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;
357 // Thread::search() is the main iterative deepening loop. It calls search()
358 // repeatedly with increasing depth until the allocated thinking time has been
359 // consumed, the user stops the search, or the maximum search depth is reached.
361 void Thread::search() {
363 Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
364 Value bestValue, alpha, beta, delta;
365 Move easyMove = MOVE_NONE;
366 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
368 std::memset(ss-2, 0, 5 * sizeof(Stack));
370 bestValue = delta = alpha = -VALUE_INFINITE;
371 beta = VALUE_INFINITE;
372 completedDepth = DEPTH_ZERO;
376 easyMove = EasyMove.get(rootPos.key());
378 mainThread->easyMovePlayed = mainThread->failedLow = false;
379 mainThread->bestMoveChanges = 0;
383 size_t multiPV = Options["MultiPV"];
384 Skill skill(Options["Skill Level"]);
386 // When playing with strength handicap enable MultiPV search that we will
387 // use behind the scenes to retrieve a set of possible moves.
389 multiPV = std::max(multiPV, (size_t)4);
391 multiPV = std::min(multiPV, rootMoves.size());
393 // Iterative deepening loop until requested to stop or the target depth is reached.
394 while (++rootDepth < DEPTH_MAX && !Signals.stop && (!Limits.depth || rootDepth <= Limits.depth))
396 // Set up the new depths for the helper threads skipping on average every
397 // 2nd ply (using a half-density map similar to a Hadamard matrix).
400 int d = rootDepth + rootPos.game_ply();
402 if (idx <= 6 || idx > 24)
404 if (((d + idx) >> (msb(idx + 1) - 1)) % 2)
409 // Table of values of 6 bits with 3 of them set
410 static const int HalfDensityMap[] = {
411 0x07, 0x0b, 0x0d, 0x0e, 0x13, 0x16, 0x19, 0x1a, 0x1c,
412 0x23, 0x25, 0x26, 0x29, 0x2c, 0x31, 0x32, 0x34, 0x38
415 if ((HalfDensityMap[idx - 7] >> (d % 6)) & 1)
420 // Age out PV variability metric
422 mainThread->bestMoveChanges *= 0.505, mainThread->failedLow = false;
424 // Save the last iteration's scores before first PV line is searched and
425 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
426 for (RootMove& rm : rootMoves)
427 rm.previousScore = rm.score;
429 // MultiPV loop. We perform a full root search for each PV line
430 for (PVIdx = 0; PVIdx < multiPV && !Signals.stop; ++PVIdx)
432 // Reset aspiration window starting size
433 if (rootDepth >= 5 * ONE_PLY)
436 alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
437 beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
440 // Start with a small aspiration window and, in the case of a fail
441 // high/low, re-search with a bigger window until we're not failing
445 bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, false);
447 // Bring the best move to the front. It is critical that sorting
448 // is done with a stable algorithm because all the values but the
449 // first and eventually the new best one are set to -VALUE_INFINITE
450 // and we want to keep the same order for all the moves except the
451 // new PV that goes to the front. Note that in case of MultiPV
452 // search the already searched PV lines are preserved.
453 std::stable_sort(rootMoves.begin() + PVIdx, rootMoves.end());
455 // Write PV back to the transposition table in case the relevant
456 // entries have been overwritten during the search.
457 for (size_t i = 0; i <= PVIdx; ++i)
458 rootMoves[i].insert_pv_in_tt(rootPos);
460 // If search has been stopped, break immediately. Sorting and
461 // writing PV back to TT is safe because RootMoves is still
462 // valid, although it refers to the previous iteration.
466 // When failing high/low give some update (without cluttering
467 // the UI) before a re-search.
470 && (bestValue <= alpha || bestValue >= beta)
471 && Time.elapsed() > 3000)
472 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
474 // In case of failing low/high increase aspiration window and
475 // re-search, otherwise exit the loop.
476 if (bestValue <= alpha)
478 beta = (alpha + beta) / 2;
479 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
483 mainThread->failedLow = true;
484 Signals.stopOnPonderhit = false;
487 else if (bestValue >= beta)
489 alpha = (alpha + beta) / 2;
490 beta = std::min(bestValue + delta, VALUE_INFINITE);
495 delta += delta / 4 + 5;
497 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
500 // Sort the PV lines searched so far and update the GUI
501 std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
507 sync_cout << "info nodes " << Threads.nodes_searched()
508 << " time " << Time.elapsed() << sync_endl;
510 else if (PVIdx + 1 == multiPV || Time.elapsed() > 3000)
511 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
515 completedDepth = rootDepth;
520 // If skill level is enabled and time is up, pick a sub-optimal best move
521 if (skill.enabled() && skill.time_to_pick(rootDepth))
522 skill.pick_best(multiPV);
524 // Have we found a "mate in x"?
526 && bestValue >= VALUE_MATE_IN_MAX_PLY
527 && VALUE_MATE - bestValue <= 2 * Limits.mate)
530 // Do we have time for the next iteration? Can we stop searching now?
531 if (Limits.use_time_management())
533 if (!Signals.stop && !Signals.stopOnPonderhit)
535 // Take some extra time if the best move has changed
536 if (rootDepth > 4 * ONE_PLY && multiPV == 1)
537 Time.pv_instability(mainThread->bestMoveChanges);
539 // Stop the search if only one legal move is available, or if all
540 // of the available time has been used, or if we matched an easyMove
541 // from the previous search and just did a fast verification.
542 const bool F[] = { !mainThread->failedLow,
543 bestValue >= mainThread->previousScore };
545 int improvingFactor = 640 - 160*F[0] - 126*F[1] - 124*F[0]*F[1];
547 bool doEasyMove = rootMoves[0].pv[0] == easyMove
548 && mainThread->bestMoveChanges < 0.03
549 && Time.elapsed() > Time.available() * 25 / 206;
551 if ( rootMoves.size() == 1
552 || Time.elapsed() > Time.available() * improvingFactor / 640
553 || (mainThread->easyMovePlayed = doEasyMove))
555 // If we are allowed to ponder do not stop the search now but
556 // keep pondering until the GUI sends "ponderhit" or "stop".
558 Signals.stopOnPonderhit = true;
564 if (rootMoves[0].pv.size() >= 3)
565 EasyMove.update(rootPos, rootMoves[0].pv);
574 // Clear any candidate easy move that wasn't stable for the last search
575 // iterations; the second condition prevents consecutive fast moves.
576 if (EasyMove.stableCnt < 6 || mainThread->easyMovePlayed)
579 // If skill level is enabled, swap best PV line with the sub-optimal one
581 std::swap(rootMoves[0], *std::find(rootMoves.begin(),
582 rootMoves.end(), skill.best_move(multiPV)));
588 // search<>() is the main search function for both PV and non-PV nodes
590 template <NodeType NT>
591 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
593 const bool PvNode = NT == PV;
594 const bool rootNode = PvNode && (ss-1)->ply == 0;
596 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
597 assert(PvNode || (alpha == beta - 1));
598 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
600 Move pv[MAX_PLY+1], quietsSearched[64];
604 Move ttMove, move, excludedMove, bestMove;
605 Depth extension, newDepth, predictedDepth;
606 Value bestValue, value, ttValue, eval, nullValue, futilityValue;
607 bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
608 bool captureOrPromotion, doFullDepthSearch;
609 int moveCount, quietCount;
611 // Step 1. Initialize node
612 Thread* thisThread = pos.this_thread();
613 inCheck = pos.checkers();
614 moveCount = quietCount = ss->moveCount = 0;
615 bestValue = -VALUE_INFINITE;
616 ss->ply = (ss-1)->ply + 1;
618 // Check for the available remaining time
619 if (thisThread->resetCalls.load(std::memory_order_relaxed))
621 thisThread->resetCalls = false;
622 thisThread->callsCnt = 0;
624 if (++thisThread->callsCnt > 4096)
626 for (Thread* th : Threads)
627 th->resetCalls = true;
632 // Used to send selDepth info to GUI
633 if (PvNode && thisThread->maxPly < ss->ply)
634 thisThread->maxPly = ss->ply;
638 // Step 2. Check for aborted search and immediate draw
639 if (Signals.stop.load(std::memory_order_relaxed) || pos.is_draw() || ss->ply >= MAX_PLY)
640 return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos)
641 : DrawValue[pos.side_to_move()];
643 // Step 3. Mate distance pruning. Even if we mate at the next move our score
644 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
645 // a shorter mate was found upward in the tree then there is no need to search
646 // because we will never beat the current alpha. Same logic but with reversed
647 // signs applies also in the opposite condition of being mated instead of giving
648 // mate. In this case return a fail-high score.
649 alpha = std::max(mated_in(ss->ply), alpha);
650 beta = std::min(mate_in(ss->ply+1), beta);
655 assert(0 <= ss->ply && ss->ply < MAX_PLY);
657 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
658 (ss+1)->skipEarlyPruning = false;
659 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
661 // Step 4. Transposition table lookup. We don't want the score of a partial
662 // search to overwrite a previous full search TT value, so we use a different
663 // position key in case of an excluded move.
664 excludedMove = ss->excludedMove;
665 posKey = excludedMove ? pos.exclusion_key() : pos.key();
666 tte = TT.probe(posKey, ttHit);
667 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
668 ttMove = rootNode ? thisThread->rootMoves[thisThread->PVIdx].pv[0]
669 : ttHit ? tte->move() : MOVE_NONE;
671 // At non-PV nodes we check for an early TT cutoff
674 && tte->depth() >= depth
675 && ttValue != VALUE_NONE // Possible in case of TT access race
676 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
677 : (tte->bound() & BOUND_UPPER)))
679 ss->currentMove = ttMove; // Can be MOVE_NONE
681 // If ttMove is quiet, update killers, history, counter move on TT hit
682 if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove))
683 update_stats(pos, ss, ttMove, depth, nullptr, 0);
688 // Step 4a. Tablebase probe
689 if (!rootNode && TB::Cardinality)
691 int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
693 if ( piecesCnt <= TB::Cardinality
694 && (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
695 && pos.rule50_count() == 0
696 && !pos.can_castle(ANY_CASTLING))
698 int found, v = Tablebases::probe_wdl(pos, &found);
704 int drawScore = TB::UseRule50 ? 1 : 0;
706 value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
707 : v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
708 : VALUE_DRAW + 2 * v * drawScore;
710 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
711 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
712 MOVE_NONE, VALUE_NONE, TT.generation());
719 // Step 5. Evaluate the position statically
722 ss->staticEval = eval = VALUE_NONE;
728 // Never assume anything on values stored in TT
729 if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
730 eval = ss->staticEval = evaluate(pos);
732 // Can ttValue be used as a better position evaluation?
733 if (ttValue != VALUE_NONE)
734 if (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))
739 eval = ss->staticEval =
740 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
741 : -(ss-1)->staticEval + 2 * Eval::Tempo;
743 tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
744 ss->staticEval, TT.generation());
747 if (ss->skipEarlyPruning)
750 // Step 6. Razoring (skipped when in check)
752 && depth < 4 * ONE_PLY
753 && eval + razor_margin[depth] <= alpha
754 && ttMove == MOVE_NONE)
756 if ( depth <= ONE_PLY
757 && eval + razor_margin[3 * ONE_PLY] <= alpha)
758 return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
760 Value ralpha = alpha - razor_margin[depth];
761 Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
766 // Step 7. Futility pruning: child node (skipped when in check)
768 && depth < 7 * ONE_PLY
769 && eval - futility_margin(depth) >= beta
770 && eval < VALUE_KNOWN_WIN // Do not return unproven wins
771 && pos.non_pawn_material(pos.side_to_move()))
772 return eval - futility_margin(depth);
774 // Step 8. Null move search with verification search (is omitted in PV nodes)
776 && depth >= 2 * ONE_PLY
778 && pos.non_pawn_material(pos.side_to_move()))
780 ss->currentMove = MOVE_NULL;
782 assert(eval - beta >= 0);
784 // Null move dynamic reduction based on depth and value
785 Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
787 pos.do_null_move(st);
788 (ss+1)->skipEarlyPruning = true;
789 nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
790 : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
791 (ss+1)->skipEarlyPruning = false;
792 pos.undo_null_move();
794 if (nullValue >= beta)
796 // Do not return unproven mate scores
797 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
800 if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
803 // Do verification search at high depths
804 ss->skipEarlyPruning = true;
805 Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
806 : search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
807 ss->skipEarlyPruning = false;
814 // Step 9. ProbCut (skipped when in check)
815 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
816 // and a reduced search returns a value much above beta, we can (almost)
817 // safely prune the previous move.
819 && depth >= 5 * ONE_PLY
820 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
822 Value rbeta = std::min(beta + 200, VALUE_INFINITE);
823 Depth rdepth = depth - 4 * ONE_PLY;
825 assert(rdepth >= ONE_PLY);
826 assert((ss-1)->currentMove != MOVE_NONE);
827 assert((ss-1)->currentMove != MOVE_NULL);
829 MovePicker mp(pos, ttMove, thisThread->history, PieceValue[MG][pos.captured_piece_type()]);
832 while ((move = mp.next_move()) != MOVE_NONE)
833 if (pos.legal(move, ci.pinned))
835 ss->currentMove = move;
836 pos.do_move(move, st, pos.gives_check(move, ci));
837 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
844 // Step 10. Internal iterative deepening (skipped when in check)
845 if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
847 && (PvNode || ss->staticEval + 256 >= beta))
849 Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
850 ss->skipEarlyPruning = true;
851 search<NT>(pos, ss, alpha, beta, d, true);
852 ss->skipEarlyPruning = false;
854 tte = TT.probe(posKey, ttHit);
855 ttMove = ttHit ? tte->move() : MOVE_NONE;
858 moves_loop: // When in check search starts from here
860 Square prevSq = to_sq((ss-1)->currentMove);
861 Move cm = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
862 const CounterMoveStats& cmh = CounterMoveHistory[pos.piece_on(prevSq)][prevSq];
864 MovePicker mp(pos, ttMove, depth, thisThread->history, cmh, cm, ss);
866 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
867 improving = ss->staticEval >= (ss-2)->staticEval
868 || ss->staticEval == VALUE_NONE
869 ||(ss-2)->staticEval == VALUE_NONE;
871 singularExtensionNode = !rootNode
872 && depth >= 8 * ONE_PLY
873 && ttMove != MOVE_NONE
874 /* && ttValue != VALUE_NONE Already implicit in the next condition */
875 && abs(ttValue) < VALUE_KNOWN_WIN
876 && !excludedMove // Recursive singular search is not allowed
877 && (tte->bound() & BOUND_LOWER)
878 && tte->depth() >= depth - 3 * ONE_PLY;
880 // Step 11. Loop through moves
881 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
882 while ((move = mp.next_move()) != MOVE_NONE)
886 if (move == excludedMove)
889 // At root obey the "searchmoves" option and skip moves not listed in Root
890 // Move List. As a consequence any illegal move is also skipped. In MultiPV
891 // mode we also skip PV moves which have been already searched.
892 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
893 thisThread->rootMoves.end(), move))
896 ss->moveCount = ++moveCount;
898 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
899 sync_cout << "info depth " << depth / ONE_PLY
900 << " currmove " << UCI::move(move, pos.is_chess960())
901 << " currmovenumber " << moveCount + thisThread->PVIdx << sync_endl;
904 (ss+1)->pv = nullptr;
906 extension = DEPTH_ZERO;
907 captureOrPromotion = pos.capture_or_promotion(move);
909 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
910 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
911 : pos.gives_check(move, ci);
913 // Step 12. Extend checks
914 if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
917 // Singular extension search. If all moves but one fail low on a search of
918 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
919 // is singular and should be extended. To verify this we do a reduced search
920 // on all the other moves but the ttMove and if the result is lower than
921 // ttValue minus a margin then we extend the ttMove.
922 if ( singularExtensionNode
925 && pos.legal(move, ci.pinned))
927 Value rBeta = ttValue - 2 * depth / ONE_PLY;
928 ss->excludedMove = move;
929 ss->skipEarlyPruning = true;
930 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
931 ss->skipEarlyPruning = false;
932 ss->excludedMove = MOVE_NONE;
938 // Update the current move (this must be done after singular extension search)
939 newDepth = depth - ONE_PLY + extension;
941 // Step 13. Pruning at shallow depth
943 && !captureOrPromotion
946 && !pos.advanced_pawn_push(move)
947 && bestValue > VALUE_MATED_IN_MAX_PLY)
949 // Move count based pruning
950 if ( depth < 16 * ONE_PLY
951 && moveCount >= FutilityMoveCounts[improving][depth])
954 // History based pruning
955 if ( depth <= 4 * ONE_PLY
956 && move != ss->killers[0]
957 && thisThread->history[pos.moved_piece(move)][to_sq(move)] < VALUE_ZERO
958 && cmh[pos.moved_piece(move)][to_sq(move)] < VALUE_ZERO)
961 predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
963 // Futility pruning: parent node
964 if (predictedDepth < 7 * ONE_PLY)
966 futilityValue = ss->staticEval + futility_margin(predictedDepth) + 256;
968 if (futilityValue <= alpha)
970 bestValue = std::max(bestValue, futilityValue);
975 // Prune moves with negative SEE at low depths
976 if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
980 // Speculative prefetch as early as possible
981 prefetch(TT.first_entry(pos.key_after(move)));
983 // Check for legality just before making the move
984 if (!rootNode && !pos.legal(move, ci.pinned))
986 ss->moveCount = --moveCount;
990 ss->currentMove = move;
992 // Step 14. Make the move
993 pos.do_move(move, st, givesCheck);
995 // Step 15. Reduced depth search (LMR). If the move fails high it will be
996 // re-searched at full depth.
997 if ( depth >= 3 * ONE_PLY
999 && !captureOrPromotion)
1001 Depth r = reduction<PvNode>(improving, depth, moveCount);
1003 // Increase reduction for cut nodes and moves with a bad history
1004 if ( (!PvNode && cutNode)
1005 || ( thisThread->history[pos.piece_on(to_sq(move))][to_sq(move)] < VALUE_ZERO
1006 && cmh[pos.piece_on(to_sq(move))][to_sq(move)] <= VALUE_ZERO))
1009 // Decrease/increase reduction for moves with a good/bad history
1010 int rHist = ( thisThread->history[pos.piece_on(to_sq(move))][to_sq(move)]
1011 + cmh[pos.piece_on(to_sq(move))][to_sq(move)]) / 14980;
1012 r = std::max(DEPTH_ZERO, r - rHist * ONE_PLY);
1014 // Decrease reduction for moves that escape a capture. Filter out
1015 // castling moves, because they are coded as "king captures rook" and
1016 // hence break make_move(). Also use see() instead of see_sign(),
1017 // because the destination square is empty.
1019 && type_of(move) == NORMAL
1020 && type_of(pos.piece_on(to_sq(move))) != PAWN
1021 && pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
1022 r = std::max(DEPTH_ZERO, r - ONE_PLY);
1024 Depth d = std::max(newDepth - r, ONE_PLY);
1026 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1028 doFullDepthSearch = (value > alpha && r != DEPTH_ZERO);
1031 doFullDepthSearch = !PvNode || moveCount > 1;
1033 // Step 16. Full depth search when LMR is skipped or fails high
1034 if (doFullDepthSearch)
1035 value = newDepth < ONE_PLY ?
1036 givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1037 : -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
1038 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1040 // For PV nodes only, do a full PV search on the first move or after a fail
1041 // high (in the latter case search only if value < beta), otherwise let the
1042 // parent node fail low with value <= alpha and try another move.
1043 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1046 (ss+1)->pv[0] = MOVE_NONE;
1048 value = newDepth < ONE_PLY ?
1049 givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1050 : -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
1051 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1054 // Step 17. Undo move
1055 pos.undo_move(move);
1057 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1059 // Step 18. Check for a new best move
1060 // Finished searching the move. If a stop occurred, the return value of
1061 // the search cannot be trusted, and we return immediately without
1062 // updating best move, PV and TT.
1063 if (Signals.stop.load(std::memory_order_relaxed))
1068 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1069 thisThread->rootMoves.end(), move);
1071 // PV move or new best move ?
1072 if (moveCount == 1 || value > alpha)
1079 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1080 rm.pv.push_back(*m);
1082 // We record how often the best move has been changed in each
1083 // iteration. This information is used for time management: When
1084 // the best move changes frequently, we allocate some more time.
1085 if (moveCount > 1 && thisThread == Threads.main())
1086 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1089 // All other moves but the PV are set to the lowest value: this is
1090 // not a problem when sorting because the sort is stable and the
1091 // move position in the list is preserved - just the PV is pushed up.
1092 rm.score = -VALUE_INFINITE;
1095 if (value > bestValue)
1101 // If there is an easy move for this position, clear it if unstable
1103 && thisThread == Threads.main()
1104 && EasyMove.get(pos.key())
1105 && (move != EasyMove.get(pos.key()) || moveCount > 1))
1110 if (PvNode && !rootNode) // Update pv even in fail-high case
1111 update_pv(ss->pv, move, (ss+1)->pv);
1113 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1117 assert(value >= beta); // Fail high
1123 if (!captureOrPromotion && move != bestMove && quietCount < 64)
1124 quietsSearched[quietCount++] = move;
1127 // The following condition would detect a stop only after move loop has been
1128 // completed. But in this case bestValue is valid because we have fully
1129 // searched our subtree, and we can anyhow save the result in TT.
1135 // Step 20. Check for mate and stalemate
1136 // All legal moves have been searched and if there are no legal moves, it
1137 // must be a mate or a stalemate. If we are in a singular extension search then
1138 // return a fail low score.
1140 bestValue = excludedMove ? alpha
1141 : inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
1143 // Quiet best move: update killers, history and countermoves
1144 else if (bestMove && !pos.capture_or_promotion(bestMove))
1145 update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount);
1147 // Bonus for prior countermove that caused the fail low
1148 else if ( depth >= 3 * ONE_PLY
1151 && !pos.captured_piece_type()
1152 && is_ok((ss - 1)->currentMove)
1153 && is_ok((ss - 2)->currentMove))
1155 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY - 1);
1156 Square prevPrevSq = to_sq((ss - 2)->currentMove);
1157 CounterMoveStats& prevCmh = CounterMoveHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
1158 prevCmh.update(pos.piece_on(prevSq), prevSq, bonus);
1161 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1162 bestValue >= beta ? BOUND_LOWER :
1163 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1164 depth, bestMove, ss->staticEval, TT.generation());
1166 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1172 // qsearch() is the quiescence search function, which is called by the main
1173 // search function when the remaining depth is zero (or, to be more precise,
1174 // less than ONE_PLY).
1176 template <NodeType NT, bool InCheck>
1177 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1179 const bool PvNode = NT == PV;
1181 assert(InCheck == !!pos.checkers());
1182 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1183 assert(PvNode || (alpha == beta - 1));
1184 assert(depth <= DEPTH_ZERO);
1190 Move ttMove, move, bestMove;
1191 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1192 bool ttHit, givesCheck, evasionPrunable;
1197 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1199 ss->pv[0] = MOVE_NONE;
1202 ss->currentMove = bestMove = MOVE_NONE;
1203 ss->ply = (ss-1)->ply + 1;
1205 // Check for an instant draw or if the maximum ply has been reached
1206 if (pos.is_draw() || ss->ply >= MAX_PLY)
1207 return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos)
1208 : DrawValue[pos.side_to_move()];
1210 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1212 // Decide whether or not to include checks: this fixes also the type of
1213 // TT entry depth that we are going to use. Note that in qsearch we use
1214 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1215 ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1216 : DEPTH_QS_NO_CHECKS;
1218 // Transposition table lookup
1220 tte = TT.probe(posKey, ttHit);
1221 ttMove = ttHit ? tte->move() : MOVE_NONE;
1222 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1226 && tte->depth() >= ttDepth
1227 && ttValue != VALUE_NONE // Only in case of TT access race
1228 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1229 : (tte->bound() & BOUND_UPPER)))
1231 ss->currentMove = ttMove; // Can be MOVE_NONE
1235 // Evaluate the position statically
1238 ss->staticEval = VALUE_NONE;
1239 bestValue = futilityBase = -VALUE_INFINITE;
1245 // Never assume anything on values stored in TT
1246 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1247 ss->staticEval = bestValue = evaluate(pos);
1249 // Can ttValue be used as a better position evaluation?
1250 if (ttValue != VALUE_NONE)
1251 if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
1252 bestValue = ttValue;
1255 ss->staticEval = bestValue =
1256 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1257 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1259 // Stand pat. Return immediately if static value is at least beta
1260 if (bestValue >= beta)
1263 tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
1264 DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
1269 if (PvNode && bestValue > alpha)
1272 futilityBase = bestValue + 128;
1275 // Initialize a MovePicker object for the current position, and prepare
1276 // to search the moves. Because the depth is <= 0 here, only captures,
1277 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1279 MovePicker mp(pos, ttMove, depth, pos.this_thread()->history, to_sq((ss-1)->currentMove));
1282 // Loop through the moves until no moves remain or a beta cutoff occurs
1283 while ((move = mp.next_move()) != MOVE_NONE)
1285 assert(is_ok(move));
1287 givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
1288 ? ci.checkSquares[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
1289 : pos.gives_check(move, ci);
1294 && futilityBase > -VALUE_KNOWN_WIN
1295 && !pos.advanced_pawn_push(move))
1297 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1299 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1301 if (futilityValue <= alpha)
1303 bestValue = std::max(bestValue, futilityValue);
1307 if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
1309 bestValue = std::max(bestValue, futilityBase);
1314 // Detect non-capture evasions that are candidates to be pruned
1315 evasionPrunable = InCheck
1316 && bestValue > VALUE_MATED_IN_MAX_PLY
1317 && !pos.capture(move);
1319 // Don't search moves with negative SEE values
1320 if ( (!InCheck || evasionPrunable)
1321 && type_of(move) != PROMOTION
1322 && pos.see_sign(move) < VALUE_ZERO)
1325 // Speculative prefetch as early as possible
1326 prefetch(TT.first_entry(pos.key_after(move)));
1328 // Check for legality just before making the move
1329 if (!pos.legal(move, ci.pinned))
1332 ss->currentMove = move;
1334 // Make and search the move
1335 pos.do_move(move, st, givesCheck);
1336 value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
1337 : -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1338 pos.undo_move(move);
1340 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1342 // Check for a new best move
1343 if (value > bestValue)
1349 if (PvNode) // Update pv even in fail-high case
1350 update_pv(ss->pv, move, (ss+1)->pv);
1352 if (PvNode && value < beta) // Update alpha here!
1359 tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
1360 ttDepth, move, ss->staticEval, TT.generation());
1368 // All legal moves have been searched. A special case: If we're in check
1369 // and no legal moves were found, it is checkmate.
1370 if (InCheck && bestValue == -VALUE_INFINITE)
1371 return mated_in(ss->ply); // Plies to mate from the root
1373 tte->save(posKey, value_to_tt(bestValue, ss->ply),
1374 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1375 ttDepth, bestMove, ss->staticEval, TT.generation());
1377 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1383 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1384 // "plies to mate from the current position". Non-mate scores are unchanged.
1385 // The function is called before storing a value in the transposition table.
1387 Value value_to_tt(Value v, int ply) {
1389 assert(v != VALUE_NONE);
1391 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1392 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1396 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1397 // from the transposition table (which refers to the plies to mate/be mated
1398 // from current position) to "plies to mate/be mated from the root".
1400 Value value_from_tt(Value v, int ply) {
1402 return v == VALUE_NONE ? VALUE_NONE
1403 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1404 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1408 // update_pv() adds current move and appends child pv[]
1410 void update_pv(Move* pv, Move move, Move* childPv) {
1412 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1418 // update_stats() updates killers, history, countermove and countermove
1419 // history when a new quiet best move is found.
1421 void update_stats(const Position& pos, Stack* ss, Move move,
1422 Depth depth, Move* quiets, int quietsCnt) {
1424 if (ss->killers[0] != move)
1426 ss->killers[1] = ss->killers[0];
1427 ss->killers[0] = move;
1430 Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY - 1);
1432 Square prevSq = to_sq((ss-1)->currentMove);
1433 CounterMoveStats& cmh = CounterMoveHistory[pos.piece_on(prevSq)][prevSq];
1434 Thread* thisThread = pos.this_thread();
1436 thisThread->history.update(pos.moved_piece(move), to_sq(move), bonus);
1438 if (is_ok((ss-1)->currentMove))
1440 thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
1441 cmh.update(pos.moved_piece(move), to_sq(move), bonus);
1444 // Decrease all the other played quiet moves
1445 for (int i = 0; i < quietsCnt; ++i)
1447 thisThread->history.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1449 if (is_ok((ss-1)->currentMove))
1450 cmh.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1453 // Extra penalty for a quiet TT move in previous ply when it gets refuted
1454 if ( (ss-1)->moveCount == 1
1455 && !pos.captured_piece_type()
1456 && is_ok((ss-2)->currentMove))
1458 Square prevPrevSq = to_sq((ss-2)->currentMove);
1459 CounterMoveStats& prevCmh = CounterMoveHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
1460 prevCmh.update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY);
1465 // When playing with strength handicap, choose best move among a set of RootMoves
1466 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1468 Move Skill::pick_best(size_t multiPV) {
1470 const Search::RootMoveVector& rootMoves = Threads.main()->rootMoves;
1471 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1473 // RootMoves are already sorted by score in descending order
1474 Value topScore = rootMoves[0].score;
1475 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1476 int weakness = 120 - 2 * level;
1477 int maxScore = -VALUE_INFINITE;
1479 // Choose best move. For each move score we add two terms, both dependent on
1480 // weakness. One is deterministic and bigger for weaker levels, and one is
1481 // random. Then we choose the move with the resulting highest score.
1482 for (size_t i = 0; i < multiPV; ++i)
1484 // This is our magic formula
1485 int push = ( weakness * int(topScore - rootMoves[i].score)
1486 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1488 if (rootMoves[i].score + push > maxScore)
1490 maxScore = rootMoves[i].score + push;
1491 best = rootMoves[i].pv[0];
1499 // check_time() is used to print debug info and, more importantly, to detect
1500 // when we are out of available time and thus stop the search.
1504 static TimePoint lastInfoTime = now();
1506 int elapsed = Time.elapsed();
1507 TimePoint tick = Limits.startTime + elapsed;
1509 if (tick - lastInfoTime >= 1000)
1511 lastInfoTime = tick;
1515 // An engine may not stop pondering until told so by the GUI
1519 if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
1520 || (Limits.movetime && elapsed >= Limits.movetime)
1521 || (Limits.nodes && Threads.nodes_searched() >= Limits.nodes))
1522 Signals.stop = true;
1528 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1529 /// that all (if any) unsearched PV lines are sent using a previous search score.
1531 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1533 std::stringstream ss;
1534 int elapsed = Time.elapsed() + 1;
1535 const Search::RootMoveVector& rootMoves = pos.this_thread()->rootMoves;
1536 size_t PVIdx = pos.this_thread()->PVIdx;
1537 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1538 uint64_t nodes_searched = Threads.nodes_searched();
1540 for (size_t i = 0; i < multiPV; ++i)
1542 bool updated = (i <= PVIdx);
1544 if (depth == ONE_PLY && !updated)
1547 Depth d = updated ? depth : depth - ONE_PLY;
1548 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1550 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1551 v = tb ? TB::Score : v;
1553 if (ss.rdbuf()->in_avail()) // Not at first line
1557 << " depth " << d / ONE_PLY
1558 << " seldepth " << pos.this_thread()->maxPly
1559 << " multipv " << i + 1
1560 << " score " << UCI::value(v);
1562 if (!tb && i == PVIdx)
1563 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1565 ss << " nodes " << nodes_searched
1566 << " nps " << nodes_searched * 1000 / elapsed;
1568 if (elapsed > 1000) // Earlier makes little sense
1569 ss << " hashfull " << TT.hashfull();
1571 ss << " tbhits " << TB::Hits
1572 << " time " << elapsed
1575 for (Move m : rootMoves[i].pv)
1576 ss << " " << UCI::move(m, pos.is_chess960());
1583 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1584 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1585 /// first, even if the old TT entries have been overwritten.
1587 void RootMove::insert_pv_in_tt(Position& pos) {
1589 StateInfo state[MAX_PLY], *st = state;
1594 assert(MoveList<LEGAL>(pos).contains(m));
1596 TTEntry* tte = TT.probe(pos.key(), ttHit);
1598 if (!ttHit || tte->move() != m) // Don't overwrite correct entries
1599 tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE,
1600 m, VALUE_NONE, TT.generation());
1602 pos.do_move(m, *st++, pos.gives_check(m, CheckInfo(pos)));
1605 for (size_t i = pv.size(); i > 0; )
1606 pos.undo_move(pv[--i]);
1610 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1611 /// before exiting the search, for instance, in case we stop the search during a
1612 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1613 /// otherwise in case of 'ponder on' we have nothing to think on.
1615 bool RootMove::extract_ponder_from_tt(Position& pos)
1620 assert(pv.size() == 1);
1622 pos.do_move(pv[0], st, pos.gives_check(pv[0], CheckInfo(pos)));
1623 TTEntry* tte = TT.probe(pos.key(), ttHit);
1624 pos.undo_move(pv[0]);
1628 Move m = tte->move(); // Local copy to be SMP safe
1629 if (MoveList<LEGAL>(pos).contains(m))
1630 return pv.push_back(m), true;