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
609 Move pv[MAX_PLY+1], quietsSearched[64];
613 Move ttMove, move, excludedMove, bestMove;
614 Depth extension, newDepth, predictedDepth;
615 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
616 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
617 bool captureOrPromotion, doFullDepthSearch;
618 int moveCount, quietCount;
620 // Step 1. Initialize node
621 Thread* thisThread = pos.this_thread();
622 inCheck = pos.checkers();
623 moveCount = quietCount = ss->moveCount = 0;
624 bestValue = -VALUE_INFINITE;
625 ss->ply = (ss-1)->ply + 1;
627 // Check for the available remaining time
628 if (thisThread->resetCalls.load(std::memory_order_relaxed))
630 thisThread->resetCalls = false;
631 thisThread->callsCnt = 0;
633 if (++thisThread->callsCnt > 4096)
635 for (Thread* th : Threads)
636 th->resetCalls = true;
641 // Used to send selDepth info to GUI
642 if (PvNode && thisThread->maxPly < ss->ply)
643 thisThread->maxPly = ss->ply;
647 // Step 2. Check for aborted search and immediate draw
648 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw() || ss->ply >= MAX_PLY)
649 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
650 : DrawValue[pos.side_to_move()];
652 // Step 3. Mate distance pruning. Even if we mate at the next move our score
653 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
654 // a shorter mate was found upward in the tree then there is no need to search
655 // because we will never beat the current alpha. Same logic but with reversed
656 // signs applies also in the opposite condition of being mated instead of giving
657 // mate. In this case return a fail-high score.
658 alpha = std::max(mated_in(ss->ply), alpha);
659 beta = std::min(mate_in(ss->ply+1), beta);
664 assert(0 <= ss->ply && ss->ply < MAX_PLY);
666 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
667 (ss+1)->skipEarlyPruning = false;
668 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
670 // Step 4. Transposition table lookup. We don't want the score of a partial
671 // search to overwrite a previous full search TT value, so we use a different
672 // position key in case of an excluded move.
673 excludedMove = ss->excludedMove;
674 posKey = excludedMove ? pos.exclusion_key() : pos.key();
675 tte = TT.probe(posKey, ttHit);
676 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
677 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
678 : ttHit ? tte->move() : MOVE_NONE;
680 // At non-PV nodes we check for an early TT cutoff
683 && tte->depth() >= depth
684 && ttValue != VALUE_NONE // Possible in case of TT access race
685 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
686 : (tte->bound() & BOUND_UPPER)))
688 ss->currentMove = ttMove; // Can be MOVE_NONE
690 // If ttMove is quiet, update killers, history, counter move on TT hit
691 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove))
692 update_stats(pos, ss, ttMove, depth, nullptr, 0);
697 // Step 4a. Tablebase probe
698 if (!rootNode && TB::Cardinality)
700 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
702 if ( piecesCnt <= TB::Cardinality
703 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
704 && pos.rule50_count() == 0
705 && !pos.can_castle(ANY_CASTLING))
707 int found, v = Tablebases::probe_wdl(pos, &found);
713 int drawScore = TB::UseRule50 ? 1 : 0;
715 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
716 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
717 : VALUE_DRAW + 2 * v * drawScore;
719 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
720 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
721 MOVE_NONE, VALUE_NONE, TT.generation());
728 // Step 5. Evaluate the position statically
731 ss->staticEval = eval = VALUE_NONE;
737 // Never assume anything on values stored in TT
738 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
739 eval = ss->staticEval = evaluate(pos);
741 // Can ttValue be used as a better position evaluation?
742 if (ttValue != VALUE_NONE)
743 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
748 eval = ss->staticEval =
749 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
750 : -(ss-1)->staticEval + 2 * Eval::Tempo;
752 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
753 ss->staticEval, TT.generation());
756 if (ss->skipEarlyPruning)
759 // Step 6. Razoring (skipped when in check)
761 && depth < 4 * ONE_PLY
762 && eval + razor_margin[depth] <= alpha
763 && ttMove == MOVE_NONE)
765 if ( depth <= ONE_PLY
766 && eval + razor_margin[3 * ONE_PLY] <= alpha)
767 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
769 Value ralpha = alpha - razor_margin[depth];
770 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
775 // Step 7. Futility pruning: child node (skipped when in check)
777 && depth < 7 * ONE_PLY
778 && eval - futility_margin(depth) >= beta
779 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
780 && pos.non_pawn_material(pos.side_to_move()))
781 return eval - futility_margin(depth);
783 // Step 8. Null move search with verification search (is omitted in PV nodes)
785 && depth >= 2 * ONE_PLY
787 && pos.non_pawn_material(pos.side_to_move()))
789 ss->currentMove = MOVE_NULL;
791 assert(eval - beta >= 0);
793 // Null move dynamic reduction based on depth and value
794 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
796 pos.do_null_move(st);
797 (ss+1)->skipEarlyPruning = true;
798 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
799 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
800 (ss+1)->skipEarlyPruning = false;
801 pos.undo_null_move();
803 if (nullValue >= beta)
805 // Do not return unproven mate scores
806 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
809 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
812 // Do verification search at high depths
813 ss->skipEarlyPruning = true;
814 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
815 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
816 ss->skipEarlyPruning = false;
823 // Step 9. ProbCut (skipped when in check)
824 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
825 // and a reduced search returns a value much above beta, we can (almost)
826 // safely prune the previous move.
828 && depth >= 5 * ONE_PLY
829 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
831 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
832 Depth rdepth = depth - 4 * ONE_PLY;
834 assert(rdepth >= ONE_PLY);
835 assert((ss-1)->currentMove != MOVE_NONE);
836 assert((ss-1)->currentMove != MOVE_NULL);
838 MovePicker mp(pos, ttMove, thisThread->history, PieceValue[MG][pos.captured_piece_type()]);
841 while ((move = mp.next_move()) != MOVE_NONE)
842 if (pos.legal(move, ci.pinned))
844 ss->currentMove = move;
845 pos.do_move(move, st, pos.gives_check(move, ci));
846 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
853 // Step 10. Internal iterative deepening (skipped when in check)
854 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
856 && (PvNode || ss->staticEval + 256 >= beta))
858 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
859 ss->skipEarlyPruning = true;
860 search<NT>(pos, ss, alpha, beta, d, true);
861 ss->skipEarlyPruning = false;
863 tte = TT.probe(posKey, ttHit);
864 ttMove = ttHit ? tte->move() : MOVE_NONE;
867 moves_loop: // When in check search starts from here
869 Square prevSq = to_sq((ss-1)->currentMove);
870 Move cm = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
871 const CounterMoveStats& cmh = CounterMoveHistory[pos.piece_on(prevSq)][prevSq];
873 MovePicker mp(pos, ttMove, depth, thisThread->history, cmh, cm, ss);
875 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
876 improving = ss->staticEval >= (ss-2)->staticEval
877 || ss->staticEval == VALUE_NONE
878 ||(ss-2)->staticEval == VALUE_NONE;
880 singularExtensionNode = !rootNode
881 && depth >= 8 * ONE_PLY
882 && ttMove != MOVE_NONE
883 /* && ttValue != VALUE_NONE Already implicit in the next condition */
884 && abs(ttValue) < VALUE_KNOWN_WIN
885 && !excludedMove // Recursive singular search is not allowed
886 && (tte->bound() & BOUND_LOWER)
887 && tte->depth() >= depth - 3 * ONE_PLY;
889 // Step 11. Loop through moves
890 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
891 while ((move = mp.next_move()) != MOVE_NONE)
895 if (move == excludedMove)
898 // At root obey the "searchmoves" option and skip moves not listed in Root
899 // Move List. As a consequence any illegal move is also skipped. In MultiPV
900 // mode we also skip PV moves which have been already searched.
901 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
902 thisThread->rootMoves.end(), move))
905 ss->moveCount = ++moveCount;
907 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
908 sync_cout << "info depth " << depth / ONE_PLY
909 << " currmove " << UCI::move(move, pos.is_chess960())
910 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
913 (ss+1)->pv = nullptr;
915 extension = DEPTH_ZERO;
916 captureOrPromotion = pos.capture_or_promotion(move);
918 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
919 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
920 : pos.gives_check(move, ci);
922 // Step 12. Extend checks
923 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
926 // Singular extension search. If all moves but one fail low on a search of
927 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
928 // is singular and should be extended. To verify this we do a reduced search
929 // on all the other moves but the ttMove and if the result is lower than
930 // ttValue minus a margin then we extend the ttMove.
931 if ( singularExtensionNode
934 && pos.legal(move, ci.pinned))
936 Value rBeta = ttValue - 2 * depth / ONE_PLY;
937 ss->excludedMove = move;
938 ss->skipEarlyPruning = true;
939 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
940 ss->skipEarlyPruning = false;
941 ss->excludedMove = MOVE_NONE;
947 // Update the current move (this must be done after singular extension search)
948 newDepth = depth - ONE_PLY + extension;
950 // Step 13. Pruning at shallow depth
952 && !captureOrPromotion
955 && !pos.advanced_pawn_push(move)
956 && bestValue > VALUE_MATED_IN_MAX_PLY)
958 // Move count based pruning
959 if ( depth < 16 * ONE_PLY
960 && moveCount >= FutilityMoveCounts[improving][depth])
963 // History based pruning
964 if ( depth <= 4 * ONE_PLY
965 && move != ss->killers[0]
966 && thisThread->history[pos.moved_piece(move)][to_sq(move)] < VALUE_ZERO
967 && cmh[pos.moved_piece(move)][to_sq(move)] < VALUE_ZERO)
970 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
972 // Futility pruning: parent node
973 if (predictedDepth < 7 * ONE_PLY)
975 futilityValue = ss->staticEval + futility_margin(predictedDepth) + 256;
977 if (futilityValue <= alpha)
979 bestValue = std::max(bestValue, futilityValue);
984 // Prune moves with negative SEE at low depths
985 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
989 // Speculative prefetch as early as possible
990 prefetch(TT.first_entry(pos.key_after(move)));
992 // Check for legality just before making the move
993 if (!rootNode && !pos.legal(move, ci.pinned))
995 ss->moveCount = --moveCount;
999 ss->currentMove = move;
1001 // Step 14. Make the move
1002 pos.do_move(move, st, givesCheck);
1004 // Step 15. Reduced depth search (LMR). If the move fails high it will be
1005 // re-searched at full depth.
1006 if ( depth >= 3 * ONE_PLY
1008 && !captureOrPromotion)
1010 Depth r = reduction<PvNode>(improving, depth, moveCount);
1012 // Increase reduction for cut nodes and moves with a bad history
1013 if ( (!PvNode && cutNode)
1014 || ( thisThread->history[pos.piece_on(to_sq(move))][to_sq(move)] < VALUE_ZERO
1015 && cmh[pos.piece_on(to_sq(move))][to_sq(move)] <= VALUE_ZERO))
1018 // Decrease/increase reduction for moves with a good/bad history
1019 int rHist = ( thisThread->history[pos.piece_on(to_sq(move))][to_sq(move)]
1020 + cmh[pos.piece_on(to_sq(move))][to_sq(move)]) / 14980;
1021 r = std::max(DEPTH_ZERO, r - rHist * ONE_PLY);
1023 // Decrease reduction for moves that escape a capture. Filter out
1024 // castling moves, because they are coded as "king captures rook" and
1025 // hence break make_move(). Also use see() instead of see_sign(),
1026 // because the destination square is empty.
1028 && type_of(move) == NORMAL
1029 && type_of(pos.piece_on(to_sq(move))) != PAWN
1030 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
1031 r = std::max(DEPTH_ZERO, r - ONE_PLY);
1033 Depth d = std::max(newDepth - r, ONE_PLY);
1035 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1037 doFullDepthSearch = (value > alpha && r != DEPTH_ZERO);
1040 doFullDepthSearch = !PvNode || moveCount > 1;
1042 // Step 16. Full depth search when LMR is skipped or fails high
1043 if (doFullDepthSearch)
1044 value = newDepth < ONE_PLY ?
1045 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1046 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1047 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1049 // For PV nodes only, do a full PV search on the first move or after a fail
1050 // high (in the latter case search only if value < beta), otherwise let the
1051 // parent node fail low with value <= alpha and try another move.
1052 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1055 (ss+1)->pv[0] = MOVE_NONE;
1057 value = newDepth < ONE_PLY ?
1058 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1059 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1060 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1063 // Step 17. Undo move
1064 pos.undo_move(move);
1066 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1068 // Step 18. Check for a new best move
1069 // Finished searching the move. If a stop occurred, the return value of
1070 // the search cannot be trusted, and we return immediately without
1071 // updating best move, PV and TT.
1072 if (Signals.stop.load(std::memory_order_relaxed))
1077 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1078 thisThread->rootMoves.end(), move);
1080 // PV move or new best move ?
1081 if (moveCount == 1 || value > alpha)
1088 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1089 rm.pv.push_back(*m);
1091 // We record how often the best move has been changed in each
1092 // iteration. This information is used for time management: When
1093 // the best move changes frequently, we allocate some more time.
1094 if (moveCount > 1 && thisThread == Threads.main())
1095 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1098 // All other moves but the PV are set to the lowest value: this is
1099 // not a problem when sorting because the sort is stable and the
1100 // move position in the list is preserved - just the PV is pushed up.
1101 rm.score = -VALUE_INFINITE;
1104 if (value > bestValue)
1110 // If there is an easy move for this position, clear it if unstable
1112 && thisThread == Threads.main()
1113 && EasyMove.get(pos.key())
1114 && (move != EasyMove.get(pos.key()) || moveCount > 1))
1119 if (PvNode && !rootNode) // Update pv even in fail-high case
1120 update_pv(ss->pv, move, (ss+1)->pv);
1122 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1126 assert(value >= beta); // Fail high
1132 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1133 quietsSearched[quietCount++] = move;
1136 // The following condition would detect a stop only after move loop has been
1137 // completed. But in this case bestValue is valid because we have fully
1138 // searched our subtree, and we can anyhow save the result in TT.
1144 // Step 20. Check for mate and stalemate
1145 // All legal moves have been searched and if there are no legal moves, it
1146 // must be a mate or a stalemate. If we are in a singular extension search then
1147 // return a fail low score.
1149 bestValue = excludedMove ? alpha
1150 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1152 // Quiet best move: update killers, history and countermoves
1153 else if (bestMove && !pos.capture_or_promotion(bestMove))
1154 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount);
1156 // Bonus for prior countermove that caused the fail low
1157 else if ( depth >= 3 * ONE_PLY
1160 && !pos.captured_piece_type()
1161 && is_ok((ss - 1)->currentMove)
1162 && is_ok((ss - 2)->currentMove))
1164 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY - 1);
1165 Square prevPrevSq = to_sq((ss - 2)->currentMove);
1166 CounterMoveStats& prevCmh = CounterMoveHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
1167 prevCmh.update(pos.piece_on(prevSq), prevSq, bonus);
1170 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1171 bestValue >= beta ? BOUND_LOWER :
1172 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1173 depth, bestMove, ss->staticEval, TT.generation());
1175 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1181 // qsearch() is the quiescence search function, which is called by the main
1182 // search function when the remaining depth is zero (or, to be more precise,
1183 // less than ONE_PLY).
1185 template <NodeType NT, bool InCheck>
1186 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1188 const bool PvNode = NT == PV;
1190 assert(InCheck == !!pos.checkers());
1191 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1192 assert(PvNode || (alpha == beta - 1));
1193 assert(depth <= DEPTH_ZERO);
1199 Move ttMove, move, bestMove;
1200 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1201 bool ttHit, givesCheck, evasionPrunable;
1206 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1208 ss->pv[0] = MOVE_NONE;
1211 ss->currentMove = bestMove = MOVE_NONE;
1212 ss->ply = (ss-1)->ply + 1;
1214 // Check for an instant draw or if the maximum ply has been reached
1215 if (pos.is_draw() || ss->ply >= MAX_PLY)
1216 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1217 : DrawValue[pos.side_to_move()];
1219 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1221 // Decide whether or not to include checks: this fixes also the type of
1222 // TT entry depth that we are going to use. Note that in qsearch we use
1223 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1224 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1225 : DEPTH_QS_NO_CHECKS;
1227 // Transposition table lookup
1229 tte = TT.probe(posKey, ttHit);
1230 ttMove = ttHit ? tte->move() : MOVE_NONE;
1231 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1235 && tte->depth() >= ttDepth
1236 && ttValue != VALUE_NONE // Only in case of TT access race
1237 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1238 : (tte->bound() & BOUND_UPPER)))
1240 ss->currentMove = ttMove; // Can be MOVE_NONE
1244 // Evaluate the position statically
1247 ss->staticEval = VALUE_NONE;
1248 bestValue = futilityBase = -VALUE_INFINITE;
1254 // Never assume anything on values stored in TT
1255 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1256 ss->staticEval = bestValue = evaluate(pos);
1258 // Can ttValue be used as a better position evaluation?
1259 if (ttValue != VALUE_NONE)
1260 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1261 bestValue = ttValue;
1264 ss->staticEval = bestValue =
1265 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1266 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1268 // Stand pat. Return immediately if static value is at least beta
1269 if (bestValue >= beta)
1272 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1273 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1278 if (PvNode && bestValue > alpha)
1281 futilityBase = bestValue + 128;
1284 // Initialize a MovePicker object for the current position, and prepare
1285 // to search the moves. Because the depth is <= 0 here, only captures,
1286 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1288 MovePicker mp(pos, ttMove, depth, pos.this_thread()->history, to_sq((ss-1)->currentMove));
1291 // Loop through the moves until no moves remain or a beta cutoff occurs
1292 while ((move = mp.next_move()) != MOVE_NONE)
1294 assert(is_ok(move));
1296 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1297 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1298 : pos.gives_check(move, ci);
1303 && futilityBase > -VALUE_KNOWN_WIN
1304 && !pos.advanced_pawn_push(move))
1306 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1308 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1310 if (futilityValue <= alpha)
1312 bestValue = std::max(bestValue, futilityValue);
1316 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1318 bestValue = std::max(bestValue, futilityBase);
1323 // Detect non-capture evasions that are candidates to be pruned
1324 evasionPrunable = InCheck
1325 && bestValue > VALUE_MATED_IN_MAX_PLY
1326 && !pos.capture(move);
1328 // Don't search moves with negative SEE values
1329 if ( (!InCheck || evasionPrunable)
1330 && type_of(move) != PROMOTION
1331 && pos.see_sign(move) < VALUE_ZERO)
1334 // Speculative prefetch as early as possible
1335 prefetch(TT.first_entry(pos.key_after(move)));
1337 // Check for legality just before making the move
1338 if (!pos.legal(move, ci.pinned))
1341 ss->currentMove = move;
1343 // Make and search the move
1344 pos.do_move(move, st, givesCheck);
1345 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1346 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1347 pos.undo_move(move);
1349 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1351 // Check for a new best move
1352 if (value > bestValue)
1358 if (PvNode) // Update pv even in fail-high case
1359 update_pv(ss->pv, move, (ss+1)->pv);
1361 if (PvNode && value < beta) // Update alpha here!
1368 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1369 ttDepth, move, ss->staticEval, TT.generation());
1377 // All legal moves have been searched. A special case: If we're in check
1378 // and no legal moves were found, it is checkmate.
1379 if (InCheck && bestValue == -VALUE_INFINITE)
1380 return mated_in(ss->ply); // Plies to mate from the root
1382 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1383 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1384 ttDepth, bestMove, ss->staticEval, TT.generation());
1386 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1392 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1393 // "plies to mate from the current position". Non-mate scores are unchanged.
1394 // The function is called before storing a value in the transposition table.
1396 Value value_to_tt(Value v, int ply) {
1398 assert(v != VALUE_NONE);
1400 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1401 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1405 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1406 // from the transposition table (which refers to the plies to mate/be mated
1407 // from current position) to "plies to mate/be mated from the root".
1409 Value value_from_tt(Value v, int ply) {
1411 return v == VALUE_NONE ? VALUE_NONE
1412 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1413 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1417 // update_pv() adds current move and appends child pv[]
1419 void update_pv(Move* pv, Move move, Move* childPv) {
1421 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1427 // update_stats() updates killers, history, countermove and countermove
1428 // history when a new quiet best move is found.
1430 void update_stats(const Position& pos, Stack* ss, Move move,
1431 Depth depth, Move* quiets, int quietsCnt) {
1433 if (ss->killers[0] != move)
1435 ss->killers[1] = ss->killers[0];
1436 ss->killers[0] = move;
1439 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY - 1);
1441 Square prevSq = to_sq((ss-1)->currentMove);
1442 CounterMoveStats& cmh = CounterMoveHistory[pos.piece_on(prevSq)][prevSq];
1443 Thread* thisThread = pos.this_thread();
1445 thisThread->history.update(pos.moved_piece(move), to_sq(move), bonus);
1447 if (is_ok((ss-1)->currentMove))
1449 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1450 cmh.update(pos.moved_piece(move), to_sq(move), bonus);
1453 // Decrease all the other played quiet moves
1454 for (int i = 0; i < quietsCnt; ++i)
1456 thisThread->history.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1458 if (is_ok((ss-1)->currentMove))
1459 cmh.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1462 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1463 if ( (ss-1)->moveCount == 1
1464 && !pos.captured_piece_type()
1465 && is_ok((ss-2)->currentMove))
1467 Square prevPrevSq = to_sq((ss-2)->currentMove);
1468 CounterMoveStats& prevCmh = CounterMoveHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
1469 prevCmh.update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY);
1474 // When playing with strength handicap, choose best move among a set of RootMoves
1475 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1477 Move Skill::pick_best(size_t multiPV) {
1479 const Search::RootMoveVector& rootMoves = Threads.main()->rootMoves;
1480 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1482 // RootMoves are already sorted by score in descending order
1483 Value topScore = rootMoves[0].score;
1484 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1485 int weakness = 120 - 2 * level;
1486 int maxScore = -VALUE_INFINITE;
1488 // Choose best move. For each move score we add two terms, both dependent on
1489 // weakness. One is deterministic and bigger for weaker levels, and one is
1490 // random. Then we choose the move with the resulting highest score.
1491 for (size_t i = 0; i < multiPV; ++i)
1493 // This is our magic formula
1494 int push = ( weakness * int(topScore - rootMoves[i].score)
1495 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1497 if (rootMoves[i].score + push > maxScore)
1499 maxScore = rootMoves[i].score + push;
1500 best = rootMoves[i].pv[0];
1508 // check_time() is used to print debug info and, more importantly, to detect
1509 // when we are out of available time and thus stop the search.
1513 static TimePoint lastInfoTime = now();
1515 int elapsed = Time.elapsed();
1516 TimePoint tick = Limits.startTime + elapsed;
1518 if (tick - lastInfoTime >= 1000)
1520 lastInfoTime = tick;
1524 // An engine may not stop pondering until told so by the GUI
1528 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1529 || (Limits.movetime && elapsed >= Limits.movetime)
1530 || (Limits.nodes && Threads.nodes_searched() >= Limits.nodes))
1531 Signals.stop = true;
1537 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1538 /// that all (if any) unsearched PV lines are sent using a previous search score.
1540 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1542 std::stringstream ss;
1543 int elapsed = Time.elapsed() + 1;
1544 const Search::RootMoveVector& rootMoves = pos.this_thread()->rootMoves;
1545 size_t PVIdx = pos.this_thread()->PVIdx;
1546 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1547 uint64_t nodes_searched = Threads.nodes_searched();
1549 for (size_t i = 0; i < multiPV; ++i)
1551 bool updated = (i <= PVIdx);
1553 if (depth == ONE_PLY && !updated)
1556 Depth d = updated ? depth : depth - ONE_PLY;
1557 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1559 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1560 v = tb ? TB::Score : v;
1562 if (ss.rdbuf()->in_avail()) // Not at first line
1566 << " depth " << d / ONE_PLY
1567 << " seldepth " << pos.this_thread()->maxPly
1568 << " multipv " << i + 1
1569 << " score " << UCI::value(v);
1571 if (!tb && i == PVIdx)
1572 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1574 ss << " nodes " << nodes_searched
1575 << " nps " << nodes_searched * 1000 / elapsed;
1577 if (elapsed > 1000) // Earlier makes little sense
1578 ss << " hashfull " << TT.hashfull();
1580 ss << " tbhits " << TB::Hits
1581 << " time " << elapsed
1584 for (Move m : rootMoves[i].pv)
1585 ss << " " << UCI::move(m, pos.is_chess960());
1592 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1593 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1594 /// first, even if the old TT entries have been overwritten.
1596 void RootMove::insert_pv_in_tt(Position& pos) {
1598 StateInfo state[MAX_PLY], *st = state;
1603 assert(MoveList<LEGAL>(pos).contains(m));
1605 TTEntry* tte = TT.probe(pos.key(), ttHit);
1607 if (!ttHit || tte->move() != m) // Don't overwrite correct entries
1608 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE,
1609 m, VALUE_NONE, TT.generation());
1611 pos.do_move(m, *st++, pos.gives_check(m, CheckInfo(pos)));
1614 for (size_t i = pv.size(); i > 0; )
1615 pos.undo_move(pv[--i]);
1619 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1620 /// before exiting the search, for instance, in case we stop the search during a
1621 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1622 /// otherwise in case of 'ponder on' we have nothing to think on.
1624 bool RootMove::extract_ponder_from_tt(Position& pos)
1629 assert(pv.size() == 1);
1631 pos.do_move(pv[0], st, pos.gives_check(pv[0], CheckInfo(pos)));
1632 TTEntry* tte = TT.probe(pos.key(), ttHit);
1633 pos.undo_move(pv[0]);
1637 Move m = tte->move(); // Local copy to be SMP safe
1638 if (MoveList<LEGAL>(pos).contains(m))
1639 return pv.push_back(m), true;