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 Value DrawValue[COLOR_NB];
131 CounterMovesHistoryStats CounterMovesHistory;
133 template <NodeType NT>
134 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
136 template <NodeType NT, bool InCheck>
137 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
139 Value value_to_tt(Value v, int ply);
140 Value value_from_tt(Value v, int ply);
141 void update_pv(Move* pv, Move move, Move* childPv);
142 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::clear() resets to zero search state, to obtain reproducible results
179 void Search::clear() {
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&, Depth);
222 /// MainThread::search() 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::search() {
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;
294 th->rootPos = Position(rootPos, th);
295 th->rootMoves = rootMoves;
296 th->start_searching();
300 Thread::search(); // Let's start searching!
303 // When playing in 'nodes as time' mode, subtract the searched nodes from
304 // the available ones before to exit.
306 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
308 // When we reach the maximum depth, we can arrive here without a raise of
309 // Signals.stop. However, if we are pondering or in an infinite search,
310 // the UCI protocol states that we shouldn't print the best move before the
311 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
312 // until the GUI sends one of those commands (which also raises Signals.stop).
313 if (!Signals.stop && (Limits.ponder || Limits.infinite))
315 Signals.stopOnPonderhit = true;
319 // Stop the threads if not already stopped
322 // Wait until all threads have finished
323 for (Thread* th : Threads)
325 th->wait_for_search_finished();
327 // Check if there are threads with a better score than main thread
328 Thread* bestThread = this;
329 if ( !this->easyMovePlayed
330 && Options["MultiPV"] == 1
331 && !Skill(Options["Skill Level"]).enabled())
333 for (Thread* th : Threads)
334 if ( th->completedDepth > bestThread->completedDepth
335 && th->rootMoves[0].score > bestThread->rootMoves[0].score)
339 // Send new PV when needed
340 if (bestThread != this)
341 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
343 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
345 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
346 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
348 std::cout << sync_endl;
352 // Thread::search() is the main iterative deepening loop. It calls search()
353 // repeatedly with increasing depth until the allocated thinking time has been
354 // consumed, user stops the search, or the maximum search depth is reached.
356 void Thread::search() {
358 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
359 Value bestValue, alpha, beta, delta;
360 Move easyMove = MOVE_NONE;
361 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
363 std::memset(ss-2, 0, 5 * sizeof(Stack));
365 bestValue = delta = alpha = -VALUE_INFINITE;
366 beta = VALUE_INFINITE;
367 completedDepth = DEPTH_ZERO;
371 easyMove = EasyMove.get(rootPos.key());
373 mainThread->easyMovePlayed = mainThread->failedLow = false;
374 mainThread->bestMoveChanges = 0;
378 size_t multiPV = Options["MultiPV"];
379 Skill skill(Options["Skill Level"]);
381 // When playing with strength handicap enable MultiPV search that we will
382 // use behind the scenes to retrieve a set of possible moves.
384 multiPV = std::max(multiPV, (size_t)4);
386 multiPV = std::min(multiPV, rootMoves.size());
388 // Iterative deepening loop until requested to stop or target depth reached
389 while (++rootDepth < DEPTH_MAX && !Signals.stop && (!Limits.depth || rootDepth <= Limits.depth))
391 // Set up the new depth for the helper threads skipping in average each
392 // 2nd ply (using a half density map similar to a Hadamard matrix).
395 int d = rootDepth + rootPos.game_ply();
397 if (idx <= 6 || idx > 24)
399 if (((d + idx) >> (msb(idx + 1) - 1)) % 2)
404 // Table of values of 6 bits with 3 of them set
405 static const int HalfDensityMap[] = {
406 0x07, 0x0b, 0x0d, 0x0e, 0x13, 0x16, 0x19, 0x1a, 0x1c,
407 0x23, 0x25, 0x26, 0x29, 0x2c, 0x31, 0x32, 0x34, 0x38
410 if ((HalfDensityMap[idx - 7] >> (d % 6)) & 1)
415 // Age out PV variability metric
417 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
419 // Save the last iteration's scores before first PV line is searched and
420 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
421 for (RootMove& rm : rootMoves)
422 rm.previousScore = rm.score;
424 // MultiPV loop. We perform a full root search for each PV line
425 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
427 // Reset aspiration window starting size
428 if (rootDepth >= 5 * ONE_PLY)
431 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
432 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
435 // Start with a small aspiration window and, in the case of a fail
436 // high/low, re-search with a bigger window until we're not failing
440 bestValue = ::search<Root>(rootPos, ss, alpha, beta, rootDepth, false);
442 // Bring the best move to the front. It is critical that sorting
443 // is done with a stable algorithm because all the values but the
444 // first and eventually the new best one are set to -VALUE_INFINITE
445 // and we want to keep the same order for all the moves except the
446 // new PV that goes to the front. Note that in case of MultiPV
447 // search the already searched PV lines are preserved.
448 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
450 // Write PV back to transposition table in case the relevant
451 // entries have been overwritten during the search.
452 for (size_t i = 0; i <= PVIdx; ++i)
453 rootMoves[i].insert_pv_in_tt(rootPos);
455 // If search has been stopped break immediately. Sorting and
456 // writing PV back to TT is safe because RootMoves is still
457 // valid, although it refers to previous iteration.
461 // When failing high/low give some update (without cluttering
462 // the UI) before a re-search.
465 && (bestValue <= alpha || bestValue >= beta)
466 && Time.elapsed() > 3000)
467 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
469 // In case of failing low/high increase aspiration window and
470 // re-search, otherwise exit the loop.
471 if (bestValue <= alpha)
473 beta = (alpha + beta) / 2;
474 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
478 mainThread->failedLow = true;
479 Signals.stopOnPonderhit = false;
482 else if (bestValue >= beta)
484 alpha = (alpha + beta) / 2;
485 beta = std::min(bestValue + delta, VALUE_INFINITE);
490 delta += delta / 4 + 5;
492 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
495 // Sort the PV lines searched so far and update the GUI
496 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
502 sync_cout << "info nodes " << Threads.nodes_searched()
503 << " time " << Time.elapsed() << sync_endl;
505 else if (PVIdx + 1 == multiPV || Time.elapsed() > 3000)
506 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
510 completedDepth = rootDepth;
515 // If skill level is enabled and time is up, pick a sub-optimal best move
516 if (skill.enabled() && skill.time_to_pick(rootDepth))
517 skill.pick_best(multiPV);
519 // Have we found a "mate in x"?
521 && bestValue >= VALUE_MATE_IN_MAX_PLY
522 && VALUE_MATE - bestValue <= 2 * Limits.mate)
525 // Do we have time for the next iteration? Can we stop searching now?
526 if (Limits.use_time_management())
528 if (!Signals.stop && !Signals.stopOnPonderhit)
530 // Take some extra time if the best move has changed
531 if (rootDepth > 4 * ONE_PLY && multiPV == 1)
532 Time.pv_instability(mainThread->bestMoveChanges);
534 // Stop the search if only one legal move is available or all
535 // of the available time has been used or we matched an easyMove
536 // from the previous search and just did a fast verification.
537 if ( rootMoves.size() == 1
538 || Time.elapsed() > Time.available() * (mainThread->failedLow ? 641 : 315) / 640
539 || (mainThread->easyMovePlayed = ( rootMoves[0].pv[0] == easyMove
540 && mainThread->bestMoveChanges < 0.03
541 && Time.elapsed() > Time.available() / 8)))
543 // If we are allowed to ponder do not stop the search now but
544 // keep pondering until the GUI sends "ponderhit" or "stop".
546 Signals.stopOnPonderhit = true;
552 if (rootMoves[0].pv.size() >= 3)
553 EasyMove.update(rootPos, rootMoves[0].pv);
562 // Clear any candidate easy move that wasn't stable for the last search
563 // iterations; the second condition prevents consecutive fast moves.
564 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
567 // If skill level is enabled, swap best PV line with the sub-optimal one
569 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
570 rootMoves.end(), skill.best_move(multiPV)));
576 // search<>() is the main search function for both PV and non-PV nodes
578 template <NodeType NT>
579 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
581 const bool RootNode = NT == Root;
582 const bool PvNode = NT == PV || NT == Root;
584 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
585 assert(PvNode || (alpha == beta - 1));
586 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
588 Move pv[MAX_PLY+1], quietsSearched[64];
592 Move ttMove, move, excludedMove, bestMove;
593 Depth extension, newDepth, predictedDepth;
594 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
595 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
596 bool captureOrPromotion, doFullDepthSearch;
597 int moveCount, quietCount;
599 // Step 1. Initialize node
600 Thread* thisThread = pos.this_thread();
601 inCheck = pos.checkers();
602 moveCount = quietCount = ss->moveCount = 0;
603 bestValue = -VALUE_INFINITE;
604 ss->ply = (ss-1)->ply + 1;
606 // Check for available remaining time
607 if (thisThread->resetCalls.load(std::memory_order_relaxed))
609 thisThread->resetCalls = false;
610 thisThread->callsCnt = 0;
612 if (++thisThread->callsCnt > 4096)
614 for (Thread* th : Threads)
615 th->resetCalls = true;
620 // Used to send selDepth info to GUI
621 if (PvNode && thisThread->maxPly < ss->ply)
622 thisThread->maxPly = ss->ply;
626 // Step 2. Check for aborted search and immediate draw
627 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw() || ss->ply >= MAX_PLY)
628 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
629 : DrawValue[pos.side_to_move()];
631 // Step 3. Mate distance pruning. Even if we mate at the next move our score
632 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
633 // a shorter mate was found upward in the tree then there is no need to search
634 // because we will never beat the current alpha. Same logic but with reversed
635 // signs applies also in the opposite condition of being mated instead of giving
636 // mate. In this case return a fail-high score.
637 alpha = std::max(mated_in(ss->ply), alpha);
638 beta = std::min(mate_in(ss->ply+1), beta);
643 assert(0 <= ss->ply && ss->ply < MAX_PLY);
645 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
646 (ss+1)->skipEarlyPruning = false;
647 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
649 // Step 4. Transposition table lookup. We don't want the score of a partial
650 // search to overwrite a previous full search TT value, so we use a different
651 // position key in case of an excluded move.
652 excludedMove = ss->excludedMove;
653 posKey = excludedMove ? pos.exclusion_key() : pos.key();
654 tte = TT.probe(posKey, ttHit);
655 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
656 ttMove = RootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
657 : ttHit ? tte->move() : MOVE_NONE;
659 // At non-PV nodes we check for an early TT cutoff
662 && tte->depth() >= depth
663 && ttValue != VALUE_NONE // Possible in case of TT access race
664 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
665 : (tte->bound() & BOUND_UPPER)))
667 ss->currentMove = ttMove; // Can be MOVE_NONE
669 // If ttMove is quiet, update killers, history, counter move on TT hit
670 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove))
671 update_stats(pos, ss, ttMove, depth, nullptr, 0);
676 // Step 4a. Tablebase probe
677 if (!RootNode && TB::Cardinality)
679 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
681 if ( piecesCnt <= TB::Cardinality
682 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
683 && pos.rule50_count() == 0)
685 int found, v = Tablebases::probe_wdl(pos, &found);
691 int drawScore = TB::UseRule50 ? 1 : 0;
693 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
694 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
695 : VALUE_DRAW + 2 * v * drawScore;
697 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
698 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
699 MOVE_NONE, VALUE_NONE, TT.generation());
706 // Step 5. Evaluate the position statically
709 ss->staticEval = eval = VALUE_NONE;
715 // Never assume anything on values stored in TT
716 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
717 eval = ss->staticEval = evaluate(pos);
719 // Can ttValue be used as a better position evaluation?
720 if (ttValue != VALUE_NONE)
721 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
726 eval = ss->staticEval =
727 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
728 : -(ss-1)->staticEval + 2 * Eval::Tempo;
730 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
731 ss->staticEval, TT.generation());
734 if (ss->skipEarlyPruning)
737 // Step 6. Razoring (skipped when in check)
739 && depth < 4 * ONE_PLY
740 && eval + razor_margin[depth] <= alpha
741 && ttMove == MOVE_NONE)
743 if ( depth <= ONE_PLY
744 && eval + razor_margin[3 * ONE_PLY] <= alpha)
745 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
747 Value ralpha = alpha - razor_margin[depth];
748 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
753 // Step 7. Futility pruning: child node (skipped when in check)
755 && depth < 7 * ONE_PLY
756 && eval - futility_margin(depth) >= beta
757 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
758 && pos.non_pawn_material(pos.side_to_move()))
759 return eval - futility_margin(depth);
761 // Step 8. Null move search with verification search (is omitted in PV nodes)
763 && depth >= 2 * ONE_PLY
765 && pos.non_pawn_material(pos.side_to_move()))
767 ss->currentMove = MOVE_NULL;
769 assert(eval - beta >= 0);
771 // Null move dynamic reduction based on depth and value
772 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
774 pos.do_null_move(st);
775 (ss+1)->skipEarlyPruning = true;
776 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
777 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
778 (ss+1)->skipEarlyPruning = false;
779 pos.undo_null_move();
781 if (nullValue >= beta)
783 // Do not return unproven mate scores
784 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
787 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
790 // Do verification search at high depths
791 ss->skipEarlyPruning = true;
792 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
793 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
794 ss->skipEarlyPruning = false;
801 // Step 9. ProbCut (skipped when in check)
802 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
803 // and a reduced search returns a value much above beta, we can (almost)
804 // safely prune the previous move.
806 && depth >= 5 * ONE_PLY
807 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
809 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
810 Depth rdepth = depth - 4 * ONE_PLY;
812 assert(rdepth >= ONE_PLY);
813 assert((ss-1)->currentMove != MOVE_NONE);
814 assert((ss-1)->currentMove != MOVE_NULL);
816 MovePicker mp(pos, ttMove, thisThread->history, PieceValue[MG][pos.captured_piece_type()]);
819 while ((move = mp.next_move()) != MOVE_NONE)
820 if (pos.legal(move, ci.pinned))
822 ss->currentMove = move;
823 pos.do_move(move, st, pos.gives_check(move, ci));
824 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
831 // Step 10. Internal iterative deepening (skipped when in check)
832 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
834 && (PvNode || ss->staticEval + 256 >= beta))
836 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
837 ss->skipEarlyPruning = true;
838 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d, true);
839 ss->skipEarlyPruning = false;
841 tte = TT.probe(posKey, ttHit);
842 ttMove = ttHit ? tte->move() : MOVE_NONE;
845 moves_loop: // When in check search starts from here
847 Square prevSq = to_sq((ss-1)->currentMove);
848 Move cm = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
849 const CounterMovesStats& cmh = CounterMovesHistory[pos.piece_on(prevSq)][prevSq];
851 MovePicker mp(pos, ttMove, depth, thisThread->history, cmh, cm, ss);
853 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
854 improving = ss->staticEval >= (ss-2)->staticEval
855 || ss->staticEval == VALUE_NONE
856 ||(ss-2)->staticEval == VALUE_NONE;
858 singularExtensionNode = !RootNode
859 && depth >= 8 * ONE_PLY
860 && ttMove != MOVE_NONE
861 /* && ttValue != VALUE_NONE Already implicit in the next condition */
862 && abs(ttValue) < VALUE_KNOWN_WIN
863 && !excludedMove // Recursive singular search is not allowed
864 && (tte->bound() & BOUND_LOWER)
865 && tte->depth() >= depth - 3 * ONE_PLY;
867 // Step 11. Loop through moves
868 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
869 while ((move = mp.next_move()) != MOVE_NONE)
873 if (move == excludedMove)
876 // At root obey the "searchmoves" option and skip moves not listed in Root
877 // Move List. As a consequence any illegal move is also skipped. In MultiPV
878 // mode we also skip PV moves which have been already searched.
879 if (RootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
880 thisThread->rootMoves.end(), move))
883 ss->moveCount = ++moveCount;
885 if (RootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
886 sync_cout << "info depth " << depth / ONE_PLY
887 << " currmove " << UCI::move(move, pos.is_chess960())
888 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
891 (ss+1)->pv = nullptr;
893 extension = DEPTH_ZERO;
894 captureOrPromotion = pos.capture_or_promotion(move);
896 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
897 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
898 : pos.gives_check(move, ci);
900 // Step 12. Extend checks
901 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
904 // Singular extension search. If all moves but one fail low on a search of
905 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
906 // is singular and should be extended. To verify this we do a reduced search
907 // on all the other moves but the ttMove and if the result is lower than
908 // ttValue minus a margin then we extend the ttMove.
909 if ( singularExtensionNode
912 && pos.legal(move, ci.pinned))
914 Value rBeta = ttValue - 2 * depth / ONE_PLY;
915 ss->excludedMove = move;
916 ss->skipEarlyPruning = true;
917 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
918 ss->skipEarlyPruning = false;
919 ss->excludedMove = MOVE_NONE;
925 // Update the current move (this must be done after singular extension search)
926 newDepth = depth - ONE_PLY + extension;
928 // Step 13. Pruning at shallow depth
930 && !captureOrPromotion
933 && !pos.advanced_pawn_push(move)
934 && bestValue > VALUE_MATED_IN_MAX_PLY)
936 // Move count based pruning
937 if ( depth < 16 * ONE_PLY
938 && moveCount >= FutilityMoveCounts[improving][depth])
941 // History based pruning
942 if ( depth <= 4 * ONE_PLY
943 && move != ss->killers[0]
944 && thisThread->history[pos.moved_piece(move)][to_sq(move)] < VALUE_ZERO
945 && cmh[pos.moved_piece(move)][to_sq(move)] < VALUE_ZERO)
948 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
950 // Futility pruning: parent node
951 if (predictedDepth < 7 * ONE_PLY)
953 futilityValue = ss->staticEval + futility_margin(predictedDepth) + 256;
955 if (futilityValue <= alpha)
957 bestValue = std::max(bestValue, futilityValue);
962 // Prune moves with negative SEE at low depths
963 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
967 // Speculative prefetch as early as possible
968 prefetch(TT.first_entry(pos.key_after(move)));
970 // Check for legality just before making the move
971 if (!RootNode && !pos.legal(move, ci.pinned))
973 ss->moveCount = --moveCount;
977 ss->currentMove = move;
979 // Step 14. Make the move
980 pos.do_move(move, st, givesCheck);
982 // Step 15. Reduced depth search (LMR). If the move fails high it will be
983 // re-searched at full depth.
984 if ( depth >= 3 * ONE_PLY
986 && !captureOrPromotion)
988 Depth r = reduction<PvNode>(improving, depth, moveCount);
990 // Increase reduction for cut nodes and moves with a bad history
991 if ( (!PvNode && cutNode)
992 || ( thisThread->history[pos.piece_on(to_sq(move))][to_sq(move)] < VALUE_ZERO
993 && cmh[pos.piece_on(to_sq(move))][to_sq(move)] <= VALUE_ZERO))
996 // Decrease reduction for moves with a good history
997 if ( thisThread->history[pos.piece_on(to_sq(move))][to_sq(move)] > VALUE_ZERO
998 && cmh[pos.piece_on(to_sq(move))][to_sq(move)] > VALUE_ZERO)
999 r = std::max(DEPTH_ZERO, r - ONE_PLY);
1001 // Decrease reduction for moves that escape a capture
1003 && type_of(move) == NORMAL
1004 && type_of(pos.piece_on(to_sq(move))) != PAWN
1005 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
1006 r = std::max(DEPTH_ZERO, r - ONE_PLY);
1008 Depth d = std::max(newDepth - r, ONE_PLY);
1010 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1012 doFullDepthSearch = (value > alpha && r != DEPTH_ZERO);
1015 doFullDepthSearch = !PvNode || moveCount > 1;
1017 // Step 16. Full depth search, when LMR is skipped or fails high
1018 if (doFullDepthSearch)
1019 value = newDepth < ONE_PLY ?
1020 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1021 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1022 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1024 // For PV nodes only, do a full PV search on the first move or after a fail
1025 // high (in the latter case search only if value < beta), otherwise let the
1026 // parent node fail low with value <= alpha and to try another move.
1027 if (PvNode && (moveCount == 1 || (value > alpha && (RootNode || value < beta))))
1030 (ss+1)->pv[0] = MOVE_NONE;
1032 value = newDepth < ONE_PLY ?
1033 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1034 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1035 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1038 // Step 17. Undo move
1039 pos.undo_move(move);
1041 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1043 // Step 18. Check for new best move
1044 // Finished searching the move. If a stop occurred, the return value of
1045 // the search cannot be trusted, and we return immediately without
1046 // updating best move, PV and TT.
1047 if (Signals.stop.load(std::memory_order_relaxed))
1052 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1053 thisThread->rootMoves.end(), move);
1055 // PV move or new best move ?
1056 if (moveCount == 1 || value > alpha)
1063 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1064 rm.pv.push_back(*m);
1066 // We record how often the best move has been changed in each
1067 // iteration. This information is used for time management: When
1068 // the best move changes frequently, we allocate some more time.
1069 if (moveCount > 1 && thisThread == Threads.main())
1070 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1073 // All other moves but the PV are set to the lowest value: this is
1074 // not a problem when sorting because the sort is stable and the
1075 // move position in the list is preserved - just the PV is pushed up.
1076 rm.score = -VALUE_INFINITE;
1079 if (value > bestValue)
1085 // If there is an easy move for this position, clear it if unstable
1087 && thisThread == Threads.main()
1088 && EasyMove.get(pos.key())
1089 && (move != EasyMove.get(pos.key()) || moveCount > 1))
1094 if (PvNode && !RootNode) // Update pv even in fail-high case
1095 update_pv(ss->pv, move, (ss+1)->pv);
1097 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1101 assert(value >= beta); // Fail high
1107 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1108 quietsSearched[quietCount++] = move;
1111 // Following condition would detect a stop only after move loop has been
1112 // completed. But in this case bestValue is valid because we have fully
1113 // searched our subtree, and we can anyhow save the result in TT.
1119 // Step 20. Check for mate and stalemate
1120 // All legal moves have been searched and if there are no legal moves, it
1121 // must be mate or stalemate. If we are in a singular extension search then
1122 // return a fail low score.
1124 bestValue = excludedMove ? alpha
1125 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1127 // Quiet best move: update killers, history and countermoves
1128 else if (bestMove && !pos.capture_or_promotion(bestMove))
1129 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount);
1131 // Bonus for prior countermove that caused the fail low
1132 else if ( depth >= 3 * ONE_PLY
1135 && !pos.captured_piece_type()
1136 && is_ok((ss - 1)->currentMove)
1137 && is_ok((ss - 2)->currentMove))
1139 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY - 1);
1140 Square prevPrevSq = to_sq((ss - 2)->currentMove);
1141 CounterMovesStats& prevCmh = CounterMovesHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
1142 prevCmh.update(pos.piece_on(prevSq), prevSq, bonus);
1145 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1146 bestValue >= beta ? BOUND_LOWER :
1147 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1148 depth, bestMove, ss->staticEval, TT.generation());
1150 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1156 // qsearch() is the quiescence search function, which is called by the main
1157 // search function when the remaining depth is zero (or, to be more precise,
1158 // less than ONE_PLY).
1160 template <NodeType NT, bool InCheck>
1161 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1163 const bool PvNode = NT == PV;
1165 assert(NT == PV || NT == NonPV);
1166 assert(InCheck == !!pos.checkers());
1167 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1168 assert(PvNode || (alpha == beta - 1));
1169 assert(depth <= DEPTH_ZERO);
1175 Move ttMove, move, bestMove;
1176 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1177 bool ttHit, givesCheck, evasionPrunable;
1182 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1184 ss->pv[0] = MOVE_NONE;
1187 ss->currentMove = bestMove = MOVE_NONE;
1188 ss->ply = (ss-1)->ply + 1;
1190 // Check for an instant draw or if the maximum ply has been reached
1191 if (pos.is_draw() || ss->ply >= MAX_PLY)
1192 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1193 : DrawValue[pos.side_to_move()];
1195 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1197 // Decide whether or not to include checks: this fixes also the type of
1198 // TT entry depth that we are going to use. Note that in qsearch we use
1199 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1200 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1201 : DEPTH_QS_NO_CHECKS;
1203 // Transposition table lookup
1205 tte = TT.probe(posKey, ttHit);
1206 ttMove = ttHit ? tte->move() : MOVE_NONE;
1207 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1211 && tte->depth() >= ttDepth
1212 && ttValue != VALUE_NONE // Only in case of TT access race
1213 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1214 : (tte->bound() & BOUND_UPPER)))
1216 ss->currentMove = ttMove; // Can be MOVE_NONE
1220 // Evaluate the position statically
1223 ss->staticEval = VALUE_NONE;
1224 bestValue = futilityBase = -VALUE_INFINITE;
1230 // Never assume anything on values stored in TT
1231 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1232 ss->staticEval = bestValue = evaluate(pos);
1234 // Can ttValue be used as a better position evaluation?
1235 if (ttValue != VALUE_NONE)
1236 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1237 bestValue = ttValue;
1240 ss->staticEval = bestValue =
1241 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1242 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1244 // Stand pat. Return immediately if static value is at least beta
1245 if (bestValue >= beta)
1248 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1249 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1254 if (PvNode && bestValue > alpha)
1257 futilityBase = bestValue + 128;
1260 // Initialize a MovePicker object for the current position, and prepare
1261 // to search the moves. Because the depth is <= 0 here, only captures,
1262 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1264 MovePicker mp(pos, ttMove, depth, pos.this_thread()->history, to_sq((ss-1)->currentMove));
1267 // Loop through the moves until no moves remain or a beta cutoff occurs
1268 while ((move = mp.next_move()) != MOVE_NONE)
1270 assert(is_ok(move));
1272 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1273 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1274 : pos.gives_check(move, ci);
1279 && futilityBase > -VALUE_KNOWN_WIN
1280 && !pos.advanced_pawn_push(move))
1282 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1284 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1286 if (futilityValue <= alpha)
1288 bestValue = std::max(bestValue, futilityValue);
1292 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1294 bestValue = std::max(bestValue, futilityBase);
1299 // Detect non-capture evasions that are candidates to be pruned
1300 evasionPrunable = InCheck
1301 && bestValue > VALUE_MATED_IN_MAX_PLY
1302 && !pos.capture(move);
1304 // Don't search moves with negative SEE values
1305 if ( (!InCheck || evasionPrunable)
1306 && type_of(move) != PROMOTION
1307 && pos.see_sign(move) < VALUE_ZERO)
1310 // Speculative prefetch as early as possible
1311 prefetch(TT.first_entry(pos.key_after(move)));
1313 // Check for legality just before making the move
1314 if (!pos.legal(move, ci.pinned))
1317 ss->currentMove = move;
1319 // Make and search the move
1320 pos.do_move(move, st, givesCheck);
1321 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1322 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1323 pos.undo_move(move);
1325 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1327 // Check for new best move
1328 if (value > bestValue)
1334 if (PvNode) // Update pv even in fail-high case
1335 update_pv(ss->pv, move, (ss+1)->pv);
1337 if (PvNode && value < beta) // Update alpha here!
1344 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1345 ttDepth, move, ss->staticEval, TT.generation());
1353 // All legal moves have been searched. A special case: If we're in check
1354 // and no legal moves were found, it is checkmate.
1355 if (InCheck && bestValue == -VALUE_INFINITE)
1356 return mated_in(ss->ply); // Plies to mate from the root
1358 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1359 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1360 ttDepth, bestMove, ss->staticEval, TT.generation());
1362 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1368 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1369 // "plies to mate from the current position". Non-mate scores are unchanged.
1370 // The function is called before storing a value in the transposition table.
1372 Value value_to_tt(Value v, int ply) {
1374 assert(v != VALUE_NONE);
1376 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1377 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1381 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1382 // from the transposition table (which refers to the plies to mate/be mated
1383 // from current position) to "plies to mate/be mated from the root".
1385 Value value_from_tt(Value v, int ply) {
1387 return v == VALUE_NONE ? VALUE_NONE
1388 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1389 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1393 // update_pv() adds current move and appends child pv[]
1395 void update_pv(Move* pv, Move move, Move* childPv) {
1397 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1403 // update_stats() updates killers, history, countermove and countermove
1404 // history when a new quiet best move is found.
1406 void update_stats(const Position& pos, Stack* ss, Move move,
1407 Depth depth, Move* quiets, int quietsCnt) {
1409 if (ss->killers[0] != move)
1411 ss->killers[1] = ss->killers[0];
1412 ss->killers[0] = move;
1415 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY - 1);
1417 Square prevSq = to_sq((ss-1)->currentMove);
1418 CounterMovesStats& cmh = CounterMovesHistory[pos.piece_on(prevSq)][prevSq];
1419 Thread* thisThread = pos.this_thread();
1421 thisThread->history.update(pos.moved_piece(move), to_sq(move), bonus);
1423 if (is_ok((ss-1)->currentMove))
1425 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1426 cmh.update(pos.moved_piece(move), to_sq(move), bonus);
1429 // Decrease all the other played quiet moves
1430 for (int i = 0; i < quietsCnt; ++i)
1432 thisThread->history.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1434 if (is_ok((ss-1)->currentMove))
1435 cmh.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1438 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1439 if ( (ss-1)->moveCount == 1
1440 && !pos.captured_piece_type()
1441 && is_ok((ss-2)->currentMove))
1443 Square prevPrevSq = to_sq((ss-2)->currentMove);
1444 CounterMovesStats& prevCmh = CounterMovesHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
1445 prevCmh.update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY);
1450 // When playing with strength handicap, choose best move among a set of RootMoves
1451 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1453 Move Skill::pick_best(size_t multiPV) {
1455 const Search::RootMoveVector& rootMoves = Threads.main()->rootMoves;
1456 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1458 // RootMoves are already sorted by score in descending order
1459 Value topScore = rootMoves[0].score;
1460 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1461 int weakness = 120 - 2 * level;
1462 int maxScore = -VALUE_INFINITE;
1464 // Choose best move. For each move score we add two terms, both dependent on
1465 // weakness. One deterministic and bigger for weaker levels, and one random,
1466 // then we choose the move with the resulting highest score.
1467 for (size_t i = 0; i < multiPV; ++i)
1469 // This is our magic formula
1470 int push = ( weakness * int(topScore - rootMoves[i].score)
1471 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1473 if (rootMoves[i].score + push > maxScore)
1475 maxScore = rootMoves[i].score + push;
1476 best = rootMoves[i].pv[0];
1484 // check_time() is used to print debug info and, more importantly, to detect
1485 // when we are out of available time and thus stop the search.
1489 static TimePoint lastInfoTime = now();
1491 int elapsed = Time.elapsed();
1492 TimePoint tick = Limits.startTime + elapsed;
1494 if (tick - lastInfoTime >= 1000)
1496 lastInfoTime = tick;
1500 // An engine may not stop pondering until told so by the GUI
1504 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1505 || (Limits.movetime && elapsed >= Limits.movetime)
1506 || (Limits.nodes && Threads.nodes_searched() >= Limits.nodes))
1507 Signals.stop = true;
1513 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1514 /// that all (if any) unsearched PV lines are sent using a previous search score.
1516 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1518 std::stringstream ss;
1519 int elapsed = Time.elapsed() + 1;
1520 const Search::RootMoveVector& rootMoves = pos.this_thread()->rootMoves;
1521 size_t PVIdx = pos.this_thread()->PVIdx;
1522 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1523 uint64_t nodes_searched = Threads.nodes_searched();
1525 for (size_t i = 0; i < multiPV; ++i)
1527 bool updated = (i <= PVIdx);
1529 if (depth == ONE_PLY && !updated)
1532 Depth d = updated ? depth : depth - ONE_PLY;
1533 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1535 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1536 v = tb ? TB::Score : v;
1538 if (ss.rdbuf()->in_avail()) // Not at first line
1542 << " depth " << d / ONE_PLY
1543 << " seldepth " << pos.this_thread()->maxPly
1544 << " multipv " << i + 1
1545 << " score " << UCI::value(v);
1547 if (!tb && i == PVIdx)
1548 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1550 ss << " nodes " << nodes_searched
1551 << " nps " << nodes_searched * 1000 / elapsed;
1553 if (elapsed > 1000) // Earlier makes little sense
1554 ss << " hashfull " << TT.hashfull();
1556 ss << " tbhits " << TB::Hits
1557 << " time " << elapsed
1560 for (Move m : rootMoves[i].pv)
1561 ss << " " << UCI::move(m, pos.is_chess960());
1568 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1569 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1570 /// first, even if the old TT entries have been overwritten.
1572 void RootMove::insert_pv_in_tt(Position& pos) {
1574 StateInfo state[MAX_PLY], *st = state;
1579 assert(MoveList<LEGAL>(pos).contains(m));
1581 TTEntry* tte = TT.probe(pos.key(), ttHit);
1583 if (!ttHit || tte->move() != m) // Don't overwrite correct entries
1584 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE,
1585 m, VALUE_NONE, TT.generation());
1587 pos.do_move(m, *st++, pos.gives_check(m, CheckInfo(pos)));
1590 for (size_t i = pv.size(); i > 0; )
1591 pos.undo_move(pv[--i]);
1595 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1596 /// before exiting the search, for instance in case we stop the search during a
1597 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1598 /// otherwise in case of 'ponder on' we have nothing to think on.
1600 bool RootMove::extract_ponder_from_tt(Position& pos)
1605 assert(pv.size() == 1);
1607 pos.do_move(pv[0], st, pos.gives_check(pv[0], CheckInfo(pos)));
1608 TTEntry* tte = TT.probe(pos.key(), ttHit);
1609 pos.undo_move(pv[0]);
1613 Move m = tte->move(); // Local copy to be SMP safe
1614 if (MoveList<LEGAL>(pos).contains(m))
1615 return pv.push_back(m), true;