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) 2015-2019 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
38 #include "syzygy/tbprobe.h"
45 namespace Tablebases {
53 namespace TB = Tablebases;
57 using namespace Search;
61 // Different node types, used as a template parameter
62 enum NodeType { NonPV, PV };
64 // Sizes and phases of the skip-blocks, used for distributing search depths across the threads
65 constexpr int SkipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
66 constexpr int SkipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
68 // Razor and futility margins
69 constexpr int RazorMargin = 600;
70 Value futility_margin(Depth d, bool improving) {
71 return Value((175 - 50 * improving) * d / ONE_PLY);
74 // Futility and reductions lookup tables, initialized at startup
75 int FutilityMoveCounts[2][16]; // [improving][depth]
76 int Reductions[64]; // [depth or moveNumber]
78 template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
79 int r = Reductions[std::min(d / ONE_PLY, 63)] * Reductions[std::min(mn, 63)] / 1024;
80 return ((r + 512) / 1024 + (!i && r > 1024) - PvNode) * ONE_PLY;
83 // History and stats update bonus, based on depth
84 int stat_bonus(Depth depth) {
85 int d = depth / ONE_PLY;
86 return d > 17 ? 0 : 29 * d * d + 138 * d - 134;
89 // Add a small random component to draw evaluations to keep search dynamic
90 // and to avoid 3fold-blindness.
91 Value value_draw(Depth depth, Thread* thisThread) {
92 return depth < 4 ? VALUE_DRAW
93 : VALUE_DRAW + Value(2 * (thisThread->nodes.load(std::memory_order_relaxed) % 2) - 1);
96 // Skill structure is used to implement strength limit
98 explicit Skill(int l) : level(l) {}
99 bool enabled() const { return level < 20; }
100 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
101 Move pick_best(size_t multiPV);
104 Move best = MOVE_NONE;
107 template <NodeType NT>
108 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
110 template <NodeType NT>
111 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
113 Value value_to_tt(Value v, int ply);
114 Value value_from_tt(Value v, int ply);
115 void update_pv(Move* pv, Move move, Move* childPv);
116 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
117 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietCount, int bonus);
118 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCount, int bonus);
120 inline bool gives_check(const Position& pos, Move move) {
121 Color us = pos.side_to_move();
122 return type_of(move) == NORMAL && !(pos.blockers_for_king(~us) & pos.pieces(us))
123 ? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
124 : pos.gives_check(move);
127 // perft() is our utility to verify move generation. All the leaf nodes up
128 // to the given depth are generated and counted, and the sum is returned.
130 uint64_t perft(Position& pos, Depth depth) {
133 uint64_t cnt, nodes = 0;
134 const bool leaf = (depth == 2 * ONE_PLY);
136 for (const auto& m : MoveList<LEGAL>(pos))
138 if (Root && depth <= ONE_PLY)
143 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
148 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
156 /// Search::init() is called at startup to initialize various lookup tables
158 void Search::init() {
160 for (int i = 1; i < 64; ++i)
161 Reductions[i] = int(1024 * std::log(i) / std::sqrt(1.95));
163 for (int d = 0; d < 16; ++d)
165 FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
166 FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
171 /// Search::clear() resets search state to its initial value
173 void Search::clear() {
175 Threads.main()->wait_for_search_finished();
177 Time.availableNodes = 0;
180 Tablebases::init(Options["SyzygyPath"]); // Free up mapped files
184 /// MainThread::search() is called by the main thread when the program receives
185 /// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
187 void MainThread::search() {
191 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
192 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
196 Color us = rootPos.side_to_move();
197 Time.init(Limits, us, rootPos.game_ply());
200 if (rootMoves.empty())
202 rootMoves.emplace_back(MOVE_NONE);
203 sync_cout << "info depth 0 score "
204 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
209 for (Thread* th : Threads)
211 th->start_searching();
213 Thread::search(); // Let's start searching!
216 // When we reach the maximum depth, we can arrive here without a raise of
217 // Threads.stop. However, if we are pondering or in an infinite search,
218 // the UCI protocol states that we shouldn't print the best move before the
219 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
220 // until the GUI sends one of those commands.
222 while (!Threads.stop && (ponder || Limits.infinite))
223 {} // Busy wait for a stop or a ponder reset
225 // Stop the threads if not already stopped (also raise the stop if
226 // "ponderhit" just reset Threads.ponder).
229 // Wait until all threads have finished
230 for (Thread* th : Threads)
232 th->wait_for_search_finished();
234 // When playing in 'nodes as time' mode, subtract the searched nodes from
235 // the available ones before exiting.
237 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
239 // Check if there are threads with a better score than main thread
240 Thread* bestThread = this;
241 if ( Options["MultiPV"] == 1
243 && !Skill(Options["Skill Level"]).enabled()
244 && rootMoves[0].pv[0] != MOVE_NONE)
246 std::map<Move, int64_t> votes;
247 Value minScore = this->rootMoves[0].score;
249 // Find out minimum score and reset votes for moves which can be voted
250 for (Thread* th: Threads)
251 minScore = std::min(minScore, th->rootMoves[0].score);
253 // Vote according to score and depth
254 for (Thread* th : Threads)
256 int64_t s = th->rootMoves[0].score - minScore + 1;
257 votes[th->rootMoves[0].pv[0]] += 200 + s * s * int(th->completedDepth);
260 // Select best thread
261 auto bestVote = votes[this->rootMoves[0].pv[0]];
262 for (Thread* th : Threads)
263 if (votes[th->rootMoves[0].pv[0]] > bestVote)
265 bestVote = votes[th->rootMoves[0].pv[0]];
270 previousScore = bestThread->rootMoves[0].score;
272 // Send again PV info if we have a new best thread
273 if (bestThread != this)
274 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
276 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
278 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
279 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
281 std::cout << sync_endl;
285 /// Thread::search() is the main iterative deepening loop. It calls search()
286 /// repeatedly with increasing depth until the allocated thinking time has been
287 /// consumed, the user stops the search, or the maximum search depth is reached.
289 void Thread::search() {
291 // To allow access to (ss-5) up to (ss+2), the stack must be oversized.
292 // The former is needed to allow update_continuation_histories(ss-1, ...),
293 // which accesses its argument at ss-4, also near the root.
294 // The latter is needed for statScores and killer initialization.
295 Stack stack[MAX_PLY+10], *ss = stack+7;
297 Value bestValue, alpha, beta, delta;
298 Move lastBestMove = MOVE_NONE;
299 Depth lastBestMoveDepth = DEPTH_ZERO;
300 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
301 double timeReduction = 1.0;
302 Color us = rootPos.side_to_move();
305 std::memset(ss-7, 0, 10 * sizeof(Stack));
306 for (int i = 7; i > 0; i--)
307 (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
310 bestValue = delta = alpha = -VALUE_INFINITE;
311 beta = VALUE_INFINITE;
314 mainThread->bestMoveChanges = 0, failedLow = false;
316 size_t multiPV = Options["MultiPV"];
317 Skill skill(Options["Skill Level"]);
319 // When playing with strength handicap enable MultiPV search that we will
320 // use behind the scenes to retrieve a set of possible moves.
322 multiPV = std::max(multiPV, (size_t)4);
324 multiPV = std::min(multiPV, rootMoves.size());
326 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
328 // In analysis mode, adjust contempt in accordance with user preference
329 if (Limits.infinite || Options["UCI_AnalyseMode"])
330 ct = Options["Analysis Contempt"] == "Off" ? 0
331 : Options["Analysis Contempt"] == "Both" ? ct
332 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
333 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
336 // In evaluate.cpp the evaluation is from the white point of view
337 contempt = (us == WHITE ? make_score(ct, ct / 2)
338 : -make_score(ct, ct / 2));
340 // Iterative deepening loop until requested to stop or the target depth is reached
341 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
343 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
345 // Distribute search depths across the helper threads
348 int i = (idx - 1) % 20;
349 if (((rootDepth / ONE_PLY + SkipPhase[i]) / SkipSize[i]) % 2)
350 continue; // Retry with an incremented rootDepth
353 // Age out PV variability metric
355 mainThread->bestMoveChanges *= 0.517, failedLow = false;
357 // Save the last iteration's scores before first PV line is searched and
358 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
359 for (RootMove& rm : rootMoves)
360 rm.previousScore = rm.score;
365 // MultiPV loop. We perform a full root search for each PV line
366 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
371 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
372 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
376 // Reset UCI info selDepth for each depth and each PV line
379 // Reset aspiration window starting size
380 if (rootDepth >= 5 * ONE_PLY)
382 Value previousScore = rootMoves[pvIdx].previousScore;
384 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
385 beta = std::min(previousScore + delta, VALUE_INFINITE);
387 // Adjust contempt based on root move's previousScore (dynamic contempt)
388 int dct = ct + 88 * previousScore / (abs(previousScore) + 200);
390 contempt = (us == WHITE ? make_score(dct, dct / 2)
391 : -make_score(dct, dct / 2));
394 // Start with a small aspiration window and, in the case of a fail
395 // high/low, re-search with a bigger window until we don't fail
397 int failedHighCnt = 0;
400 Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
401 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
403 // Bring the best move to the front. It is critical that sorting
404 // is done with a stable algorithm because all the values but the
405 // first and eventually the new best one are set to -VALUE_INFINITE
406 // and we want to keep the same order for all the moves except the
407 // new PV that goes to the front. Note that in case of MultiPV
408 // search the already searched PV lines are preserved.
409 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
411 // If search has been stopped, we break immediately. Sorting is
412 // safe because RootMoves is still valid, although it refers to
413 // the previous iteration.
417 // When failing high/low give some update (without cluttering
418 // the UI) before a re-search.
421 && (bestValue <= alpha || bestValue >= beta)
422 && Time.elapsed() > 3000)
423 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
425 // In case of failing low/high increase aspiration window and
426 // re-search, otherwise exit the loop.
427 if (bestValue <= alpha)
429 beta = (alpha + beta) / 2;
430 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
436 mainThread->stopOnPonderhit = false;
439 else if (bestValue >= beta)
441 beta = std::min(bestValue + delta, VALUE_INFINITE);
448 delta += delta / 4 + 5;
450 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
453 // Sort the PV lines searched so far and update the GUI
454 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
457 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
458 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
462 completedDepth = rootDepth;
464 if (rootMoves[0].pv[0] != lastBestMove) {
465 lastBestMove = rootMoves[0].pv[0];
466 lastBestMoveDepth = rootDepth;
469 // Have we found a "mate in x"?
471 && bestValue >= VALUE_MATE_IN_MAX_PLY
472 && VALUE_MATE - bestValue <= 2 * Limits.mate)
478 // If skill level is enabled and time is up, pick a sub-optimal best move
479 if (skill.enabled() && skill.time_to_pick(rootDepth))
480 skill.pick_best(multiPV);
482 // Do we have time for the next iteration? Can we stop searching now?
483 if ( Limits.use_time_management()
485 && !mainThread->stopOnPonderhit)
487 double fallingEval = (306 + 119 * failedLow + 6 * (mainThread->previousScore - bestValue)) / 581.0;
488 fallingEval = std::max(0.5, std::min(1.5, fallingEval));
490 // If the bestMove is stable over several iterations, reduce time accordingly
491 timeReduction = lastBestMoveDepth + 10 * ONE_PLY < completedDepth ? 1.95 : 1.0;
493 // Use part of the gained time from a previous stable move for the current move
494 double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
495 bestMoveInstability *= std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
497 // Stop the search if we have only one legal move, or if available time elapsed
498 if ( rootMoves.size() == 1
499 || Time.elapsed() > Time.optimum() * bestMoveInstability * fallingEval)
501 // If we are allowed to ponder do not stop the search now but
502 // keep pondering until the GUI sends "ponderhit" or "stop".
503 if (mainThread->ponder)
504 mainThread->stopOnPonderhit = true;
514 mainThread->previousTimeReduction = timeReduction;
516 // If skill level is enabled, swap best PV line with the sub-optimal one
518 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
519 skill.best ? skill.best : skill.pick_best(multiPV)));
525 // search<>() is the main search function for both PV and non-PV nodes
527 template <NodeType NT>
528 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
530 constexpr bool PvNode = NT == PV;
531 const bool rootNode = PvNode && ss->ply == 0;
533 // Check if we have an upcoming move which draws by repetition, or
534 // if the opponent had an alternative move earlier to this position.
535 if ( pos.rule50_count() >= 3
536 && alpha < VALUE_DRAW
538 && pos.has_game_cycle(ss->ply))
540 alpha = value_draw(depth, pos.this_thread());
545 // Dive into quiescence search when the depth reaches zero
547 return qsearch<NT>(pos, ss, alpha, beta);
549 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
550 assert(PvNode || (alpha == beta - 1));
551 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
552 assert(!(PvNode && cutNode));
553 assert(depth / ONE_PLY * ONE_PLY == depth);
555 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
559 Move ttMove, move, excludedMove, bestMove;
560 Depth extension, newDepth;
561 Value bestValue, value, ttValue, eval, maxValue, pureStaticEval;
562 bool ttHit, ttPv, inCheck, givesCheck, improving;
563 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
565 int moveCount, captureCount, quietCount;
567 // Step 1. Initialize node
568 Thread* thisThread = pos.this_thread();
569 inCheck = pos.checkers();
570 Color us = pos.side_to_move();
571 moveCount = captureCount = quietCount = ss->moveCount = 0;
572 bestValue = -VALUE_INFINITE;
573 maxValue = VALUE_INFINITE;
575 // Check for the available remaining time
576 if (thisThread == Threads.main())
577 static_cast<MainThread*>(thisThread)->check_time();
579 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
580 if (PvNode && thisThread->selDepth < ss->ply + 1)
581 thisThread->selDepth = ss->ply + 1;
585 // Step 2. Check for aborted search and immediate draw
586 if ( Threads.stop.load(std::memory_order_relaxed)
587 || pos.is_draw(ss->ply)
588 || ss->ply >= MAX_PLY)
589 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
590 : value_draw(depth, pos.this_thread());
592 // Step 3. Mate distance pruning. Even if we mate at the next move our score
593 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
594 // a shorter mate was found upward in the tree then there is no need to search
595 // because we will never beat the current alpha. Same logic but with reversed
596 // signs applies also in the opposite condition of being mated instead of giving
597 // mate. In this case return a fail-high score.
598 alpha = std::max(mated_in(ss->ply), alpha);
599 beta = std::min(mate_in(ss->ply+1), beta);
604 assert(0 <= ss->ply && ss->ply < MAX_PLY);
606 (ss+1)->ply = ss->ply + 1;
607 ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
608 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
609 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
610 Square prevSq = to_sq((ss-1)->currentMove);
612 // Initialize statScore to zero for the grandchildren of the current position.
613 // So statScore is shared between all grandchildren and only the first grandchild
614 // starts with statScore = 0. Later grandchildren start with the last calculated
615 // statScore of the previous grandchild. This influences the reduction rules in
616 // LMR which are based on the statScore of parent position.
617 (ss+2)->statScore = 0;
619 // Step 4. Transposition table lookup. We don't want the score of a partial
620 // search to overwrite a previous full search TT value, so we use a different
621 // position key in case of an excluded move.
622 excludedMove = ss->excludedMove;
623 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
624 tte = TT.probe(posKey, ttHit);
625 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
626 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
627 : ttHit ? tte->move() : MOVE_NONE;
628 ttPv = (ttHit && tte->is_pv()) || (PvNode && depth > 4 * ONE_PLY);
630 // At non-PV nodes we check for an early TT cutoff
633 && tte->depth() >= depth
634 && ttValue != VALUE_NONE // Possible in case of TT access race
635 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
636 : (tte->bound() & BOUND_UPPER)))
638 // If ttMove is quiet, update move sorting heuristics on TT hit
643 if (!pos.capture_or_promotion(ttMove))
644 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
646 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
647 if ( ((ss-1)->moveCount == 1 || (ss-1)->currentMove == (ss-1)->killers[0])
648 && !pos.captured_piece())
649 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
651 // Penalty for a quiet ttMove that fails low
652 else if (!pos.capture_or_promotion(ttMove))
654 int penalty = -stat_bonus(depth);
655 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
656 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
662 // Step 5. Tablebases probe
663 if (!rootNode && TB::Cardinality)
665 int piecesCount = pos.count<ALL_PIECES>();
667 if ( piecesCount <= TB::Cardinality
668 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
669 && pos.rule50_count() == 0
670 && !pos.can_castle(ANY_CASTLING))
673 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
675 // Force check of time on the next occasion
676 if (thisThread == Threads.main())
677 static_cast<MainThread*>(thisThread)->callsCnt = 0;
679 if (err != TB::ProbeState::FAIL)
681 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
683 int drawScore = TB::UseRule50 ? 1 : 0;
685 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
686 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
687 : VALUE_DRAW + 2 * wdl * drawScore;
689 Bound b = wdl < -drawScore ? BOUND_UPPER
690 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
692 if ( b == BOUND_EXACT
693 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
695 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
696 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
697 MOVE_NONE, VALUE_NONE);
704 if (b == BOUND_LOWER)
705 bestValue = value, alpha = std::max(alpha, bestValue);
713 // Step 6. Static evaluation of the position
716 ss->staticEval = eval = pureStaticEval = VALUE_NONE;
718 goto moves_loop; // Skip early pruning when in check
722 // Never assume anything on values stored in TT
723 ss->staticEval = eval = pureStaticEval = tte->eval();
724 if (eval == VALUE_NONE)
725 ss->staticEval = eval = pureStaticEval = evaluate(pos);
727 // Can ttValue be used as a better position evaluation?
728 if ( ttValue != VALUE_NONE
729 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
734 if ((ss-1)->currentMove != MOVE_NULL)
736 int bonus = -(ss-1)->statScore / 512;
738 pureStaticEval = evaluate(pos);
739 ss->staticEval = eval = pureStaticEval + bonus;
742 ss->staticEval = eval = pureStaticEval = -(ss-1)->staticEval + 2 * Eval::Tempo;
744 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, pureStaticEval);
747 // Step 7. Razoring (~2 Elo)
748 if ( !rootNode // The required rootNode PV handling is not available in qsearch
749 && depth < 2 * ONE_PLY
750 && eval <= alpha - RazorMargin)
751 return qsearch<NT>(pos, ss, alpha, beta);
753 improving = ss->staticEval >= (ss-2)->staticEval
754 || (ss-2)->staticEval == VALUE_NONE;
756 // Step 8. Futility pruning: child node (~30 Elo)
758 && depth < 7 * ONE_PLY
759 && eval - futility_margin(depth, improving) >= beta
760 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
763 // Step 9. Null move search with verification search (~40 Elo)
765 && (ss-1)->currentMove != MOVE_NULL
766 && (ss-1)->statScore < 23200
768 && pureStaticEval >= beta - 36 * depth / ONE_PLY + 225
770 && pos.non_pawn_material(us)
771 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
773 assert(eval - beta >= 0);
775 // Null move dynamic reduction based on depth and value
776 Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 200, 3)) * ONE_PLY;
778 ss->currentMove = MOVE_NULL;
779 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
781 pos.do_null_move(st);
783 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
785 pos.undo_null_move();
787 if (nullValue >= beta)
789 // Do not return unproven mate scores
790 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
793 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 12 * ONE_PLY))
796 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
798 // Do verification search at high depths, with null move pruning disabled
799 // for us, until ply exceeds nmpMinPly.
800 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
801 thisThread->nmpColor = us;
803 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
805 thisThread->nmpMinPly = 0;
812 // Step 10. ProbCut (~10 Elo)
813 // If we have a good enough capture and a reduced search returns a value
814 // much above beta, we can (almost) safely prune the previous move.
816 && depth >= 5 * ONE_PLY
817 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
819 Value raisedBeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
820 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
821 int probCutCount = 0;
823 while ( (move = mp.next_move()) != MOVE_NONE
824 && probCutCount < 2 + 2 * cutNode)
825 if (move != excludedMove && pos.legal(move))
829 ss->currentMove = move;
830 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
832 assert(depth >= 5 * ONE_PLY);
834 pos.do_move(move, st);
836 // Perform a preliminary qsearch to verify that the move holds
837 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
839 // If the qsearch held, perform the regular search
840 if (value >= raisedBeta)
841 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
845 if (value >= raisedBeta)
850 // Step 11. Internal iterative deepening (~2 Elo)
851 if ( depth >= 8 * ONE_PLY
854 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
856 tte = TT.probe(posKey, ttHit);
857 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
858 ttMove = ttHit ? tte->move() : MOVE_NONE;
861 moves_loop: // When in check, search starts from here
863 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
864 nullptr, (ss-4)->continuationHistory,
865 nullptr, (ss-6)->continuationHistory };
866 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
868 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
869 &thisThread->captureHistory,
873 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
875 moveCountPruning = false;
876 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
878 // Step 12. Loop through all pseudo-legal moves until no moves remain
879 // or a beta cutoff occurs.
880 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
884 if (move == excludedMove)
887 // At root obey the "searchmoves" option and skip moves not listed in Root
888 // Move List. As a consequence any illegal move is also skipped. In MultiPV
889 // mode we also skip PV moves which have been already searched and those
890 // of lower "TB rank" if we are in a TB root position.
891 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
892 thisThread->rootMoves.begin() + thisThread->pvLast, move))
895 ss->moveCount = ++moveCount;
897 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
898 sync_cout << "info depth " << depth / ONE_PLY
899 << " currmove " << UCI::move(move, pos.is_chess960())
900 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
902 (ss+1)->pv = nullptr;
904 extension = DEPTH_ZERO;
905 captureOrPromotion = pos.capture_or_promotion(move);
906 movedPiece = pos.moved_piece(move);
907 givesCheck = gives_check(pos, move);
909 // Step 13. Extensions (~70 Elo)
911 // Singular extension search (~60 Elo). If all moves but one fail low on a
912 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
913 // then that move is singular and should be extended. To verify this we do
914 // a reduced search on all the other moves but the ttMove and if the
915 // result is lower than ttValue minus a margin then we will extend the ttMove.
916 if ( depth >= 8 * ONE_PLY
919 && !excludedMove // Avoid recursive singular search
920 && ttValue != VALUE_NONE
921 && (tte->bound() & BOUND_LOWER)
922 && tte->depth() >= depth - 3 * ONE_PLY
925 Value singularBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
926 ss->excludedMove = move;
927 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, depth / 2, cutNode);
928 ss->excludedMove = MOVE_NONE;
930 if (value < singularBeta)
934 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
935 // search without the ttMove. So we assume this expected Cut-node is not singular,
936 // that is multiple moves fail high, and we can prune the whole subtree by returning
937 // the hard beta bound.
938 else if (cutNode && singularBeta > beta)
942 // Check extension (~2 Elo)
944 && (pos.blockers_for_king(~us) & from_sq(move) || pos.see_ge(move)))
947 // Castling extension
948 else if (type_of(move) == CASTLING)
951 // Calculate new depth for this move
952 newDepth = depth - ONE_PLY + extension;
954 // Step 14. Pruning at shallow depth (~170 Elo)
956 && pos.non_pawn_material(us)
957 && bestValue > VALUE_MATED_IN_MAX_PLY)
959 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
960 moveCountPruning = depth < 16 * ONE_PLY
961 && moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
963 if ( !captureOrPromotion
965 && !pos.advanced_pawn_push(move))
967 // Move count based pruning (~30 Elo)
968 if (moveCountPruning)
971 // Reduced depth of the next LMR search
972 int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
974 // Countermoves based pruning (~20 Elo)
975 if ( lmrDepth < 3 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
976 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
977 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
980 // Futility pruning: parent node (~2 Elo)
983 && ss->staticEval + 256 + 200 * lmrDepth <= alpha)
986 // Prune moves with negative SEE (~10 Elo)
987 if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
990 else if ( !extension // (~20 Elo)
991 && !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
995 // Speculative prefetch as early as possible
996 prefetch(TT.first_entry(pos.key_after(move)));
998 // Check for legality just before making the move
999 if (!rootNode && !pos.legal(move))
1001 ss->moveCount = --moveCount;
1005 // Update the current move (this must be done after singular extension search)
1006 ss->currentMove = move;
1007 ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
1009 // Step 15. Make the move
1010 pos.do_move(move, st, givesCheck);
1012 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1013 // re-searched at full depth.
1014 if ( depth >= 3 * ONE_PLY
1016 && (!captureOrPromotion || moveCountPruning))
1018 Depth r = reduction<PvNode>(improving, depth, moveCount);
1020 // Decrease reduction if position is or has been on the PV
1024 // Decrease reduction if opponent's move count is high (~10 Elo)
1025 if ((ss-1)->moveCount > 15)
1028 if (!captureOrPromotion)
1030 // Increase reduction if ttMove is a capture (~0 Elo)
1034 // Increase reduction for cut nodes (~5 Elo)
1038 // Decrease reduction for moves that escape a capture. Filter out
1039 // castling moves, because they are coded as "king captures rook" and
1040 // hence break make_move(). (~5 Elo)
1041 else if ( type_of(move) == NORMAL
1042 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1045 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1046 + (*contHist[0])[movedPiece][to_sq(move)]
1047 + (*contHist[1])[movedPiece][to_sq(move)]
1048 + (*contHist[3])[movedPiece][to_sq(move)]
1051 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1052 if (ss->statScore >= 0 && (ss-1)->statScore < 0)
1055 else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
1058 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1059 r -= ss->statScore / 20000 * ONE_PLY;
1062 Depth d = std::max(newDepth - std::max(r, DEPTH_ZERO), ONE_PLY);
1064 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1066 doFullDepthSearch = (value > alpha && d != newDepth);
1069 doFullDepthSearch = !PvNode || moveCount > 1;
1071 // Step 17. Full depth search when LMR is skipped or fails high
1072 if (doFullDepthSearch)
1073 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1075 // For PV nodes only, do a full PV search on the first move or after a fail
1076 // high (in the latter case search only if value < beta), otherwise let the
1077 // parent node fail low with value <= alpha and try another move.
1078 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1081 (ss+1)->pv[0] = MOVE_NONE;
1083 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1086 // Step 18. Undo move
1087 pos.undo_move(move);
1089 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1091 // Step 19. Check for a new best move
1092 // Finished searching the move. If a stop occurred, the return value of
1093 // the search cannot be trusted, and we return immediately without
1094 // updating best move, PV and TT.
1095 if (Threads.stop.load(std::memory_order_relaxed))
1100 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1101 thisThread->rootMoves.end(), move);
1103 // PV move or new best move?
1104 if (moveCount == 1 || value > alpha)
1107 rm.selDepth = thisThread->selDepth;
1112 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1113 rm.pv.push_back(*m);
1115 // We record how often the best move has been changed in each
1116 // iteration. This information is used for time management: When
1117 // the best move changes frequently, we allocate some more time.
1118 if (moveCount > 1 && thisThread == Threads.main())
1119 ++static_cast<MainThread*>(thisThread)->bestMoveChanges;
1122 // All other moves but the PV are set to the lowest value: this
1123 // is not a problem when sorting because the sort is stable and the
1124 // move position in the list is preserved - just the PV is pushed up.
1125 rm.score = -VALUE_INFINITE;
1128 if (value > bestValue)
1136 if (PvNode && !rootNode) // Update pv even in fail-high case
1137 update_pv(ss->pv, move, (ss+1)->pv);
1139 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1143 assert(value >= beta); // Fail high
1150 if (move != bestMove)
1152 if (captureOrPromotion && captureCount < 32)
1153 capturesSearched[captureCount++] = move;
1155 else if (!captureOrPromotion && quietCount < 64)
1156 quietsSearched[quietCount++] = move;
1160 // The following condition would detect a stop only after move loop has been
1161 // completed. But in this case bestValue is valid because we have fully
1162 // searched our subtree, and we can anyhow save the result in TT.
1168 // Step 20. Check for mate and stalemate
1169 // All legal moves have been searched and if there are no legal moves, it
1170 // must be a mate or a stalemate. If we are in a singular extension search then
1171 // return a fail low score.
1173 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1176 bestValue = excludedMove ? alpha
1177 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1180 // Quiet best move: update move sorting heuristics
1181 if (!pos.capture_or_promotion(bestMove))
1182 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1183 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1185 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1187 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1188 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1189 && !pos.captured_piece())
1190 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1193 // Bonus for prior countermove that caused the fail low
1194 else if ( (depth >= 3 * ONE_PLY || PvNode)
1195 && !pos.captured_piece())
1196 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1199 bestValue = std::min(bestValue, maxValue);
1202 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1203 bestValue >= beta ? BOUND_LOWER :
1204 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1205 depth, bestMove, pureStaticEval);
1207 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1213 // qsearch() is the quiescence search function, which is called by the main
1214 // search function with depth zero, or recursively with depth less than ONE_PLY.
1215 template <NodeType NT>
1216 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1218 constexpr bool PvNode = NT == PV;
1220 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1221 assert(PvNode || (alpha == beta - 1));
1222 assert(depth <= DEPTH_ZERO);
1223 assert(depth / ONE_PLY * ONE_PLY == depth);
1229 Move ttMove, move, bestMove;
1231 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1232 bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
1237 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1239 ss->pv[0] = MOVE_NONE;
1242 Thread* thisThread = pos.this_thread();
1243 (ss+1)->ply = ss->ply + 1;
1244 ss->currentMove = bestMove = MOVE_NONE;
1245 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
1246 inCheck = pos.checkers();
1249 // Check for an immediate draw or maximum ply reached
1250 if ( pos.is_draw(ss->ply)
1251 || ss->ply >= MAX_PLY)
1252 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1254 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1256 // Decide whether or not to include checks: this fixes also the type of
1257 // TT entry depth that we are going to use. Note that in qsearch we use
1258 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1259 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1260 : DEPTH_QS_NO_CHECKS;
1261 // Transposition table lookup
1263 tte = TT.probe(posKey, ttHit);
1264 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1265 ttMove = ttHit ? tte->move() : MOVE_NONE;
1266 pvHit = ttHit && tte->is_pv();
1270 && tte->depth() >= ttDepth
1271 && ttValue != VALUE_NONE // Only in case of TT access race
1272 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1273 : (tte->bound() & BOUND_UPPER)))
1276 // Evaluate the position statically
1279 ss->staticEval = VALUE_NONE;
1280 bestValue = futilityBase = -VALUE_INFINITE;
1286 // Never assume anything on values stored in TT
1287 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1288 ss->staticEval = bestValue = evaluate(pos);
1290 // Can ttValue be used as a better position evaluation?
1291 if ( ttValue != VALUE_NONE
1292 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1293 bestValue = ttValue;
1296 ss->staticEval = bestValue =
1297 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1298 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1300 // Stand pat. Return immediately if static value is at least beta
1301 if (bestValue >= beta)
1304 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1305 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1310 if (PvNode && bestValue > alpha)
1313 futilityBase = bestValue + 128;
1316 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1317 nullptr, (ss-4)->continuationHistory,
1318 nullptr, (ss-6)->continuationHistory };
1320 // Initialize a MovePicker object for the current position, and prepare
1321 // to search the moves. Because the depth is <= 0 here, only captures,
1322 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1324 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1325 &thisThread->captureHistory,
1327 to_sq((ss-1)->currentMove));
1329 // Loop through the moves until no moves remain or a beta cutoff occurs
1330 while ((move = mp.next_move()) != MOVE_NONE)
1332 assert(is_ok(move));
1334 givesCheck = gives_check(pos, move);
1341 && futilityBase > -VALUE_KNOWN_WIN
1342 && !pos.advanced_pawn_push(move))
1344 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1346 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1348 if (futilityValue <= alpha)
1350 bestValue = std::max(bestValue, futilityValue);
1354 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1356 bestValue = std::max(bestValue, futilityBase);
1361 // Detect non-capture evasions that are candidates to be pruned
1362 evasionPrunable = inCheck
1363 && (depth != DEPTH_ZERO || moveCount > 2)
1364 && bestValue > VALUE_MATED_IN_MAX_PLY
1365 && !pos.capture(move);
1367 // Don't search moves with negative SEE values
1368 if ( (!inCheck || evasionPrunable)
1369 && !pos.see_ge(move))
1372 // Speculative prefetch as early as possible
1373 prefetch(TT.first_entry(pos.key_after(move)));
1375 // Check for legality just before making the move
1376 if (!pos.legal(move))
1382 ss->currentMove = move;
1383 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
1385 // Make and search the move
1386 pos.do_move(move, st, givesCheck);
1387 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1388 pos.undo_move(move);
1390 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1392 // Check for a new best move
1393 if (value > bestValue)
1401 if (PvNode) // Update pv even in fail-high case
1402 update_pv(ss->pv, move, (ss+1)->pv);
1404 if (PvNode && value < beta) // Update alpha here!
1412 // All legal moves have been searched. A special case: If we're in check
1413 // and no legal moves were found, it is checkmate.
1414 if (inCheck && bestValue == -VALUE_INFINITE)
1415 return mated_in(ss->ply); // Plies to mate from the root
1417 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1418 bestValue >= beta ? BOUND_LOWER :
1419 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1420 ttDepth, bestMove, ss->staticEval);
1422 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1428 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1429 // "plies to mate from the current position". Non-mate scores are unchanged.
1430 // The function is called before storing a value in the transposition table.
1432 Value value_to_tt(Value v, int ply) {
1434 assert(v != VALUE_NONE);
1436 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1437 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1441 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1442 // from the transposition table (which refers to the plies to mate/be mated
1443 // from current position) to "plies to mate/be mated from the root".
1445 Value value_from_tt(Value v, int ply) {
1447 return v == VALUE_NONE ? VALUE_NONE
1448 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1449 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1453 // update_pv() adds current move and appends child pv[]
1455 void update_pv(Move* pv, Move move, Move* childPv) {
1457 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1463 // update_continuation_histories() updates histories of the move pairs formed
1464 // by moves at ply -1, -2, and -4 with current move.
1466 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1468 for (int i : {1, 2, 4, 6})
1469 if (is_ok((ss-i)->currentMove))
1470 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1474 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1476 void update_capture_stats(const Position& pos, Move move,
1477 Move* captures, int captureCount, int bonus) {
1479 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1480 Piece moved_piece = pos.moved_piece(move);
1481 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1483 if (pos.capture_or_promotion(move))
1484 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1486 // Decrease all the other played capture moves
1487 for (int i = 0; i < captureCount; ++i)
1489 moved_piece = pos.moved_piece(captures[i]);
1490 captured = type_of(pos.piece_on(to_sq(captures[i])));
1491 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1496 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1498 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1499 Move* quiets, int quietCount, int bonus) {
1501 if (ss->killers[0] != move)
1503 ss->killers[1] = ss->killers[0];
1504 ss->killers[0] = move;
1507 Color us = pos.side_to_move();
1508 Thread* thisThread = pos.this_thread();
1509 thisThread->mainHistory[us][from_to(move)] << bonus;
1510 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1512 if (is_ok((ss-1)->currentMove))
1514 Square prevSq = to_sq((ss-1)->currentMove);
1515 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1518 // Decrease all the other played quiet moves
1519 for (int i = 0; i < quietCount; ++i)
1521 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1522 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1526 // When playing with strength handicap, choose best move among a set of RootMoves
1527 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1529 Move Skill::pick_best(size_t multiPV) {
1531 const RootMoves& rootMoves = Threads.main()->rootMoves;
1532 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1534 // RootMoves are already sorted by score in descending order
1535 Value topScore = rootMoves[0].score;
1536 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1537 int weakness = 120 - 2 * level;
1538 int maxScore = -VALUE_INFINITE;
1540 // Choose best move. For each move score we add two terms, both dependent on
1541 // weakness. One is deterministic and bigger for weaker levels, and one is
1542 // random. Then we choose the move with the resulting highest score.
1543 for (size_t i = 0; i < multiPV; ++i)
1545 // This is our magic formula
1546 int push = ( weakness * int(topScore - rootMoves[i].score)
1547 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1549 if (rootMoves[i].score + push >= maxScore)
1551 maxScore = rootMoves[i].score + push;
1552 best = rootMoves[i].pv[0];
1561 /// MainThread::check_time() is used to print debug info and, more importantly,
1562 /// to detect when we are out of available time and thus stop the search.
1564 void MainThread::check_time() {
1569 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1570 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1572 static TimePoint lastInfoTime = now();
1574 TimePoint elapsed = Time.elapsed();
1575 TimePoint tick = Limits.startTime + elapsed;
1577 if (tick - lastInfoTime >= 1000)
1579 lastInfoTime = tick;
1583 // We should not stop pondering until told so by the GUI
1587 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1588 || (Limits.movetime && elapsed >= Limits.movetime)
1589 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1590 Threads.stop = true;
1594 /// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1595 /// that all (if any) unsearched PV lines are sent using a previous search score.
1597 string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1599 std::stringstream ss;
1600 TimePoint elapsed = Time.elapsed() + 1;
1601 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1602 size_t pvIdx = pos.this_thread()->pvIdx;
1603 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1604 uint64_t nodesSearched = Threads.nodes_searched();
1605 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1607 for (size_t i = 0; i < multiPV; ++i)
1609 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1611 if (depth == ONE_PLY && !updated)
1614 Depth d = updated ? depth : depth - ONE_PLY;
1615 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1617 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1618 v = tb ? rootMoves[i].tbScore : v;
1620 if (ss.rdbuf()->in_avail()) // Not at first line
1624 << " depth " << d / ONE_PLY
1625 << " seldepth " << rootMoves[i].selDepth
1626 << " multipv " << i + 1
1627 << " score " << UCI::value(v);
1629 if (!tb && i == pvIdx)
1630 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1632 ss << " nodes " << nodesSearched
1633 << " nps " << nodesSearched * 1000 / elapsed;
1635 if (elapsed > 1000) // Earlier makes little sense
1636 ss << " hashfull " << TT.hashfull();
1638 ss << " tbhits " << tbHits
1639 << " time " << elapsed
1642 for (Move m : rootMoves[i].pv)
1643 ss << " " << UCI::move(m, pos.is_chess960());
1650 /// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1651 /// before exiting the search, for instance, in case we stop the search during a
1652 /// fail high at root. We try hard to have a ponder move to return to the GUI,
1653 /// otherwise in case of 'ponder on' we have nothing to think on.
1655 bool RootMove::extract_ponder_from_tt(Position& pos) {
1660 assert(pv.size() == 1);
1662 if (pv[0] == MOVE_NONE)
1665 pos.do_move(pv[0], st);
1666 TTEntry* tte = TT.probe(pos.key(), ttHit);
1670 Move m = tte->move(); // Local copy to be SMP safe
1671 if (MoveList<LEGAL>(pos).contains(m))
1675 pos.undo_move(pv[0]);
1676 return pv.size() > 1;
1679 void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1682 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1683 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1684 Cardinality = int(Options["SyzygyProbeLimit"]);
1685 bool dtz_available = true;
1687 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1688 // ProbeDepth == DEPTH_ZERO
1689 if (Cardinality > MaxCardinality)
1691 Cardinality = MaxCardinality;
1692 ProbeDepth = DEPTH_ZERO;
1695 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1697 // Rank moves using DTZ tables
1698 RootInTB = root_probe(pos, rootMoves);
1702 // DTZ tables are missing; try to rank moves using WDL tables
1703 dtz_available = false;
1704 RootInTB = root_probe_wdl(pos, rootMoves);
1710 // Sort moves according to TB rank
1711 std::sort(rootMoves.begin(), rootMoves.end(),
1712 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1714 // Probe during search only if DTZ is not available and we are winning
1715 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1720 // Assign the same rank to all moves
1721 for (auto& m : rootMoves)