2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2012 Marco Costalba, Joona Kiiski, Tord Romstad
6 Stockfish is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation, either version 3 of the License, or
9 (at your option) any later version.
11 Stockfish is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>.
37 #include "ucioption.h"
41 volatile SignalsType Signals;
43 std::vector<RootMove> RootMoves;
44 Position RootPosition;
52 using namespace Search;
56 // Set to true to force running with one thread. Used for debugging
57 const bool FakeSplit = false;
59 // Different node types, used as template parameter
60 enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
62 // Lookup table to check if a Piece is a slider and its access function
63 const bool Slidings[18] = { 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1 };
64 inline bool piece_is_slider(Piece p) { return Slidings[p]; }
66 // Maximum depth for razoring
67 const Depth RazorDepth = 4 * ONE_PLY;
69 // Dynamic razoring margin based on depth
70 inline Value razor_margin(Depth d) { return Value(0x200 + 0x10 * int(d)); }
72 // Maximum depth for use of dynamic threat detection when null move fails low
73 const Depth ThreatDepth = 5 * ONE_PLY;
75 // Minimum depth for use of internal iterative deepening
76 const Depth IIDDepth[] = { 8 * ONE_PLY, 5 * ONE_PLY };
78 // At Non-PV nodes we do an internal iterative deepening search
79 // when the static evaluation is bigger then beta - IIDMargin.
80 const Value IIDMargin = Value(0x100);
82 // Minimum depth for use of singular extension
83 const Depth SingularExtensionDepth[] = { 8 * ONE_PLY, 6 * ONE_PLY };
85 // Futility margin for quiescence search
86 const Value FutilityMarginQS = Value(0x80);
88 // Futility lookup tables (initialized at startup) and their access functions
89 Value FutilityMargins[16][64]; // [depth][moveNumber]
90 int FutilityMoveCounts[32]; // [depth]
92 inline Value futility_margin(Depth d, int mn) {
94 return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
98 inline int futility_move_count(Depth d) {
100 return d < 16 * ONE_PLY ? FutilityMoveCounts[d] : MAX_MOVES;
103 // Reduction lookup tables (initialized at startup) and their access function
104 int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
106 template <bool PvNode> inline Depth reduction(Depth d, int mn) {
108 return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
111 // Easy move margin. An easy move candidate must be at least this much better
112 // than the second best move.
113 const Value EasyMoveMargin = Value(0x150);
115 // This is the minimum interval in msec between two check_time() calls
116 const int TimerResolution = 5;
119 size_t MultiPV, UCIMultiPV, PVIdx;
123 bool SkillLevelEnabled, Chess960;
127 template <NodeType NT>
128 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
130 template <NodeType NT>
131 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
133 void id_loop(Position& pos);
134 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta);
135 bool connected_moves(const Position& pos, Move m1, Move m2);
136 Value value_to_tt(Value v, int ply);
137 Value value_from_tt(Value v, int ply);
138 bool can_return_tt(const TTEntry* tte, Depth depth, Value ttValue, Value beta);
139 bool connected_threat(const Position& pos, Move m, Move threat);
140 Value refine_eval(const TTEntry* tte, Value ttValue, Value defaultEval);
141 Move do_skill_level();
142 string score_to_uci(Value v, Value alpha = -VALUE_INFINITE, Value beta = VALUE_INFINITE);
143 void pv_info_to_log(Position& pos, int depth, Value score, int time, Move pv[]);
144 void pv_info_to_uci(const Position& pos, int depth, Value alpha, Value beta);
146 // MovePickerExt class template extends MovePicker and allows to choose at
147 // compile time the proper moves source according to the type of node. In the
148 // default case we simply create and use a standard MovePicker object.
149 template<bool SpNode> struct MovePickerExt : public MovePicker {
151 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
152 : MovePicker(p, ttm, d, h, ss, b) {}
155 // In case of a SpNode we use split point's shared MovePicker object as moves source
156 template<> struct MovePickerExt<true> : public MovePicker {
158 MovePickerExt(const Position& p, Move ttm, Depth d, const History& h, Stack* ss, Value b)
159 : MovePicker(p, ttm, d, h, ss, b), mp(ss->sp->mp) {}
161 Move next_move() { return mp->next_move(); }
165 // is_dangerous() checks whether a move belongs to some classes of known
166 // 'dangerous' moves so that we avoid to prune it.
167 FORCE_INLINE bool is_dangerous(const Position& pos, Move m, bool captureOrPromotion) {
169 // Test for a pawn pushed to 7th or a passed pawn move
170 if (type_of(pos.piece_moved(m)) == PAWN)
172 Color c = pos.side_to_move();
173 if ( relative_rank(c, to_sq(m)) == RANK_7
174 || pos.pawn_is_passed(c, to_sq(m)))
178 // Test for a capture that triggers a pawn endgame
179 if ( captureOrPromotion
180 && type_of(pos.piece_on(to_sq(m))) != PAWN
181 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
182 - PieceValueMidgame[pos.piece_on(to_sq(m))] == VALUE_ZERO)
192 /// Search::init() is called during startup to initialize various lookup tables
194 void Search::init() {
196 int d; // depth (ONE_PLY == 2)
197 int hd; // half depth (ONE_PLY == 1)
200 // Init reductions array
201 for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
203 double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
204 double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
205 Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
206 Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
209 // Init futility margins array
210 for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
211 FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
213 // Init futility move count array
214 for (d = 0; d < 32; d++)
215 FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(d, 2.0));
219 /// Search::perft() is our utility to verify move generation. All the leaf nodes
220 /// up to the given depth are generated and counted and the sum returned.
222 int64_t Search::perft(Position& pos, Depth depth) {
227 MoveList<MV_LEGAL> ml(pos);
229 // At the last ply just return the number of moves (leaf nodes)
230 if (depth == ONE_PLY)
234 for ( ; !ml.end(); ++ml)
236 pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
237 cnt += perft(pos, depth - ONE_PLY);
238 pos.undo_move(ml.move());
244 /// Search::think() is the external interface to Stockfish's search, and is
245 /// called by the main thread when the program receives the UCI 'go' command. It
246 /// searches from RootPosition and at the end prints the "bestmove" to output.
248 void Search::think() {
250 static Book book; // Defined static to initialize the PRNG only once
252 Position& pos = RootPosition;
253 Chess960 = pos.is_chess960();
254 Eval::RootColor = pos.side_to_move();
255 TimeMgr.init(Limits, pos.startpos_ply_counter(), pos.side_to_move());
259 if (RootMoves.empty())
261 cout << "info depth 0 score "
262 << score_to_uci(pos.in_check() ? -VALUE_MATE : VALUE_DRAW) << endl;
264 RootMoves.push_back(MOVE_NONE);
268 if (Options["OwnBook"])
270 Move bookMove = book.probe(pos, Options["Book File"], Options["Best Book Move"]);
272 if (bookMove && count(RootMoves.begin(), RootMoves.end(), bookMove))
274 std::swap(RootMoves[0], *find(RootMoves.begin(), RootMoves.end(), bookMove));
279 UCIMultiPV = Options["MultiPV"];
280 SkillLevel = Options["Skill Level"];
282 // Do we have to play with skill handicap? In this case enable MultiPV that
283 // we will use behind the scenes to retrieve a set of possible moves.
284 SkillLevelEnabled = (SkillLevel < 20);
285 MultiPV = (SkillLevelEnabled ? std::max(UCIMultiPV, (size_t)4) : UCIMultiPV);
287 if (Options["Use Search Log"])
289 Log log(Options["Search Log Filename"]);
290 log << "\nSearching: " << pos.to_fen()
291 << "\ninfinite: " << Limits.infinite
292 << " ponder: " << Limits.ponder
293 << " time: " << Limits.time[pos.side_to_move()]
294 << " increment: " << Limits.inc[pos.side_to_move()]
295 << " moves to go: " << Limits.movestogo
301 // Set best timer interval to avoid lagging under time pressure. Timer is
302 // used to check for remaining available thinking time.
303 if (Limits.use_time_management())
304 Threads.set_timer(std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)));
306 Threads.set_timer(100);
308 // We're ready to start searching. Call the iterative deepening loop function
311 Threads.set_timer(0); // Stop timer
314 if (Options["Use Search Log"])
316 int e = SearchTime.elapsed();
318 Log log(Options["Search Log Filename"]);
319 log << "Nodes: " << pos.nodes_searched()
320 << "\nNodes/second: " << (e > 0 ? pos.nodes_searched() * 1000 / e : 0)
321 << "\nBest move: " << move_to_san(pos, RootMoves[0].pv[0]);
324 pos.do_move(RootMoves[0].pv[0], st);
325 log << "\nPonder move: " << move_to_san(pos, RootMoves[0].pv[1]) << endl;
326 pos.undo_move(RootMoves[0].pv[0]);
331 // When we reach max depth we arrive here even without Signals.stop is raised,
332 // but if we are pondering or in infinite search, we shouldn't print the best
333 // move before we are told to do so.
334 if (!Signals.stop && (Limits.ponder || Limits.infinite))
335 Threads[pos.this_thread()].wait_for_stop_or_ponderhit();
337 // Best move could be MOVE_NONE when searching on a stalemate position
338 cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], Chess960)
339 << " ponder " << move_to_uci(RootMoves[0].pv[1], Chess960) << endl;
345 // id_loop() is the main iterative deepening loop. It calls search() repeatedly
346 // with increasing depth until the allocated thinking time has been consumed,
347 // user stops the search, or the maximum search depth is reached.
349 void id_loop(Position& pos) {
351 Stack ss[MAX_PLY_PLUS_2];
352 int depth, prevBestMoveChanges;
353 Value bestValue, alpha, beta, delta;
354 bool bestMoveNeverChanged = true;
355 Move skillBest = MOVE_NONE;
357 memset(ss, 0, 4 * sizeof(Stack));
358 depth = BestMoveChanges = 0;
359 bestValue = delta = -VALUE_INFINITE;
360 ss->currentMove = MOVE_NULL; // Hack to skip update gains
362 // Iterative deepening loop until requested to stop or target depth reached
363 while (!Signals.stop && ++depth <= MAX_PLY && (!Limits.depth || depth <= Limits.depth))
365 // Save last iteration's scores before first PV line is searched and all
366 // the move scores but the (new) PV are set to -VALUE_INFINITE.
367 for (size_t i = 0; i < RootMoves.size(); i++)
368 RootMoves[i].prevScore = RootMoves[i].score;
370 prevBestMoveChanges = BestMoveChanges;
373 // MultiPV loop. We perform a full root search for each PV line
374 for (PVIdx = 0; PVIdx < std::min(MultiPV, RootMoves.size()); PVIdx++)
376 // Set aspiration window default width
377 if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
380 alpha = RootMoves[PVIdx].prevScore - delta;
381 beta = RootMoves[PVIdx].prevScore + delta;
385 alpha = -VALUE_INFINITE;
386 beta = VALUE_INFINITE;
389 // Start with a small aspiration window and, in case of fail high/low,
390 // research with bigger window until not failing high/low anymore.
392 // Search starts from ss+1 to allow referencing (ss-1). This is
393 // needed by update gains and ss copy when splitting at Root.
394 bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
396 // Bring to front the best move. It is critical that sorting is
397 // done with a stable algorithm because all the values but the first
398 // and eventually the new best one are set to -VALUE_INFINITE and
399 // we want to keep the same order for all the moves but the new
400 // PV that goes to the front. Note that in case of MultiPV search
401 // the already searched PV lines are preserved.
402 sort<RootMove>(RootMoves.begin() + PVIdx, RootMoves.end());
404 // In case we have found an exact score and we are going to leave
405 // the fail high/low loop then reorder the PV moves, otherwise
406 // leave the last PV move in its position so to be searched again.
407 // Of course this is needed only in MultiPV search.
408 if (PVIdx && bestValue > alpha && bestValue < beta)
409 sort<RootMove>(RootMoves.begin(), RootMoves.begin() + PVIdx);
411 // Write PV back to transposition table in case the relevant
412 // entries have been overwritten during the search.
413 for (size_t i = 0; i <= PVIdx; i++)
414 RootMoves[i].insert_pv_in_tt(pos);
416 // If search has been stopped exit the aspiration window loop.
417 // Sorting and writing PV back to TT is safe becuase RootMoves
418 // is still valid, although refers to previous iteration.
422 // Send full PV info to GUI if we are going to leave the loop or
423 // if we have a fail high/low and we are deep in the search.
424 if ((bestValue > alpha && bestValue < beta) || SearchTime.elapsed() > 2000)
425 pv_info_to_uci(pos, depth, alpha, beta);
427 // In case of failing high/low increase aspiration window and
428 // research, otherwise exit the fail high/low loop.
429 if (bestValue >= beta)
434 else if (bestValue <= alpha)
436 Signals.failedLowAtRoot = true;
437 Signals.stopOnPonderhit = false;
445 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
447 } while (abs(bestValue) < VALUE_KNOWN_WIN);
450 // Skills: Do we need to pick now the best move ?
451 if (SkillLevelEnabled && depth == 1 + SkillLevel)
452 skillBest = do_skill_level();
454 if (!Signals.stop && Options["Use Search Log"])
455 pv_info_to_log(pos, depth, bestValue, SearchTime.elapsed(), &RootMoves[0].pv[0]);
457 // Filter out startup noise when monitoring best move stability
458 if (depth > 2 && BestMoveChanges)
459 bestMoveNeverChanged = false;
461 // Do we have time for the next iteration? Can we stop searching now?
462 if (!Signals.stop && !Signals.stopOnPonderhit && Limits.use_time_management())
464 bool stop = false; // Local variable, not the volatile Signals.stop
466 // Take in account some extra time if the best move has changed
467 if (depth > 4 && depth < 50)
468 TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
470 // Stop search if most of available time is already consumed. We
471 // probably don't have enough time to search the first move at the
472 // next iteration anyway.
473 if (SearchTime.elapsed() > (TimeMgr.available_time() * 62) / 100)
476 // Stop search early if one move seems to be much better than others
479 && ( (bestMoveNeverChanged && pos.captured_piece_type())
480 || SearchTime.elapsed() > (TimeMgr.available_time() * 40) / 100))
482 Value rBeta = bestValue - EasyMoveMargin;
483 (ss+1)->excludedMove = RootMoves[0].pv[0];
484 (ss+1)->skipNullMove = true;
485 Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth - 3) * ONE_PLY);
486 (ss+1)->skipNullMove = false;
487 (ss+1)->excludedMove = MOVE_NONE;
495 // If we are allowed to ponder do not stop the search now but
496 // keep pondering until GUI sends "ponderhit" or "stop".
498 Signals.stopOnPonderhit = true;
505 // When using skills swap best PV line with the sub-optimal one
506 if (SkillLevelEnabled)
508 if (skillBest == MOVE_NONE) // Still unassigned ?
509 skillBest = do_skill_level();
511 std::swap(RootMoves[0], *find(RootMoves.begin(), RootMoves.end(), skillBest));
516 // search<>() is the main search function for both PV and non-PV nodes and for
517 // normal and SplitPoint nodes. When called just after a split point the search
518 // is simpler because we have already probed the hash table, done a null move
519 // search, and searched the first move before splitting, we don't have to repeat
520 // all this work again. We also don't need to store anything to the hash table
521 // here: This is taken care of after we return from the split point.
523 template <NodeType NT>
524 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
526 const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
527 const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
528 const bool RootNode = (NT == Root || NT == SplitPointRoot);
530 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
531 assert((alpha == beta - 1) || PvNode);
532 assert(depth > DEPTH_ZERO);
533 assert(pos.this_thread() >= 0 && pos.this_thread() < Threads.size());
535 Move movesSearched[MAX_MOVES];
539 Move ttMove, move, excludedMove, bestMove, threatMove;
542 Value bestValue, value, oldAlpha, ttValue;
543 Value refinedValue, nullValue, futilityBase, futilityValue;
544 bool isPvMove, inCheck, singularExtensionNode, givesCheck;
545 bool captureOrPromotion, dangerous, doFullDepthSearch;
546 int moveCount = 0, playedMoveCount = 0;
547 Thread& thread = Threads[pos.this_thread()];
548 SplitPoint* sp = NULL;
550 refinedValue = bestValue = value = -VALUE_INFINITE;
552 inCheck = pos.in_check();
553 ss->ply = (ss-1)->ply + 1;
555 // Used to send selDepth info to GUI
556 if (PvNode && thread.maxPly < ss->ply)
557 thread.maxPly = ss->ply;
559 // Step 1. Initialize node
563 ttMove = excludedMove = MOVE_NONE;
564 ttValue = VALUE_ZERO;
566 bestMove = sp->bestMove;
567 threatMove = sp->threatMove;
568 bestValue = sp->bestValue;
569 moveCount = sp->moveCount; // Lock must be held here
571 assert(bestValue > -VALUE_INFINITE && moveCount > 0);
573 goto split_point_start;
577 ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
578 (ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
579 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
583 // Step 2. Check for aborted search and immediate draw
584 // Enforce node limit here. FIXME: This only works with 1 search thread.
585 if (Limits.nodes && pos.nodes_searched() >= Limits.nodes)
589 || pos.is_draw<false>()
590 || ss->ply > MAX_PLY) && !RootNode)
593 // Step 3. Mate distance pruning. Even if we mate at the next move our score
594 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
595 // a shorter mate was found upward in the tree then there is no need to search
596 // further, we will never beat current alpha. Same logic but with reversed signs
597 // applies also in the opposite condition of being mated instead of giving mate,
598 // in this case return a fail-high score.
601 alpha = std::max(mated_in(ss->ply), alpha);
602 beta = std::min(mate_in(ss->ply+1), beta);
607 // Step 4. Transposition table lookup
608 // We don't want the score of a partial search to overwrite a previous full search
609 // TT value, so we use a different position key in case of an excluded move.
610 excludedMove = ss->excludedMove;
611 posKey = excludedMove ? pos.exclusion_key() : pos.key();
612 tte = TT.probe(posKey);
613 ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
614 ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_ZERO;
616 // At PV nodes we check for exact scores, while at non-PV nodes we check for
617 // a fail high/low. Biggest advantage at probing at PV nodes is to have a
618 // smooth experience in analysis mode. We don't probe at Root nodes otherwise
619 // we should also update RootMoveList to avoid bogus output.
620 if (!RootNode && tte && (PvNode ? tte->depth() >= depth && tte->type() == BOUND_EXACT
621 : can_return_tt(tte, depth, ttValue, beta)))
624 ss->currentMove = ttMove; // Can be MOVE_NONE
628 && !pos.is_capture_or_promotion(ttMove)
629 && ttMove != ss->killers[0])
631 ss->killers[1] = ss->killers[0];
632 ss->killers[0] = ttMove;
637 // Step 5. Evaluate the position statically and update parent's gain statistics
639 ss->eval = ss->evalMargin = VALUE_NONE;
642 assert(tte->static_value() != VALUE_NONE);
644 ss->eval = tte->static_value();
645 ss->evalMargin = tte->static_value_margin();
646 refinedValue = refine_eval(tte, ttValue, ss->eval);
650 refinedValue = ss->eval = evaluate(pos, ss->evalMargin);
651 TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, ss->eval, ss->evalMargin);
654 // Update gain for the parent non-capture move given the static position
655 // evaluation before and after the move.
656 if ( (move = (ss-1)->currentMove) != MOVE_NULL
657 && (ss-1)->eval != VALUE_NONE
658 && ss->eval != VALUE_NONE
659 && !pos.captured_piece_type()
660 && !is_special(move))
662 Square to = to_sq(move);
663 H.update_gain(pos.piece_on(to), to, -(ss-1)->eval - ss->eval);
666 // Step 6. Razoring (is omitted in PV nodes)
668 && depth < RazorDepth
670 && refinedValue + razor_margin(depth) < beta
671 && ttMove == MOVE_NONE
672 && abs(beta) < VALUE_MATE_IN_MAX_PLY
673 && !pos.pawn_on_7th(pos.side_to_move()))
675 Value rbeta = beta - razor_margin(depth);
676 Value v = qsearch<NonPV>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
678 // Logically we should return (v + razor_margin(depth)), but
679 // surprisingly this did slightly weaker in tests.
683 // Step 7. Static null move pruning (is omitted in PV nodes)
684 // We're betting that the opponent doesn't have a move that will reduce
685 // the score by more than futility_margin(depth) if we do a null move.
688 && depth < RazorDepth
690 && refinedValue - futility_margin(depth, 0) >= beta
691 && abs(beta) < VALUE_MATE_IN_MAX_PLY
692 && pos.non_pawn_material(pos.side_to_move()))
693 return refinedValue - futility_margin(depth, 0);
695 // Step 8. Null move search with verification search (is omitted in PV nodes)
700 && refinedValue >= beta
701 && abs(beta) < VALUE_MATE_IN_MAX_PLY
702 && pos.non_pawn_material(pos.side_to_move()))
704 ss->currentMove = MOVE_NULL;
706 // Null move dynamic reduction based on depth
707 int R = 3 + (depth >= 5 * ONE_PLY ? depth / 8 : 0);
709 // Null move dynamic reduction based on value
710 if (refinedValue - PawnValueMidgame > beta)
713 pos.do_null_move<true>(st);
714 (ss+1)->skipNullMove = true;
715 nullValue = depth-R*ONE_PLY < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
716 : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R*ONE_PLY);
717 (ss+1)->skipNullMove = false;
718 pos.do_null_move<false>(st);
720 if (nullValue >= beta)
722 // Do not return unproven mate scores
723 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
726 if (depth < 6 * ONE_PLY)
729 // Do verification search at high depths
730 ss->skipNullMove = true;
731 Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*ONE_PLY);
732 ss->skipNullMove = false;
739 // The null move failed low, which means that we may be faced with
740 // some kind of threat. If the previous move was reduced, check if
741 // the move that refuted the null move was somehow connected to the
742 // move which was reduced. If a connection is found, return a fail
743 // low score (which will cause the reduced move to fail high in the
744 // parent node, which will trigger a re-search with full depth).
745 threatMove = (ss+1)->currentMove;
747 if ( depth < ThreatDepth
749 && threatMove != MOVE_NONE
750 && connected_moves(pos, (ss-1)->currentMove, threatMove))
755 // Step 9. ProbCut (is omitted in PV nodes)
756 // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
757 // and a reduced search returns a value much above beta, we can (almost) safely
758 // prune the previous move.
760 && depth >= RazorDepth + ONE_PLY
763 && excludedMove == MOVE_NONE
764 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
766 Value rbeta = beta + 200;
767 Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
769 assert(rdepth >= ONE_PLY);
770 assert((ss-1)->currentMove != MOVE_NONE);
771 assert((ss-1)->currentMove != MOVE_NULL);
773 MovePicker mp(pos, ttMove, H, pos.captured_piece_type());
776 while ((move = mp.next_move()) != MOVE_NONE)
777 if (pos.pl_move_is_legal(move, ci.pinned))
779 ss->currentMove = move;
780 pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
781 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
788 // Step 10. Internal iterative deepening
789 if ( depth >= IIDDepth[PvNode]
790 && ttMove == MOVE_NONE
791 && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
793 Depth d = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
795 ss->skipNullMove = true;
796 search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
797 ss->skipNullMove = false;
799 tte = TT.probe(posKey);
800 ttMove = tte ? tte->move() : MOVE_NONE;
803 split_point_start: // At split points actual search starts from here
805 MovePickerExt<SpNode> mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
807 futilityBase = ss->eval + ss->evalMargin;
808 singularExtensionNode = !RootNode
810 && depth >= SingularExtensionDepth[PvNode]
811 && ttMove != MOVE_NONE
812 && !excludedMove // Recursive singular search is not allowed
813 && (tte->type() & BOUND_LOWER)
814 && tte->depth() >= depth - 3 * ONE_PLY;
816 // Step 11. Loop through moves
817 // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
818 while ( bestValue < beta
819 && (move = mp.next_move()) != MOVE_NONE
820 && !thread.cutoff_occurred()
825 if (move == excludedMove)
828 // At root obey the "searchmoves" option and skip moves not listed in Root
829 // Move List, as a consequence any illegal move is also skipped. In MultiPV
830 // mode we also skip PV moves which have been already searched.
831 if (RootNode && !count(RootMoves.begin() + PVIdx, RootMoves.end(), move))
834 // At PV and SpNode nodes we want all moves to be legal since the beginning
835 if ((PvNode || SpNode) && !pos.pl_move_is_legal(move, ci.pinned))
840 moveCount = ++sp->moveCount;
841 lock_release(sp->lock);
848 Signals.firstRootMove = (moveCount == 1);
850 if (pos.this_thread() == 0 && SearchTime.elapsed() > 2000)
851 cout << "info depth " << depth / ONE_PLY
852 << " currmove " << move_to_uci(move, Chess960)
853 << " currmovenumber " << moveCount + PVIdx << endl;
856 isPvMove = (PvNode && moveCount <= 1);
857 captureOrPromotion = pos.is_capture_or_promotion(move);
858 givesCheck = pos.move_gives_check(move, ci);
859 dangerous = givesCheck || is_dangerous(pos, move, captureOrPromotion);
862 // Step 12. Extend checks and, in PV nodes, also dangerous moves
863 if (PvNode && dangerous)
866 else if (givesCheck && pos.see_sign(move) >= 0)
867 ext = PvNode ? ONE_PLY : ONE_PLY / 2;
869 // Singular extension search. If all moves but one fail low on a search of
870 // (alpha-s, beta-s), and just one fails high on (alpha, beta), then that move
871 // is singular and should be extended. To verify this we do a reduced search
872 // on all the other moves but the ttMove, if result is lower than ttValue minus
873 // a margin then we extend ttMove.
874 if ( singularExtensionNode
877 && pos.pl_move_is_legal(move, ci.pinned))
879 if (abs(ttValue) < VALUE_KNOWN_WIN)
881 Value rBeta = ttValue - int(depth);
882 ss->excludedMove = move;
883 ss->skipNullMove = true;
884 value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
885 ss->skipNullMove = false;
886 ss->excludedMove = MOVE_NONE;
892 // Update current move (this must be done after singular extension search)
893 newDepth = depth - ONE_PLY + ext;
895 // Step 13. Futility pruning (is omitted in PV nodes)
897 && !captureOrPromotion
902 && (bestValue > VALUE_MATED_IN_MAX_PLY || bestValue == -VALUE_INFINITE))
904 // Move count based pruning
905 if ( moveCount >= futility_move_count(depth)
906 && (!threatMove || !connected_threat(pos, move, threatMove)))
914 // Value based pruning
915 // We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
916 // but fixing this made program slightly weaker.
917 Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
918 futilityValue = futilityBase + futility_margin(predictedDepth, moveCount)
919 + H.gain(pos.piece_moved(move), to_sq(move));
921 if (futilityValue < beta)
929 // Prune moves with negative SEE at low depths
930 if ( predictedDepth < 2 * ONE_PLY
931 && pos.see_sign(move) < 0)
940 // Check for legality only before to do the move
941 if (!pos.pl_move_is_legal(move, ci.pinned))
947 ss->currentMove = move;
948 if (!SpNode && !captureOrPromotion)
949 movesSearched[playedMoveCount++] = move;
951 // Step 14. Make the move
952 pos.do_move(move, st, ci, givesCheck);
954 // Step 15. Reduced depth search (LMR). If the move fails high will be
955 // re-searched at full depth.
956 if ( depth > 3 * ONE_PLY
958 && !captureOrPromotion
961 && ss->killers[0] != move
962 && ss->killers[1] != move)
964 ss->reduction = reduction<PvNode>(depth, moveCount);
965 Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
966 alpha = SpNode ? sp->alpha : alpha;
968 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
970 doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
971 ss->reduction = DEPTH_ZERO;
974 doFullDepthSearch = !isPvMove;
976 // Step 16. Full depth search, when LMR is skipped or fails high
977 if (doFullDepthSearch)
979 alpha = SpNode ? sp->alpha : alpha;
980 value = newDepth < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
981 : - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth);
984 // Only for PV nodes do a full PV search on the first move or after a fail
985 // high, in the latter case search only if value < beta, otherwise let the
986 // parent node to fail low with value <= alpha and to try another move.
987 if (PvNode && (isPvMove || (value > alpha && (RootNode || value < beta))))
988 value = newDepth < ONE_PLY ? -qsearch<PV>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
989 : - search<PV>(pos, ss+1, -beta, -alpha, newDepth);
991 // Step 17. Undo move
994 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
996 // Step 18. Check for new best move
1000 bestValue = sp->bestValue;
1004 // Finished searching the move. If Signals.stop is true, the search
1005 // was aborted because the user interrupted the search or because we
1006 // ran out of time. In this case, the return value of the search cannot
1007 // be trusted, and we don't update the best move and/or PV.
1008 if (RootNode && !Signals.stop)
1010 RootMove& rm = *find(RootMoves.begin(), RootMoves.end(), move);
1012 // PV move or new best move ?
1013 if (isPvMove || value > alpha)
1016 rm.extract_pv_from_tt(pos);
1018 // We record how often the best move has been changed in each
1019 // iteration. This information is used for time management: When
1020 // the best move changes frequently, we allocate some more time.
1021 if (!isPvMove && MultiPV == 1)
1025 // All other moves but the PV are set to the lowest value, this
1026 // is not a problem when sorting becuase sort is stable and move
1027 // position in the list is preserved, just the PV is pushed up.
1028 rm.score = -VALUE_INFINITE;
1032 if (value > bestValue)
1039 && value < beta) // We want always alpha < beta
1042 if (SpNode && !thread.cutoff_occurred())
1044 sp->bestValue = value;
1045 sp->bestMove = move;
1053 // Step 19. Check for split
1055 && depth >= Threads.min_split_depth()
1057 && Threads.available_slave_exists(pos.this_thread())
1059 && !thread.cutoff_occurred())
1060 bestValue = Threads.split<FakeSplit>(pos, ss, alpha, beta, bestValue, &bestMove,
1061 depth, threatMove, moveCount, &mp, NT);
1064 // Step 20. Check for mate and stalemate
1065 // All legal moves have been searched and if there are no legal moves, it
1066 // must be mate or stalemate. Note that we can have a false positive in
1067 // case of Signals.stop or thread.cutoff_occurred() are set, but this is
1068 // harmless because return value is discarded anyhow in the parent nodes.
1069 // If we are in a singular extension search then return a fail low score.
1071 return excludedMove ? oldAlpha : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1073 // If we have pruned all the moves without searching return a fail-low score
1074 if (bestValue == -VALUE_INFINITE)
1076 assert(!playedMoveCount);
1078 bestValue = oldAlpha;
1081 // Step 21. Update tables
1082 // Update transposition table entry, killers and history
1083 if (!SpNode && !Signals.stop && !thread.cutoff_occurred())
1085 move = bestValue <= oldAlpha ? MOVE_NONE : bestMove;
1086 bt = bestValue <= oldAlpha ? BOUND_UPPER
1087 : bestValue >= beta ? BOUND_LOWER : BOUND_EXACT;
1089 TT.store(posKey, value_to_tt(bestValue, ss->ply), bt, depth, move, ss->eval, ss->evalMargin);
1091 // Update killers and history for non capture cut-off moves
1092 if ( bestValue >= beta
1093 && !pos.is_capture_or_promotion(move)
1096 if (move != ss->killers[0])
1098 ss->killers[1] = ss->killers[0];
1099 ss->killers[0] = move;
1102 // Increase history value of the cut-off move
1103 Value bonus = Value(int(depth) * int(depth));
1104 H.add(pos.piece_moved(move), to_sq(move), bonus);
1106 // Decrease history of all the other played non-capture moves
1107 for (int i = 0; i < playedMoveCount - 1; i++)
1109 Move m = movesSearched[i];
1110 H.add(pos.piece_moved(m), to_sq(m), -bonus);
1115 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1121 // qsearch() is the quiescence search function, which is called by the main
1122 // search function when the remaining depth is zero (or, to be more precise,
1123 // less than ONE_PLY).
1125 template <NodeType NT>
1126 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1128 const bool PvNode = (NT == PV);
1130 assert(NT == PV || NT == NonPV);
1131 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1132 assert((alpha == beta - 1) || PvNode);
1133 assert(depth <= DEPTH_ZERO);
1134 assert(pos.this_thread() >= 0 && pos.this_thread() < Threads.size());
1137 Move ttMove, move, bestMove;
1138 Value ttValue, bestValue, value, evalMargin, futilityValue, futilityBase;
1139 bool inCheck, enoughMaterial, givesCheck, evasionPrunable;
1143 Value oldAlpha = alpha;
1145 ss->currentMove = bestMove = MOVE_NONE;
1146 ss->ply = (ss-1)->ply + 1;
1148 // Check for an instant draw or maximum ply reached
1149 if (pos.is_draw<true>() || ss->ply > MAX_PLY)
1152 // Decide whether or not to include checks, this fixes also the type of
1153 // TT entry depth that we are going to use. Note that in qsearch we use
1154 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1155 inCheck = pos.in_check();
1156 ttDepth = (inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS);
1158 // Transposition table lookup. At PV nodes, we don't use the TT for
1159 // pruning, but only for move ordering.
1160 tte = TT.probe(pos.key());
1161 ttMove = (tte ? tte->move() : MOVE_NONE);
1162 ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_ZERO;
1164 if (!PvNode && tte && can_return_tt(tte, ttDepth, ttValue, beta))
1166 ss->currentMove = ttMove; // Can be MOVE_NONE
1170 // Evaluate the position statically
1173 bestValue = futilityBase = -VALUE_INFINITE;
1174 ss->eval = evalMargin = VALUE_NONE;
1175 enoughMaterial = false;
1181 assert(tte->static_value() != VALUE_NONE);
1183 evalMargin = tte->static_value_margin();
1184 ss->eval = bestValue = tte->static_value();
1187 ss->eval = bestValue = evaluate(pos, evalMargin);
1189 // Stand pat. Return immediately if static value is at least beta
1190 if (bestValue >= beta)
1193 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER, DEPTH_NONE, MOVE_NONE, ss->eval, evalMargin);
1198 if (PvNode && bestValue > alpha)
1201 futilityBase = ss->eval + evalMargin + FutilityMarginQS;
1202 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1205 // Initialize a MovePicker object for the current position, and prepare
1206 // to search the moves. Because the depth is <= 0 here, only captures,
1207 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1209 MovePicker mp(pos, ttMove, depth, H, to_sq((ss-1)->currentMove));
1212 // Loop through the moves until no moves remain or a beta cutoff occurs
1213 while ( bestValue < beta
1214 && (move = mp.next_move()) != MOVE_NONE)
1216 assert(is_ok(move));
1218 givesCheck = pos.move_gives_check(move, ci);
1226 && !is_promotion(move)
1227 && !pos.is_passed_pawn_push(move))
1229 futilityValue = futilityBase
1230 + PieceValueEndgame[pos.piece_on(to_sq(move))]
1231 + (is_enpassant(move) ? PawnValueEndgame : VALUE_ZERO);
1233 if (futilityValue < beta)
1235 if (futilityValue > bestValue)
1236 bestValue = futilityValue;
1241 // Prune moves with negative or equal SEE
1242 if ( futilityBase < beta
1243 && depth < DEPTH_ZERO
1244 && pos.see(move) <= 0)
1248 // Detect non-capture evasions that are candidate to be pruned
1249 evasionPrunable = !PvNode
1251 && bestValue > VALUE_MATED_IN_MAX_PLY
1252 && !pos.is_capture(move)
1253 && !pos.can_castle(pos.side_to_move());
1255 // Don't search moves with negative SEE values
1257 && (!inCheck || evasionPrunable)
1259 && !is_promotion(move)
1260 && pos.see_sign(move) < 0)
1263 // Don't search useless checks
1268 && !pos.is_capture_or_promotion(move)
1269 && ss->eval + PawnValueMidgame / 4 < beta
1270 && !check_is_dangerous(pos, move, futilityBase, beta))
1273 // Check for legality only before to do the move
1274 if (!pos.pl_move_is_legal(move, ci.pinned))
1277 ss->currentMove = move;
1279 // Make and search the move
1280 pos.do_move(move, st, ci, givesCheck);
1281 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth-ONE_PLY);
1282 pos.undo_move(move);
1284 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1287 if (value > bestValue)
1294 && value < beta) // We want always alpha < beta
1299 // All legal moves have been searched. A special case: If we're in check
1300 // and no legal moves were found, it is checkmate.
1301 if (inCheck && bestValue == -VALUE_INFINITE)
1302 return mated_in(ss->ply); // Plies to mate from the root
1304 // Update transposition table
1305 move = bestValue <= oldAlpha ? MOVE_NONE : bestMove;
1306 bt = bestValue <= oldAlpha ? BOUND_UPPER
1307 : bestValue >= beta ? BOUND_LOWER : BOUND_EXACT;
1309 TT.store(pos.key(), value_to_tt(bestValue, ss->ply), bt, ttDepth, move, ss->eval, evalMargin);
1311 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1317 // check_is_dangerous() tests if a checking move can be pruned in qsearch().
1318 // bestValue is updated only when returning false because in that case move
1321 bool check_is_dangerous(Position &pos, Move move, Value futilityBase, Value beta)
1323 Bitboard b, occ, oldAtt, newAtt, kingAtt;
1324 Square from, to, ksq;
1328 from = from_sq(move);
1330 them = ~pos.side_to_move();
1331 ksq = pos.king_square(them);
1332 kingAtt = pos.attacks_from<KING>(ksq);
1333 pc = pos.piece_moved(move);
1335 occ = pos.pieces() ^ from ^ ksq;
1336 oldAtt = pos.attacks_from(pc, from, occ);
1337 newAtt = pos.attacks_from(pc, to, occ);
1339 // Rule 1. Checks which give opponent's king at most one escape square are dangerous
1340 b = kingAtt & ~pos.pieces(them) & ~newAtt & ~(1ULL << to);
1342 if (single_bit(b)) // Catches also !b
1345 // Rule 2. Queen contact check is very dangerous
1346 if (type_of(pc) == QUEEN && (kingAtt & to))
1349 // Rule 3. Creating new double threats with checks
1350 b = pos.pieces(them) & newAtt & ~oldAtt & ~(1ULL << ksq);
1353 // Note that here we generate illegal "double move"!
1354 if (futilityBase + PieceValueEndgame[pos.piece_on(pop_1st_bit(&b))] >= beta)
1362 // connected_moves() tests whether two moves are 'connected' in the sense
1363 // that the first move somehow made the second move possible (for instance
1364 // if the moving piece is the same in both moves). The first move is assumed
1365 // to be the move that was made to reach the current position, while the
1366 // second move is assumed to be a move from the current position.
1368 bool connected_moves(const Position& pos, Move m1, Move m2) {
1370 Square f1, t1, f2, t2;
1377 // Case 1: The moving piece is the same in both moves
1383 // Case 2: The destination square for m2 was vacated by m1
1389 // Case 3: Moving through the vacated square
1390 p2 = pos.piece_on(f2);
1391 if (piece_is_slider(p2) && (squares_between(f2, t2) & f1))
1394 // Case 4: The destination square for m2 is defended by the moving piece in m1
1395 p1 = pos.piece_on(t1);
1396 if (pos.attacks_from(p1, t1) & t2)
1399 // Case 5: Discovered check, checking piece is the piece moved in m1
1400 ksq = pos.king_square(pos.side_to_move());
1401 if ( piece_is_slider(p1)
1402 && (squares_between(t1, ksq) & f2)
1403 && (pos.attacks_from(p1, t1, pos.pieces() ^ f2) & ksq))
1410 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1411 // "plies to mate from the current position". Non-mate scores are unchanged.
1412 // The function is called before storing a value to the transposition table.
1414 Value value_to_tt(Value v, int ply) {
1416 if (v >= VALUE_MATE_IN_MAX_PLY)
1419 if (v <= VALUE_MATED_IN_MAX_PLY)
1426 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1427 // from the transposition table (where refers to the plies to mate/be mated
1428 // from current position) to "plies to mate/be mated from the root".
1430 Value value_from_tt(Value v, int ply) {
1432 if (v >= VALUE_MATE_IN_MAX_PLY)
1435 if (v <= VALUE_MATED_IN_MAX_PLY)
1442 // connected_threat() tests whether it is safe to forward prune a move or if
1443 // is somehow connected to the threat move returned by null search.
1445 bool connected_threat(const Position& pos, Move m, Move threat) {
1448 assert(is_ok(threat));
1449 assert(!pos.is_capture_or_promotion(m));
1450 assert(!pos.is_passed_pawn_push(m));
1452 Square mfrom, mto, tfrom, tto;
1456 tfrom = from_sq(threat);
1457 tto = to_sq(threat);
1459 // Case 1: Don't prune moves which move the threatened piece
1463 // Case 2: If the threatened piece has value less than or equal to the
1464 // value of the threatening piece, don't prune moves which defend it.
1465 if ( pos.is_capture(threat)
1466 && ( PieceValueMidgame[pos.piece_on(tfrom)] >= PieceValueMidgame[pos.piece_on(tto)]
1467 || type_of(pos.piece_on(tfrom)) == KING)
1468 && pos.move_attacks_square(m, tto))
1471 // Case 3: If the moving piece in the threatened move is a slider, don't
1472 // prune safe moves which block its ray.
1473 if ( piece_is_slider(pos.piece_on(tfrom))
1474 && (squares_between(tfrom, tto) & mto)
1475 && pos.see_sign(m) >= 0)
1482 // can_return_tt() returns true if a transposition table score can be used to
1483 // cut-off at a given point in search.
1485 bool can_return_tt(const TTEntry* tte, Depth depth, Value v, Value beta) {
1487 return ( tte->depth() >= depth
1488 || v >= std::max(VALUE_MATE_IN_MAX_PLY, beta)
1489 || v < std::min(VALUE_MATED_IN_MAX_PLY, beta))
1491 && ( ((tte->type() & BOUND_LOWER) && v >= beta)
1492 || ((tte->type() & BOUND_UPPER) && v < beta));
1496 // refine_eval() returns the transposition table score if possible, otherwise
1497 // falls back on static position evaluation.
1499 Value refine_eval(const TTEntry* tte, Value v, Value defaultEval) {
1503 if ( ((tte->type() & BOUND_LOWER) && v >= defaultEval)
1504 || ((tte->type() & BOUND_UPPER) && v < defaultEval))
1511 // score_to_uci() converts a value to a string suitable for use with the UCI
1512 // protocol specifications:
1514 // cp <x> The score from the engine's point of view in centipawns.
1515 // mate <y> Mate in y moves, not plies. If the engine is getting mated
1516 // use negative values for y.
1518 string score_to_uci(Value v, Value alpha, Value beta) {
1520 std::stringstream s;
1522 if (abs(v) < VALUE_MATE_IN_MAX_PLY)
1523 s << "cp " << v * 100 / int(PawnValueMidgame);
1525 s << "mate " << (v > 0 ? VALUE_MATE - v + 1 : -VALUE_MATE - v) / 2;
1527 s << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1533 // pv_info_to_uci() sends search info to GUI. UCI protocol requires to send all
1534 // the PV lines also if are still to be searched and so refer to the previous
1537 void pv_info_to_uci(const Position& pos, int depth, Value alpha, Value beta) {
1539 int t = SearchTime.elapsed();
1542 for (int i = 0; i < Threads.size(); i++)
1543 if (Threads[i].maxPly > selDepth)
1544 selDepth = Threads[i].maxPly;
1546 for (size_t i = 0; i < std::min(UCIMultiPV, RootMoves.size()); i++)
1548 bool updated = (i <= PVIdx);
1550 if (depth == 1 && !updated)
1553 int d = (updated ? depth : depth - 1);
1554 Value v = (updated ? RootMoves[i].score : RootMoves[i].prevScore);
1555 std::stringstream s;
1557 for (int j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
1558 s << " " << move_to_uci(RootMoves[i].pv[j], Chess960);
1560 cout << "info depth " << d
1561 << " seldepth " << selDepth
1562 << " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
1563 << " nodes " << pos.nodes_searched()
1564 << " nps " << (t > 0 ? pos.nodes_searched() * 1000 / t : 0)
1566 << " multipv " << i + 1
1567 << " pv" << s.str() << endl;
1572 // pv_info_to_log() writes human-readable search information to the log file
1573 // (which is created when the UCI parameter "Use Search Log" is "true"). It
1574 // uses the two below helpers to pretty format time and score respectively.
1576 string time_to_string(int millisecs) {
1578 const int MSecMinute = 1000 * 60;
1579 const int MSecHour = 1000 * 60 * 60;
1581 int hours = millisecs / MSecHour;
1582 int minutes = (millisecs % MSecHour) / MSecMinute;
1583 int seconds = ((millisecs % MSecHour) % MSecMinute) / 1000;
1585 std::stringstream s;
1590 s << std::setfill('0') << std::setw(2) << minutes << ':'
1591 << std::setw(2) << seconds;
1595 string score_to_string(Value v) {
1597 std::stringstream s;
1599 if (v >= VALUE_MATE_IN_MAX_PLY)
1600 s << "#" << (VALUE_MATE - v + 1) / 2;
1601 else if (v <= VALUE_MATED_IN_MAX_PLY)
1602 s << "-#" << (VALUE_MATE + v) / 2;
1604 s << std::setprecision(2) << std::fixed << std::showpos
1605 << float(v) / PawnValueMidgame;
1610 void pv_info_to_log(Position& pos, int depth, Value value, int time, Move pv[]) {
1612 const int64_t K = 1000;
1613 const int64_t M = 1000000;
1615 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1617 string san, padding;
1619 std::stringstream s;
1621 s << std::setw(2) << depth
1622 << std::setw(8) << score_to_string(value)
1623 << std::setw(8) << time_to_string(time);
1625 if (pos.nodes_searched() < M)
1626 s << std::setw(8) << pos.nodes_searched() / 1 << " ";
1628 else if (pos.nodes_searched() < K * M)
1629 s << std::setw(7) << pos.nodes_searched() / K << "K ";
1632 s << std::setw(7) << pos.nodes_searched() / M << "M ";
1634 padding = string(s.str().length(), ' ');
1635 length = padding.length();
1637 while (*m != MOVE_NONE)
1639 san = move_to_san(pos, *m);
1641 if (length + san.length() > 80)
1643 s << "\n" + padding;
1644 length = padding.length();
1648 length += san.length() + 1;
1650 pos.do_move(*m++, *st++);
1654 pos.undo_move(*--m);
1656 Log l(Options["Search Log Filename"]);
1657 l << s.str() << endl;
1661 // When playing with strength handicap choose best move among the MultiPV set
1662 // using a statistical rule dependent on SkillLevel. Idea by Heinz van Saanen.
1664 Move do_skill_level() {
1666 assert(MultiPV > 1);
1670 // PRNG sequence should be not deterministic
1671 for (int i = Time::current_time().msec() % 50; i > 0; i--)
1672 rk.rand<unsigned>();
1674 // RootMoves are already sorted by score in descending order
1675 size_t size = std::min(MultiPV, RootMoves.size());
1676 int variance = std::min(RootMoves[0].score - RootMoves[size - 1].score, PawnValueMidgame);
1677 int weakness = 120 - 2 * SkillLevel;
1678 int max_s = -VALUE_INFINITE;
1679 Move best = MOVE_NONE;
1681 // Choose best move. For each move score we add two terms both dependent on
1682 // weakness, one deterministic and bigger for weaker moves, and one random,
1683 // then we choose the move with the resulting highest score.
1684 for (size_t i = 0; i < size; i++)
1686 int s = RootMoves[i].score;
1688 // Don't allow crazy blunders even at very low skills
1689 if (i > 0 && RootMoves[i-1].score > s + EasyMoveMargin)
1692 // This is our magic formula
1693 s += ( weakness * int(RootMoves[0].score - s)
1694 + variance * (rk.rand<unsigned>() % weakness)) / 128;
1699 best = RootMoves[i].pv[0];
1708 /// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
1709 /// We consider also failing high nodes and not only BOUND_EXACT nodes so to
1710 /// allow to always have a ponder move even when we fail high at root, and a
1711 /// long PV to print that is important for position analysis.
1713 void RootMove::extract_pv_from_tt(Position& pos) {
1715 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1720 assert(m != MOVE_NONE && pos.is_pseudo_legal(m));
1724 pos.do_move(m, *st++);
1726 while ( (tte = TT.probe(pos.key())) != NULL
1727 && (m = tte->move()) != MOVE_NONE // Local copy, TT entry could change
1728 && pos.is_pseudo_legal(m)
1729 && pos.pl_move_is_legal(m, pos.pinned_pieces())
1731 && (!pos.is_draw<false>() || ply < 2))
1734 pos.do_move(m, *st++);
1737 pv.push_back(MOVE_NONE);
1739 do pos.undo_move(pv[--ply]); while (ply);
1743 /// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
1744 /// inserts the PV back into the TT. This makes sure the old PV moves are searched
1745 /// first, even if the old TT entries have been overwritten.
1747 void RootMove::insert_pv_in_tt(Position& pos) {
1749 StateInfo state[MAX_PLY_PLUS_2], *st = state;
1752 Value v, m = VALUE_NONE;
1755 assert(pv[ply] != MOVE_NONE && pos.is_pseudo_legal(pv[ply]));
1761 // Don't overwrite existing correct entries
1762 if (!tte || tte->move() != pv[ply])
1764 v = (pos.in_check() ? VALUE_NONE : evaluate(pos, m));
1765 TT.store(k, VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], v, m);
1767 pos.do_move(pv[ply], *st++);
1769 } while (pv[++ply] != MOVE_NONE);
1771 do pos.undo_move(pv[--ply]); while (ply);
1775 /// Thread::idle_loop() is where the thread is parked when it has no work to do.
1776 /// The parameter 'master_sp', if non-NULL, is a pointer to an active SplitPoint
1777 /// object for which the thread is the master.
1779 void Thread::idle_loop(SplitPoint* sp_master) {
1781 // If this thread is the master of a split point and all slaves have
1782 // finished their work at this split point, return from the idle loop.
1783 while (!sp_master || sp_master->slavesMask)
1785 // If we are not searching, wait for a condition to be signaled
1786 // instead of wasting CPU time polling for work.
1789 || (!is_searching && Threads.use_sleeping_threads()))
1797 // Grab the lock to avoid races with Thread::wake_up()
1798 lock_grab(sleepLock);
1800 // If we are master and all slaves have finished don't go to sleep
1801 if (sp_master && !sp_master->slavesMask)
1803 lock_release(sleepLock);
1807 // Do sleep after retesting sleep conditions under lock protection, in
1808 // particular we need to avoid a deadlock in case a master thread has,
1809 // in the meanwhile, allocated us and sent the wake_up() call before we
1810 // had the chance to grab the lock.
1811 if (do_sleep || !is_searching)
1812 cond_wait(sleepCond, sleepLock);
1814 lock_release(sleepLock);
1817 // If this thread has been assigned work, launch a search
1820 assert(!do_sleep && !do_exit);
1822 lock_grab(Threads.splitLock);
1824 assert(is_searching);
1825 SplitPoint* sp = curSplitPoint;
1827 lock_release(Threads.splitLock);
1829 Stack ss[MAX_PLY_PLUS_2];
1830 Position pos(*sp->pos, threadID);
1831 int master = sp->master;
1833 memcpy(ss, sp->ss - 1, 4 * sizeof(Stack));
1836 lock_grab(sp->lock);
1838 if (sp->nodeType == Root)
1839 search<SplitPointRoot>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1840 else if (sp->nodeType == PV)
1841 search<SplitPointPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1842 else if (sp->nodeType == NonPV)
1843 search<SplitPointNonPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
1847 assert(is_searching);
1849 is_searching = false;
1850 sp->slavesMask &= ~(1ULL << threadID);
1851 sp->nodes += pos.nodes_searched();
1853 // After releasing the lock we cannot access anymore any SplitPoint
1854 // related data in a reliably way becuase it could have been released
1855 // under our feet by the sp master.
1856 lock_release(sp->lock);
1858 // Wake up master thread so to allow it to return from the idle loop in
1859 // case we are the last slave of the split point.
1860 if ( Threads.use_sleeping_threads()
1861 && threadID != master
1862 && !Threads[master].is_searching)
1863 Threads[master].wake_up();
1869 /// check_time() is called by the timer thread when the timer triggers. It is
1870 /// used to print debug info and, more important, to detect when we are out of
1871 /// available time and so stop the search.
1875 static Time lastInfoTime = Time::current_time();
1877 if (lastInfoTime.elapsed() >= 1000)
1879 lastInfoTime.restart();
1886 int e = SearchTime.elapsed();
1887 bool stillAtFirstMove = Signals.firstRootMove
1888 && !Signals.failedLowAtRoot
1889 && e > TimeMgr.available_time();
1891 bool noMoreTime = e > TimeMgr.maximum_time() - 2 * TimerResolution
1892 || stillAtFirstMove;
1894 if ( (Limits.use_time_management() && noMoreTime)
1895 || (Limits.movetime && e >= Limits.movetime))
1896 Signals.stop = true;