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
6 Stockfish is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation, either version 3 of the License, or
9 (at your option) any later version.
11 Stockfish is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>.
23 #include <cstring> // For std::memset
36 #include "syzygy/tbprobe.h"
42 StateStackPtr SetupStates;
45 namespace Tablebases {
55 namespace TB = Tablebases;
59 using namespace Search;
63 // Different node types, used as template parameter
64 enum NodeType { Root, PV, NonPV };
66 // Razoring and futility margin based on depth
67 const int razor_margin[4] = { 483, 570, 603, 554 };
68 Value futility_margin(Depth d) { return Value(200 * d); }
70 // Futility and reductions lookup tables, initialized at startup
71 int FutilityMoveCounts[2][16]; // [improving][depth]
72 Depth Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
74 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
75 return Reductions[PvNode][i][std::min(d, 63 * ONE_PLY)][std::min(mn, 63)];
78 // Skill struct is used to implement strength limiting
80 Skill(int l) : level(l) {}
81 bool enabled() const { return level < 20; }
82 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
83 Move best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
84 Move pick_best(size_t multiPV);
87 Move best = MOVE_NONE;
90 // EasyMoveManager struct is used to detect a so called 'easy move'; when PV is
91 // stable across multiple search iterations we can fast return the best move.
92 struct EasyMoveManager {
97 pv[0] = pv[1] = pv[2] = MOVE_NONE;
100 Move get(Key key) const {
101 return expectedPosKey == key ? pv[2] : MOVE_NONE;
104 void update(Position& pos, const std::vector<Move>& newPv) {
106 assert(newPv.size() >= 3);
108 // Keep track of how many times in a row 3rd ply remains stable
109 stableCnt = (newPv[2] == pv[2]) ? stableCnt + 1 : 0;
111 if (!std::equal(newPv.begin(), newPv.begin() + 3, pv))
113 std::copy(newPv.begin(), newPv.begin() + 3, pv);
116 pos.do_move(newPv[0], st[0], pos.gives_check(newPv[0], CheckInfo(pos)));
117 pos.do_move(newPv[1], st[1], pos.gives_check(newPv[1], CheckInfo(pos)));
118 expectedPosKey = pos.key();
119 pos.undo_move(newPv[1]);
120 pos.undo_move(newPv[0]);
129 EasyMoveManager EasyMove;
130 double BestMoveChanges;
131 Value DrawValue[COLOR_NB];
132 CounterMovesHistoryStats CounterMovesHistory;
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);
148 /// Search::init() is called during startup to initialize various lookup tables
150 void Search::init() {
152 const double K[][2] = {{ 0.799, 2.281 }, { 0.484, 3.023 }};
154 for (int pv = 0; pv <= 1; ++pv)
155 for (int imp = 0; imp <= 1; ++imp)
156 for (int d = 1; d < 64; ++d)
157 for (int mc = 1; mc < 64; ++mc)
159 double r = K[pv][0] + log(d) * log(mc) / K[pv][1];
162 Reductions[pv][imp][d][mc] = int(r) * ONE_PLY;
164 // Increase reduction when eval is not improving
165 if (!pv && !imp && Reductions[pv][imp][d][mc] >= 2 * ONE_PLY)
166 Reductions[pv][imp][d][mc] += ONE_PLY;
169 for (int d = 0; d < 16; ++d)
171 FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
172 FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
177 /// Search::reset() clears all search memory, to obtain reproducible search results
179 void Search::reset () {
182 CounterMovesHistory.clear();
184 for (Thread* th : Threads)
187 th->counterMoves.clear();
192 /// Search::perft() is our utility to verify move generation. All the leaf nodes
193 /// up to the given depth are generated and counted and the sum returned.
195 uint64_t Search::perft(Position& pos, Depth depth) {
198 uint64_t cnt, nodes = 0;
200 const bool leaf = (depth == 2 * ONE_PLY);
202 for (const auto& m : MoveList<LEGAL>(pos))
204 if (Root && depth <= ONE_PLY)
208 pos.do_move(m, st, pos.gives_check(m, ci));
209 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
214 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
219 template uint64_t Search::perft<true>(Position& pos, Depth depth);
222 /// MainThread::think() is called by the main thread when the program receives
223 /// the UCI 'go' command. It searches from root position and at the end prints
224 /// the "bestmove" to output.
226 void MainThread::think() {
228 Color us = rootPos.side_to_move();
229 Time.init(Limits, us, rootPos.game_ply());
231 int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
232 DrawValue[ us] = VALUE_DRAW - Value(contempt);
233 DrawValue[~us] = VALUE_DRAW + Value(contempt);
236 TB::RootInTB = false;
237 TB::UseRule50 = Options["Syzygy50MoveRule"];
238 TB::ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
239 TB::Cardinality = Options["SyzygyProbeLimit"];
241 // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
242 if (TB::Cardinality > TB::MaxCardinality)
244 TB::Cardinality = TB::MaxCardinality;
245 TB::ProbeDepth = DEPTH_ZERO;
248 if (rootMoves.empty())
250 rootMoves.push_back(RootMove(MOVE_NONE));
251 sync_cout << "info depth 0 score "
252 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
257 if (TB::Cardinality >= rootPos.count<ALL_PIECES>(WHITE)
258 + rootPos.count<ALL_PIECES>(BLACK))
260 // If the current root position is in the tablebases then RootMoves
261 // contains only moves that preserve the draw or win.
262 TB::RootInTB = Tablebases::root_probe(rootPos, rootMoves, TB::Score);
265 TB::Cardinality = 0; // Do not probe tablebases during the search
267 else // If DTZ tables are missing, use WDL tables as a fallback
269 // Filter out moves that do not preserve a draw or win
270 TB::RootInTB = Tablebases::root_probe_wdl(rootPos, rootMoves, TB::Score);
272 // Only probe during search if winning
273 if (TB::Score <= VALUE_DRAW)
279 TB::Hits = rootMoves.size();
282 TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
283 : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
288 for (Thread* th : Threads)
291 th->rootDepth = DEPTH_ZERO;
292 th->searching = true;
295 th->rootPos = Position(rootPos, th);
296 th->rootMoves = rootMoves;
297 th->notify_one(); // Wake up the thread and start searching
301 Threads.timer->run = true;
302 Threads.timer->notify_one(); // Start the recurring timer
304 search(true); // Let's start searching!
306 // Stop the threads and the timer
308 Threads.timer->run = false;
310 // Wait until all threads have finished
311 for (Thread* th : Threads)
313 th->wait_while(th->searching);
316 // When playing in 'nodes as time' mode, subtract the searched nodes from
317 // the available ones before to exit.
319 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
321 // When we reach the maximum depth, we can arrive here without a raise of
322 // Signals.stop. However, if we are pondering or in an infinite search,
323 // the UCI protocol states that we shouldn't print the best move before the
324 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
325 // until the GUI sends one of those commands (which also raises Signals.stop).
326 if (!Signals.stop && (Limits.ponder || Limits.infinite))
328 Signals.stopOnPonderhit = true;
332 sync_cout << "bestmove " << UCI::move(rootMoves[0].pv[0], rootPos.is_chess960());
334 if (rootMoves[0].pv.size() > 1 || rootMoves[0].extract_ponder_from_tt(rootPos))
335 std::cout << " ponder " << UCI::move(rootMoves[0].pv[1], rootPos.is_chess960());
337 std::cout << sync_endl;
341 // Thread::search() is the main iterative deepening loop. It calls search()
342 // repeatedly with increasing depth until the allocated thinking time has been
343 // consumed, user stops the search, or the maximum search depth is reached.
345 void Thread::search(bool isMainThread) {
347 Stack* ss = stack + 2; // To allow referencing (ss-2) and (ss+2)
348 Value bestValue, alpha, beta, delta;
349 Move easyMove = MOVE_NONE;
351 std::memset(ss-2, 0, 5 * sizeof(Stack));
353 bestValue = delta = alpha = -VALUE_INFINITE;
354 beta = VALUE_INFINITE;
358 easyMove = EasyMove.get(rootPos.key());
364 size_t multiPV = Options["MultiPV"];
365 Skill skill(Options["Skill Level"]);
367 // When playing with strength handicap enable MultiPV search that we will
368 // use behind the scenes to retrieve a set of possible moves.
370 multiPV = std::max(multiPV, (size_t)4);
372 multiPV = std::min(multiPV, rootMoves.size());
374 // Iterative deepening loop until requested to stop or target depth reached
375 while (++rootDepth < DEPTH_MAX && !Signals.stop && (!Limits.depth || rootDepth <= Limits.depth))
377 // Set up the new depth for the helper threads
379 rootDepth = Threads.main()->rootDepth + Depth(int(2.2 * log(1 + this->idx)));
381 // Age out PV variability metric
383 BestMoveChanges *= 0.5;
385 // Save the last iteration's scores before first PV line is searched and
386 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
387 for (RootMove& rm : rootMoves)
388 rm.previousScore = rm.score;
390 // MultiPV loop. We perform a full root search for each PV line
391 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
393 // Reset aspiration window starting size
394 if (rootDepth >= 5 * ONE_PLY)
397 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
398 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
401 // Start with a small aspiration window and, in the case of a fail
402 // high/low, re-search with a bigger window until we're not failing
406 bestValue = ::search<Root>(rootPos, ss, alpha, beta, rootDepth, false);
408 // Bring the best move to the front. It is critical that sorting
409 // is done with a stable algorithm because all the values but the
410 // first and eventually the new best one are set to -VALUE_INFINITE
411 // and we want to keep the same order for all the moves except the
412 // new PV that goes to the front. Note that in case of MultiPV
413 // search the already searched PV lines are preserved.
414 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
416 // Write PV back to transposition table in case the relevant
417 // entries have been overwritten during the search.
418 for (size_t i = 0; i <= PVIdx; ++i)
419 rootMoves[i].insert_pv_in_tt(rootPos);
421 // If search has been stopped break immediately. Sorting and
422 // writing PV back to TT is safe because RootMoves is still
423 // valid, although it refers to previous iteration.
427 // When failing high/low give some update (without cluttering
428 // the UI) before a re-search.
431 && (bestValue <= alpha || bestValue >= beta)
432 && Time.elapsed() > 3000)
433 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
435 // In case of failing low/high increase aspiration window and
436 // re-search, otherwise exit the loop.
437 if (bestValue <= alpha)
439 beta = (alpha + beta) / 2;
440 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
444 Signals.failedLowAtRoot = true;
445 Signals.stopOnPonderhit = false;
448 else if (bestValue >= beta)
450 alpha = (alpha + beta) / 2;
451 beta = std::min(bestValue + delta, VALUE_INFINITE);
456 delta += delta / 4 + 5;
458 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
461 // Sort the PV lines searched so far and update the GUI
462 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
468 sync_cout << "info nodes " << Threads.nodes_searched()
469 << " time " << Time.elapsed() << sync_endl;
471 else if (PVIdx + 1 == multiPV || Time.elapsed() > 3000)
472 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
478 // If skill level is enabled and time is up, pick a sub-optimal best move
479 if (skill.enabled() && skill.time_to_pick(rootDepth))
480 skill.pick_best(multiPV);
482 // Have we found a "mate in x"?
484 && bestValue >= VALUE_MATE_IN_MAX_PLY
485 && VALUE_MATE - bestValue <= 2 * Limits.mate)
488 // Do we have time for the next iteration? Can we stop searching now?
489 if (Limits.use_time_management())
491 if (!Signals.stop && !Signals.stopOnPonderhit)
493 // Take some extra time if the best move has changed
494 if (rootDepth > 4 * ONE_PLY && multiPV == 1)
495 Time.pv_instability(BestMoveChanges);
497 // Stop the search if only one legal move is available or all
498 // of the available time has been used or we matched an easyMove
499 // from the previous search and just did a fast verification.
500 if ( rootMoves.size() == 1
501 || Time.elapsed() > Time.available()
502 || ( rootMoves[0].pv[0] == easyMove
503 && BestMoveChanges < 0.03
504 && Time.elapsed() > Time.available() / 10))
506 // If we are allowed to ponder do not stop the search now but
507 // keep pondering until the GUI sends "ponderhit" or "stop".
509 Signals.stopOnPonderhit = true;
515 if (rootMoves[0].pv.size() >= 3)
516 EasyMove.update(rootPos, rootMoves[0].pv);
523 notify_one(); // Wake up main thread if is sleeping waiting for us
528 // Clear any candidate easy move that wasn't stable for the last search
529 // iterations; the second condition prevents consecutive fast moves.
530 if (EasyMove.stableCnt < 6 || Time.elapsed() < Time.available())
533 // If skill level is enabled, swap best PV line with the sub-optimal one
535 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
536 rootMoves.end(), skill.best_move(multiPV)));
542 // search<>() is the main search function for both PV and non-PV nodes
544 template <NodeType NT>
545 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
547 const bool RootNode = NT == Root;
548 const bool PvNode = NT == PV || NT == Root;
550 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
551 assert(PvNode || (alpha == beta - 1));
552 assert(depth > DEPTH_ZERO);
554 Move pv[MAX_PLY+1], quietsSearched[64];
558 Move ttMove, move, excludedMove, bestMove;
559 Depth extension, newDepth, predictedDepth;
560 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
561 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
562 bool captureOrPromotion, doFullDepthSearch;
563 int moveCount, quietCount;
565 // Step 1. Initialize node
566 Thread* thisThread = pos.this_thread();
567 inCheck = pos.checkers();
568 moveCount = quietCount = ss->moveCount = 0;
569 bestValue = -VALUE_INFINITE;
570 ss->ply = (ss-1)->ply + 1;
572 // Used to send selDepth info to GUI
573 if (PvNode && thisThread->maxPly < ss->ply)
574 thisThread->maxPly = ss->ply;
578 // Step 2. Check for aborted search and immediate draw
579 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw() || ss->ply >= MAX_PLY)
580 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
581 : DrawValue[pos.side_to_move()];
583 // Step 3. Mate distance pruning. Even if we mate at the next move our score
584 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
585 // a shorter mate was found upward in the tree then there is no need to search
586 // because we will never beat the current alpha. Same logic but with reversed
587 // signs applies also in the opposite condition of being mated instead of giving
588 // mate. In this case return a fail-high score.
589 alpha = std::max(mated_in(ss->ply), alpha);
590 beta = std::min(mate_in(ss->ply+1), beta);
595 assert(0 <= ss->ply && ss->ply < MAX_PLY);
597 ss->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
598 (ss+1)->skipEarlyPruning = false; (ss+1)->reduction = DEPTH_ZERO;
599 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
601 // Step 4. Transposition table lookup. We don't want the score of a partial
602 // search to overwrite a previous full search TT value, so we use a different
603 // position key in case of an excluded move.
604 excludedMove = ss->excludedMove;
605 posKey = excludedMove ? pos.exclusion_key() : pos.key();
606 tte = TT.probe(posKey, ttHit);
607 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
608 ss->ttMove = ttMove = RootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
609 : ttHit ? tte->move() : MOVE_NONE;
611 // At non-PV nodes we check for an early TT cutoff
614 && tte->depth() >= depth
615 && ttValue != VALUE_NONE // Possible in case of TT access race
616 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
617 : (tte->bound() & BOUND_UPPER)))
619 ss->currentMove = ttMove; // Can be MOVE_NONE
621 // If ttMove is quiet, update killers, history, counter move on TT hit
622 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove))
623 update_stats(pos, ss, ttMove, depth, nullptr, 0);
628 // Step 4a. Tablebase probe
629 if (!RootNode && TB::Cardinality)
631 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
633 if ( piecesCnt <= TB::Cardinality
634 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
635 && pos.rule50_count() == 0)
637 int found, v = Tablebases::probe_wdl(pos, &found);
643 int drawScore = TB::UseRule50 ? 1 : 0;
645 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
646 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
647 : VALUE_DRAW + 2 * v * drawScore;
649 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
650 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
651 MOVE_NONE, VALUE_NONE, TT.generation());
658 // Step 5. Evaluate the position statically
661 ss->staticEval = eval = VALUE_NONE;
667 // Never assume anything on values stored in TT
668 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
669 eval = ss->staticEval = evaluate(pos);
671 // Can ttValue be used as a better position evaluation?
672 if (ttValue != VALUE_NONE)
673 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
678 eval = ss->staticEval =
679 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
680 : -(ss-1)->staticEval + 2 * Eval::Tempo;
682 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
683 ss->staticEval, TT.generation());
686 if (ss->skipEarlyPruning)
689 // Step 6. Razoring (skipped when in check)
691 && depth < 4 * ONE_PLY
692 && eval + razor_margin[depth] <= alpha
693 && ttMove == MOVE_NONE)
695 if ( depth <= ONE_PLY
696 && eval + razor_margin[3 * ONE_PLY] <= alpha)
697 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
699 Value ralpha = alpha - razor_margin[depth];
700 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
705 // Step 7. Futility pruning: child node (skipped when in check)
707 && depth < 7 * ONE_PLY
708 && eval - futility_margin(depth) >= beta
709 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
710 && pos.non_pawn_material(pos.side_to_move()))
711 return eval - futility_margin(depth);
713 // Step 8. Null move search with verification search (is omitted in PV nodes)
715 && depth >= 2 * ONE_PLY
717 && pos.non_pawn_material(pos.side_to_move()))
719 ss->currentMove = MOVE_NULL;
721 assert(eval - beta >= 0);
723 // Null move dynamic reduction based on depth and value
724 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
726 pos.do_null_move(st);
727 (ss+1)->skipEarlyPruning = true;
728 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
729 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
730 (ss+1)->skipEarlyPruning = false;
731 pos.undo_null_move();
733 if (nullValue >= beta)
735 // Do not return unproven mate scores
736 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
739 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
742 // Do verification search at high depths
743 ss->skipEarlyPruning = true;
744 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
745 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
746 ss->skipEarlyPruning = false;
753 // Step 9. ProbCut (skipped when in check)
754 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
755 // and a reduced search returns a value much above beta, we can (almost)
756 // safely prune the previous move.
758 && depth >= 5 * ONE_PLY
759 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
761 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
762 Depth rdepth = depth - 4 * ONE_PLY;
764 assert(rdepth >= ONE_PLY);
765 assert((ss-1)->currentMove != MOVE_NONE);
766 assert((ss-1)->currentMove != MOVE_NULL);
768 MovePicker mp(pos, ttMove, thisThread->history, PieceValue[MG][pos.captured_piece_type()]);
771 while ((move = mp.next_move()) != MOVE_NONE)
772 if (pos.legal(move, ci.pinned))
774 ss->currentMove = move;
775 pos.do_move(move, st, pos.gives_check(move, ci));
776 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
783 // Step 10. Internal iterative deepening (skipped when in check)
784 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
786 && (PvNode || ss->staticEval + 256 >= beta))
788 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
789 ss->skipEarlyPruning = true;
790 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d, true);
791 ss->skipEarlyPruning = false;
793 tte = TT.probe(posKey, ttHit);
794 ttMove = ttHit ? tte->move() : MOVE_NONE;
797 moves_loop: // When in check search starts from here
799 Square prevSq = to_sq((ss-1)->currentMove);
800 Move cm = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
801 const CounterMovesStats& cmh = CounterMovesHistory[pos.piece_on(prevSq)][prevSq];
803 MovePicker mp(pos, ttMove, depth, thisThread->history, cmh, cm, ss);
805 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
806 improving = ss->staticEval >= (ss-2)->staticEval
807 || ss->staticEval == VALUE_NONE
808 ||(ss-2)->staticEval == VALUE_NONE;
810 singularExtensionNode = !RootNode
811 && depth >= 8 * ONE_PLY
812 && ttMove != MOVE_NONE
813 /* && ttValue != VALUE_NONE Already implicit in the next condition */
814 && abs(ttValue) < VALUE_KNOWN_WIN
815 && !excludedMove // Recursive singular search is not allowed
816 && (tte->bound() & BOUND_LOWER)
817 && tte->depth() >= depth - 3 * ONE_PLY;
819 // Step 11. Loop through moves
820 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
821 while ((move = mp.next_move()) != MOVE_NONE)
825 if (move == excludedMove)
828 // At root obey the "searchmoves" option and skip moves not listed in Root
829 // Move List. As a consequence any illegal move is also skipped. In MultiPV
830 // mode we also skip PV moves which have been already searched.
831 if (RootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
832 thisThread->rootMoves.end(), move))
835 ss->moveCount = ++moveCount;
837 if (RootNode && thisThread == Threads.main())
839 Signals.firstRootMove = moveCount == 1;
841 if (Time.elapsed() > 3000)
842 sync_cout << "info depth " << depth / ONE_PLY
843 << " currmove " << UCI::move(move, pos.is_chess960())
844 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
848 (ss+1)->pv = nullptr;
850 extension = DEPTH_ZERO;
851 captureOrPromotion = pos.capture_or_promotion(move);
853 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
854 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
855 : pos.gives_check(move, ci);
857 // Step 12. Extend checks
858 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
861 // Singular extension search. If all moves but one fail low on a search of
862 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
863 // is singular and should be extended. To verify this we do a reduced search
864 // on all the other moves but the ttMove and if the result is lower than
865 // ttValue minus a margin then we extend the ttMove.
866 if ( singularExtensionNode
869 && pos.legal(move, ci.pinned))
871 Value rBeta = ttValue - 2 * depth / ONE_PLY;
872 ss->excludedMove = move;
873 ss->skipEarlyPruning = true;
874 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
875 ss->skipEarlyPruning = false;
876 ss->excludedMove = MOVE_NONE;
882 // Update the current move (this must be done after singular extension search)
883 newDepth = depth - ONE_PLY + extension;
885 // Step 13. Pruning at shallow depth
887 && !captureOrPromotion
890 && !pos.advanced_pawn_push(move)
891 && bestValue > VALUE_MATED_IN_MAX_PLY)
893 // Move count based pruning
894 if ( depth < 16 * ONE_PLY
895 && moveCount >= FutilityMoveCounts[improving][depth])
898 // History based pruning
899 if ( depth <= 3 * ONE_PLY
900 && thisThread->history[pos.moved_piece(move)][to_sq(move)] < VALUE_ZERO
901 && cmh[pos.moved_piece(move)][to_sq(move)] < VALUE_ZERO)
904 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
906 // Futility pruning: parent node
907 if (predictedDepth < 7 * ONE_PLY)
909 futilityValue = ss->staticEval + futility_margin(predictedDepth) + 256;
911 if (futilityValue <= alpha)
913 bestValue = std::max(bestValue, futilityValue);
918 // Prune moves with negative SEE at low depths
919 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
923 // Speculative prefetch as early as possible
924 prefetch(TT.first_entry(pos.key_after(move)));
926 // Check for legality just before making the move
927 if (!RootNode && !pos.legal(move, ci.pinned))
929 ss->moveCount = --moveCount;
933 ss->currentMove = move;
935 // Step 14. Make the move
936 pos.do_move(move, st, givesCheck);
938 // Step 15. Reduced depth search (LMR). If the move fails high it will be
939 // re-searched at full depth.
940 if ( depth >= 3 * ONE_PLY
942 && !captureOrPromotion
943 && move != ss->killers[0]
944 && move != ss->killers[1])
946 ss->reduction = reduction<PvNode>(improving, depth, moveCount);
948 // Increase reduction for cut nodes and moves with a bad history
949 if ( (!PvNode && cutNode)
950 || ( thisThread->history[pos.piece_on(to_sq(move))][to_sq(move)] < VALUE_ZERO
951 && cmh[pos.piece_on(to_sq(move))][to_sq(move)] <= VALUE_ZERO))
952 ss->reduction += ONE_PLY;
954 // Decrease reduction for moves with a good history
955 if ( thisThread->history[pos.piece_on(to_sq(move))][to_sq(move)] > VALUE_ZERO
956 && cmh[pos.piece_on(to_sq(move))][to_sq(move)] > VALUE_ZERO)
957 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
959 // Decrease reduction for moves that escape a capture
961 && type_of(move) == NORMAL
962 && type_of(pos.piece_on(to_sq(move))) != PAWN
963 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
964 ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
966 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
968 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
970 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
971 ss->reduction = DEPTH_ZERO;
974 doFullDepthSearch = !PvNode || moveCount > 1;
976 // Step 16. Full depth search, when LMR is skipped or fails high
977 if (doFullDepthSearch)
978 value = newDepth < ONE_PLY ?
979 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
980 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
981 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
983 // For PV nodes only, do a full PV search on the first move or after a fail
984 // high (in the latter case search only if value < beta), otherwise let the
985 // parent node fail low with value <= alpha and to try another move.
986 if (PvNode && (moveCount == 1 || (value > alpha && (RootNode || value < beta))))
989 (ss+1)->pv[0] = MOVE_NONE;
991 value = newDepth < ONE_PLY ?
992 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
993 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
994 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
997 // Step 17. Undo move
1000 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1002 // Step 18. Check for new best move
1003 // Finished searching the move. If a stop occurred, the return value of
1004 // the search cannot be trusted, and we return immediately without
1005 // updating best move, PV and TT.
1006 if (Signals.stop.load(std::memory_order_relaxed))
1011 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1012 thisThread->rootMoves.end(), move);
1014 // PV move or new best move ?
1015 if (moveCount == 1 || value > alpha)
1022 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1023 rm.pv.push_back(*m);
1025 // We record how often the best move has been changed in each
1026 // iteration. This information is used for time management: When
1027 // the best move changes frequently, we allocate some more time.
1028 if (moveCount > 1 && thisThread == Threads.main())
1032 // All other moves but the PV are set to the lowest value: this is
1033 // not a problem when sorting because the sort is stable and the
1034 // move position in the list is preserved - just the PV is pushed up.
1035 rm.score = -VALUE_INFINITE;
1038 if (value > bestValue)
1044 // If there is an easy move for this position, clear it if unstable
1046 && thisThread == Threads.main()
1047 && EasyMove.get(pos.key())
1048 && (move != EasyMove.get(pos.key()) || moveCount > 1))
1053 if (PvNode && !RootNode) // Update pv even in fail-high case
1054 update_pv(ss->pv, move, (ss+1)->pv);
1056 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1060 assert(value >= beta); // Fail high
1066 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1067 quietsSearched[quietCount++] = move;
1070 // Following condition would detect a stop only after move loop has been
1071 // completed. But in this case bestValue is valid because we have fully
1072 // searched our subtree, and we can anyhow save the result in TT.
1078 // Step 20. Check for mate and stalemate
1079 // All legal moves have been searched and if there are no legal moves, it
1080 // must be mate or stalemate. If we are in a singular extension search then
1081 // return a fail low score.
1083 bestValue = excludedMove ? alpha
1084 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1086 // Quiet best move: update killers, history and countermoves
1087 else if (bestMove && !pos.capture_or_promotion(bestMove))
1088 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount);
1090 // Bonus for prior countermove that caused the fail low
1091 else if ( depth >= 3 * ONE_PLY
1094 && !pos.captured_piece_type()
1095 && is_ok((ss - 1)->currentMove)
1096 && is_ok((ss - 2)->currentMove))
1098 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY -1);
1099 Square prevPrevSq = to_sq((ss - 2)->currentMove);
1100 CounterMovesStats& prevCmh = CounterMovesHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
1101 prevCmh.update(pos.piece_on(prevSq), prevSq, bonus);
1104 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1105 bestValue >= beta ? BOUND_LOWER :
1106 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1107 depth, bestMove, ss->staticEval, TT.generation());
1109 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1115 // qsearch() is the quiescence search function, which is called by the main
1116 // search function when the remaining depth is zero (or, to be more precise,
1117 // less than ONE_PLY).
1119 template <NodeType NT, bool InCheck>
1120 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1122 const bool PvNode = NT == PV;
1124 assert(NT == PV || NT == NonPV);
1125 assert(InCheck == !!pos.checkers());
1126 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1127 assert(PvNode || (alpha == beta - 1));
1128 assert(depth <= DEPTH_ZERO);
1134 Move ttMove, move, bestMove;
1135 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1136 bool ttHit, givesCheck, evasionPrunable;
1141 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1143 ss->pv[0] = MOVE_NONE;
1146 ss->currentMove = bestMove = MOVE_NONE;
1147 ss->ply = (ss-1)->ply + 1;
1149 // Check for an instant draw or if the maximum ply has been reached
1150 if (pos.is_draw() || ss->ply >= MAX_PLY)
1151 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1152 : DrawValue[pos.side_to_move()];
1154 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1156 // Decide whether or not to include checks: this fixes also the type of
1157 // TT entry depth that we are going to use. Note that in qsearch we use
1158 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1159 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1160 : DEPTH_QS_NO_CHECKS;
1162 // Transposition table lookup
1164 tte = TT.probe(posKey, ttHit);
1165 ttMove = ttHit ? tte->move() : MOVE_NONE;
1166 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1170 && tte->depth() >= ttDepth
1171 && ttValue != VALUE_NONE // Only in case of TT access race
1172 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1173 : (tte->bound() & BOUND_UPPER)))
1175 ss->currentMove = ttMove; // Can be MOVE_NONE
1179 // Evaluate the position statically
1182 ss->staticEval = VALUE_NONE;
1183 bestValue = futilityBase = -VALUE_INFINITE;
1189 // Never assume anything on values stored in TT
1190 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1191 ss->staticEval = bestValue = evaluate(pos);
1193 // Can ttValue be used as a better position evaluation?
1194 if (ttValue != VALUE_NONE)
1195 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1196 bestValue = ttValue;
1199 ss->staticEval = bestValue =
1200 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1201 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1203 // Stand pat. Return immediately if static value is at least beta
1204 if (bestValue >= beta)
1207 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1208 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1213 if (PvNode && bestValue > alpha)
1216 futilityBase = bestValue + 128;
1219 // Initialize a MovePicker object for the current position, and prepare
1220 // to search the moves. Because the depth is <= 0 here, only captures,
1221 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1223 MovePicker mp(pos, ttMove, depth, pos.this_thread()->history, to_sq((ss-1)->currentMove));
1226 // Loop through the moves until no moves remain or a beta cutoff occurs
1227 while ((move = mp.next_move()) != MOVE_NONE)
1229 assert(is_ok(move));
1231 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1232 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1233 : pos.gives_check(move, ci);
1238 && futilityBase > -VALUE_KNOWN_WIN
1239 && !pos.advanced_pawn_push(move))
1241 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1243 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1245 if (futilityValue <= alpha)
1247 bestValue = std::max(bestValue, futilityValue);
1251 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1253 bestValue = std::max(bestValue, futilityBase);
1258 // Detect non-capture evasions that are candidates to be pruned
1259 evasionPrunable = InCheck
1260 && bestValue > VALUE_MATED_IN_MAX_PLY
1261 && !pos.capture(move);
1263 // Don't search moves with negative SEE values
1264 if ( (!InCheck || evasionPrunable)
1265 && type_of(move) != PROMOTION
1266 && pos.see_sign(move) < VALUE_ZERO)
1269 // Speculative prefetch as early as possible
1270 prefetch(TT.first_entry(pos.key_after(move)));
1272 // Check for legality just before making the move
1273 if (!pos.legal(move, ci.pinned))
1276 ss->currentMove = move;
1278 // Make and search the move
1279 pos.do_move(move, st, givesCheck);
1280 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1281 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1282 pos.undo_move(move);
1284 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1286 // Check for new best move
1287 if (value > bestValue)
1293 if (PvNode) // Update pv even in fail-high case
1294 update_pv(ss->pv, move, (ss+1)->pv);
1296 if (PvNode && value < beta) // Update alpha here!
1303 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1304 ttDepth, move, ss->staticEval, TT.generation());
1312 // All legal moves have been searched. A special case: If we're in check
1313 // and no legal moves were found, it is checkmate.
1314 if (InCheck && bestValue == -VALUE_INFINITE)
1315 return mated_in(ss->ply); // Plies to mate from the root
1317 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1318 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1319 ttDepth, bestMove, ss->staticEval, TT.generation());
1321 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1327 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1328 // "plies to mate from the current position". Non-mate scores are unchanged.
1329 // The function is called before storing a value in the transposition table.
1331 Value value_to_tt(Value v, int ply) {
1333 assert(v != VALUE_NONE);
1335 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1336 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1340 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1341 // from the transposition table (which refers to the plies to mate/be mated
1342 // from current position) to "plies to mate/be mated from the root".
1344 Value value_from_tt(Value v, int ply) {
1346 return v == VALUE_NONE ? VALUE_NONE
1347 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1348 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1352 // update_pv() adds current move and appends child pv[]
1354 void update_pv(Move* pv, Move move, Move* childPv) {
1356 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1362 // update_stats() updates killers, history, countermove history and
1363 // countermoves stats for a quiet best move.
1365 void update_stats(const Position& pos, Stack* ss, Move move,
1366 Depth depth, Move* quiets, int quietsCnt) {
1368 if (ss->killers[0] != move)
1370 ss->killers[1] = ss->killers[0];
1371 ss->killers[0] = move;
1374 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY -1);
1376 Square prevSq = to_sq((ss-1)->currentMove);
1377 CounterMovesStats& cmh = CounterMovesHistory[pos.piece_on(prevSq)][prevSq];
1378 Thread* thisThread = pos.this_thread();
1380 thisThread->history.update(pos.moved_piece(move), to_sq(move), bonus);
1382 if (is_ok((ss-1)->currentMove))
1384 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1385 cmh.update(pos.moved_piece(move), to_sq(move), bonus);
1388 // Decrease all the other played quiet moves
1389 for (int i = 0; i < quietsCnt; ++i)
1391 thisThread->history.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1393 if (is_ok((ss-1)->currentMove))
1394 cmh.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1397 // Extra penalty for TT move in previous ply when it gets refuted
1398 if ( (ss-1)->moveCount == 1
1399 && !pos.captured_piece_type()
1400 && is_ok((ss-2)->currentMove))
1402 Square prevPrevSq = to_sq((ss-2)->currentMove);
1403 CounterMovesStats& prevCmh = CounterMovesHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
1404 prevCmh.update(pos.piece_on(prevSq), prevSq, -bonus - 2 * ((depth + 1) / ONE_PLY));
1409 // When playing with strength handicap, choose best move among a set of RootMoves
1410 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1412 Move Skill::pick_best(size_t multiPV) {
1414 // PRNG sequence should be non-deterministic, so we seed it with the time at init
1415 const Search::RootMoveVector& rootMoves = Threads.main()->rootMoves;
1416 static PRNG rng(now());
1418 // RootMoves are already sorted by score in descending order
1419 int variance = std::min(rootMoves[0].score - rootMoves[multiPV - 1].score, PawnValueMg);
1420 int weakness = 120 - 2 * level;
1421 int maxScore = -VALUE_INFINITE;
1423 // Choose best move. For each move score we add two terms both dependent on
1424 // weakness. One deterministic and bigger for weaker levels, and one random,
1425 // then we choose the move with the resulting highest score.
1426 for (size_t i = 0; i < multiPV; ++i)
1428 // This is our magic formula
1429 int push = ( weakness * int(rootMoves[0].score - rootMoves[i].score)
1430 + variance * (rng.rand<unsigned>() % weakness)) / 128;
1432 if (rootMoves[i].score + push > maxScore)
1434 maxScore = rootMoves[i].score + push;
1435 best = rootMoves[i].pv[0];
1444 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1445 /// that all (if any) unsearched PV lines are sent using a previous search score.
1447 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1449 std::stringstream ss;
1450 int elapsed = Time.elapsed() + 1;
1451 const Search::RootMoveVector& rootMoves = pos.this_thread()->rootMoves;
1452 size_t PVIdx = pos.this_thread()->PVIdx;
1453 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1454 uint64_t nodes_searched = Threads.nodes_searched();
1456 for (size_t i = 0; i < multiPV; ++i)
1458 bool updated = (i <= PVIdx);
1460 if (depth == ONE_PLY && !updated)
1463 Depth d = updated ? depth : depth - ONE_PLY;
1464 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1466 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1467 v = tb ? TB::Score : v;
1469 if (ss.rdbuf()->in_avail()) // Not at first line
1473 << " depth " << d / ONE_PLY
1474 << " seldepth " << pos.this_thread()->maxPly
1475 << " multipv " << i + 1
1476 << " score " << UCI::value(v);
1478 if (!tb && i == PVIdx)
1479 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1481 ss << " nodes " << nodes_searched
1482 << " nps " << nodes_searched * 1000 / elapsed;
1484 if (elapsed > 1000) // Earlier makes little sense
1485 ss << " hashfull " << TT.hashfull();
1487 ss << " tbhits " << TB::Hits
1488 << " time " << elapsed
1491 for (Move m : rootMoves[i].pv)
1492 ss << " " << UCI::move(m, pos.is_chess960());
1499 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1500 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1501 /// first, even if the old TT entries have been overwritten.
1503 void RootMove::insert_pv_in_tt(Position& pos) {
1505 StateInfo state[MAX_PLY], *st = state;
1510 assert(MoveList<LEGAL>(pos).contains(m));
1512 TTEntry* tte = TT.probe(pos.key(), ttHit);
1514 if (!ttHit || tte->move() != m) // Don't overwrite correct entries
1515 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, m, VALUE_NONE, TT.generation());
1517 pos.do_move(m, *st++, pos.gives_check(m, CheckInfo(pos)));
1520 for (size_t i = pv.size(); i > 0; )
1521 pos.undo_move(pv[--i]);
1525 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move before
1526 /// exiting the search, for instance in case we stop the search during a fail high at
1527 /// root. We try hard to have a ponder move to return to the GUI, otherwise in case of
1528 /// 'ponder on' we have nothing to think on.
1530 bool RootMove::extract_ponder_from_tt(Position& pos)
1535 assert(pv.size() == 1);
1537 pos.do_move(pv[0], st, pos.gives_check(pv[0], CheckInfo(pos)));
1538 TTEntry* tte = TT.probe(pos.key(), ttHit);
1539 pos.undo_move(pv[0]);
1543 Move m = tte->move(); // Local copy to be SMP safe
1544 if (MoveList<LEGAL>(pos).contains(m))
1545 return pv.push_back(m), true;
1552 /// check_time() is called by the timer thread when the timer triggers. It is
1553 /// used to print debug info and, more importantly, to detect when we are out of
1554 /// available time and thus stop the search.
1558 static TimePoint lastInfoTime = now();
1559 int elapsed = Time.elapsed();
1561 if (now() - lastInfoTime >= 1000)
1563 lastInfoTime = now();
1567 // An engine may not stop pondering until told so by the GUI
1571 if (Limits.use_time_management())
1573 bool stillAtFirstMove = Signals.firstRootMove
1574 && !Signals.failedLowAtRoot
1575 && elapsed > Time.available() * 3 / 4;
1577 if ( stillAtFirstMove
1578 || elapsed > Time.maximum() - 2 * TimerThread::Resolution)
1579 Signals.stop = true;
1581 else if (Limits.movetime && elapsed >= Limits.movetime)
1582 Signals.stop = true;
1584 else if (Limits.nodes && Threads.nodes_searched() >= Limits.nodes)
1585 Signals.stop = true;