2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2010 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/>.
43 #include "ucioption.h"
49 //// Local definitions
57 // ThreadsManager class is used to handle all the threads related stuff in search,
58 // init, starting, parking and, the most important, launching a slave thread at a
59 // split point are what this class does. All the access to shared thread data is
60 // done through this class, so that we avoid using global variables instead.
62 class ThreadsManager {
63 /* As long as the single ThreadsManager object is defined as a global we don't
64 need to explicitly initialize to zero its data members because variables with
65 static storage duration are automatically set to zero before enter main()
71 int active_threads() const { return ActiveThreads; }
72 void set_active_threads(int newActiveThreads) { ActiveThreads = newActiveThreads; }
73 void incrementNodeCounter(int threadID) { threads[threadID].nodes++; }
74 void incrementBetaCounter(Color us, Depth d, int threadID) { threads[threadID].betaCutOffs[us] += unsigned(d); }
76 void resetNodeCounters();
77 void resetBetaCounters();
78 int64_t nodes_searched() const;
79 void get_beta_counters(Color us, int64_t& our, int64_t& their) const;
80 bool available_thread_exists(int master) const;
81 bool thread_is_available(int slave, int master) const;
82 bool thread_should_stop(int threadID) const;
83 void wake_sleeping_threads();
84 void put_threads_to_sleep();
85 void idle_loop(int threadID, SplitPoint* waitSp);
86 bool split(const Position& pos, SearchStack* ss, int ply, Value* alpha, const Value beta, Value* bestValue,
87 Depth depth, bool mateThreat, int* moves, MovePicker* mp, int master, bool pvNode);
90 friend void poll(SearchStack ss[], int ply);
93 volatile bool AllThreadsShouldExit, AllThreadsShouldSleep;
94 Thread threads[MAX_THREADS];
95 SplitPoint SplitPointStack[MAX_THREADS][ACTIVE_SPLIT_POINTS_MAX];
97 Lock MPLock, WaitLock;
99 #if !defined(_MSC_VER)
100 pthread_cond_t WaitCond;
102 HANDLE SitIdleEvent[MAX_THREADS];
108 // RootMove struct is used for moves at the root at the tree. For each
109 // root move, we store a score, a node count, and a PV (really a refutation
110 // in the case of moves which fail low).
114 RootMove() { nodes = cumulativeNodes = ourBeta = theirBeta = 0ULL; }
116 // RootMove::operator<() is the comparison function used when
117 // sorting the moves. A move m1 is considered to be better
118 // than a move m2 if it has a higher score, or if the moves
119 // have equal score but m1 has the higher node count.
120 bool operator<(const RootMove& m) const {
122 return score != m.score ? score < m.score : theirBeta <= m.theirBeta;
127 int64_t nodes, cumulativeNodes, ourBeta, theirBeta;
128 Move pv[PLY_MAX_PLUS_2];
132 // The RootMoveList class is essentially an array of RootMove objects, with
133 // a handful of methods for accessing the data in the individual moves.
138 RootMoveList(Position& pos, Move searchMoves[]);
140 int move_count() const { return count; }
141 Move get_move(int moveNum) const { return moves[moveNum].move; }
142 Value get_move_score(int moveNum) const { return moves[moveNum].score; }
143 void set_move_score(int moveNum, Value score) { moves[moveNum].score = score; }
144 Move get_move_pv(int moveNum, int i) const { return moves[moveNum].pv[i]; }
145 int64_t get_move_cumulative_nodes(int moveNum) const { return moves[moveNum].cumulativeNodes; }
147 void set_move_nodes(int moveNum, int64_t nodes);
148 void set_beta_counters(int moveNum, int64_t our, int64_t their);
149 void set_move_pv(int moveNum, const Move pv[]);
151 void sort_multipv(int n);
154 static const int MaxRootMoves = 500;
155 RootMove moves[MaxRootMoves];
164 // Maximum depth for razoring
165 const Depth RazorDepth = 4 * OnePly;
167 // Dynamic razoring margin based on depth
168 inline Value razor_margin(Depth d) { return Value(0x200 + 0x10 * int(d)); }
170 // Step 8. Null move search with verification search
172 // Null move margin. A null move search will not be done if the static
173 // evaluation of the position is more than NullMoveMargin below beta.
174 const Value NullMoveMargin = Value(0x200);
176 // Maximum depth for use of dynamic threat detection when null move fails low
177 const Depth ThreatDepth = 5 * OnePly;
179 // Step 9. Internal iterative deepening
181 // Minimum depth for use of internal iterative deepening
182 const Depth IIDDepthAtPVNodes = 5 * OnePly;
183 const Depth IIDDepthAtNonPVNodes = 8 * OnePly;
185 // At Non-PV nodes we do an internal iterative deepening search
186 // when the static evaluation is at most IIDMargin below beta.
187 const Value IIDMargin = Value(0x100);
189 // Step 11. Decide the new search depth
191 // Extensions. Configurable UCI options
192 // Array index 0 is used at non-PV nodes, index 1 at PV nodes.
193 Depth CheckExtension[2], SingleEvasionExtension[2], PawnPushTo7thExtension[2];
194 Depth PassedPawnExtension[2], PawnEndgameExtension[2], MateThreatExtension[2];
196 // Minimum depth for use of singular extension
197 const Depth SingularExtensionDepthAtPVNodes = 6 * OnePly;
198 const Depth SingularExtensionDepthAtNonPVNodes = 8 * OnePly;
200 // If the TT move is at least SingularExtensionMargin better then the
201 // remaining ones we will extend it.
202 const Value SingularExtensionMargin = Value(0x20);
204 // Step 12. Futility pruning
206 // Futility margin for quiescence search
207 const Value FutilityMarginQS = Value(0x80);
209 // Futility lookup tables (initialized at startup) and their getter functions
210 int32_t FutilityMarginsMatrix[16][64]; // [depth][moveNumber]
211 int FutilityMoveCountArray[32]; // [depth]
213 inline Value futility_margin(Depth d, int mn) { return Value(d < 7 * OnePly ? FutilityMarginsMatrix[Max(d, 0)][Min(mn, 63)] : 2 * VALUE_INFINITE); }
214 inline int futility_move_count(Depth d) { return d < 16 * OnePly ? FutilityMoveCountArray[d] : 512; }
216 // Step 14. Reduced search
218 int ReductionLevel; // 0 = most aggressive reductions, 7 = minimum reductions
220 // Reduction lookup tables (initialized at startup) and their getter functions
221 int8_t PVReductionMatrix[8][64][64]; // [depth][moveNumber]
222 int8_t NonPVReductionMatrix[8][64][64]; // [depth][moveNumber]
224 inline Depth pv_reduction(Depth d, int mn) { return (Depth) PVReductionMatrix[ReductionLevel][Min(d / 2, 63)][Min(mn, 63)]; }
225 inline Depth nonpv_reduction(Depth d, int mn) { return (Depth) NonPVReductionMatrix[ReductionLevel][Min(d / 2, 63)][Min(mn, 63)]; }
227 // Common adjustments
229 // Search depth at iteration 1
230 const Depth InitialDepth = OnePly;
232 // Easy move margin. An easy move candidate must be at least this much
233 // better than the second best move.
234 const Value EasyMoveMargin = Value(0x200);
236 // Last seconds noise filtering (LSN)
237 const bool UseLSNFiltering = true;
238 const int LSNTime = 4000; // In milliseconds
239 const Value LSNValue = value_from_centipawns(200);
240 bool loseOnTime = false;
248 // Scores and number of times the best move changed for each iteration
249 Value ValueByIteration[PLY_MAX_PLUS_2];
250 int BestMoveChangesByIteration[PLY_MAX_PLUS_2];
252 // Search window management
258 // Time managment variables
259 int SearchStartTime, MaxNodes, MaxDepth, MaxSearchTime;
260 int AbsoluteMaxSearchTime, ExtraSearchTime, ExactMaxTime;
261 bool UseTimeManagement, InfiniteSearch, PonderSearch, StopOnPonderhit;
262 bool FirstRootMove, AbortSearch, Quit, AspirationFailLow;
264 // Show current line?
265 bool ShowCurrentLine;
269 std::ofstream LogFile;
271 // Multi-threads related variables
272 Depth MinimumSplitDepth;
273 int MaxThreadsPerSplitPoint;
276 // Node counters, used only by thread[0] but try to keep in different cache
277 // lines (64 bytes each) from the heavy multi-thread read accessed variables.
279 int NodesBetweenPolls = 30000;
286 Value id_loop(const Position& pos, Move searchMoves[]);
287 Value root_search(Position& pos, SearchStack ss[], RootMoveList& rml, Value* alphaPtr, Value* betaPtr);
288 Value search_pv(Position& pos, SearchStack ss[], Value alpha, Value beta, Depth depth, int ply, int threadID);
289 Value search(Position& pos, SearchStack ss[], Value beta, Depth depth, int ply, bool allowNullmove, int threadID, Move excludedMove = MOVE_NONE);
290 Value qsearch(Position& pos, SearchStack ss[], Value alpha, Value beta, Depth depth, int ply, int threadID);
291 void sp_search(SplitPoint* sp, int threadID);
292 void sp_search_pv(SplitPoint* sp, int threadID);
293 void init_node(SearchStack ss[], int ply, int threadID);
294 void update_pv(SearchStack ss[], int ply);
295 void sp_update_pv(SearchStack* pss, SearchStack ss[], int ply);
296 bool connected_moves(const Position& pos, Move m1, Move m2);
297 bool value_is_mate(Value value);
298 bool move_is_killer(Move m, const SearchStack& ss);
299 Depth extension(const Position&, Move, bool, bool, bool, bool, bool, bool*);
300 bool ok_to_do_nullmove(const Position& pos);
301 bool ok_to_prune(const Position& pos, Move m, Move threat);
302 bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply);
303 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
304 void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount);
305 void update_killers(Move m, SearchStack& ss);
306 void update_gains(const Position& pos, Move move, Value before, Value after);
308 int current_search_time();
310 void poll(SearchStack ss[], int ply);
312 void wait_for_stop_or_ponderhit();
313 void init_ss_array(SearchStack ss[]);
314 void print_pv_info(const Position& pos, SearchStack ss[], Value alpha, Value beta, Value value);
316 #if !defined(_MSC_VER)
317 void *init_thread(void *threadID);
319 DWORD WINAPI init_thread(LPVOID threadID);
329 /// init_threads(), exit_threads() and nodes_searched() are helpers to
330 /// give accessibility to some TM methods from outside of current file.
332 void init_threads() { TM.init_threads(); }
333 void exit_threads() { TM.exit_threads(); }
334 int64_t nodes_searched() { return TM.nodes_searched(); }
337 /// perft() is our utility to verify move generation is bug free. All the legal
338 /// moves up to given depth are generated and counted and the sum returned.
340 int perft(Position& pos, Depth depth)
345 MovePicker mp(pos, MOVE_NONE, depth, H);
347 // If we are at the last ply we don't need to do and undo
348 // the moves, just to count them.
349 if (depth <= OnePly) // Replace with '<' to test also qsearch
351 while (mp.get_next_move()) sum++;
355 // Loop through all legal moves
357 while ((move = mp.get_next_move()) != MOVE_NONE)
359 pos.do_move(move, st, ci, pos.move_is_check(move, ci));
360 sum += perft(pos, depth - OnePly);
367 /// think() is the external interface to Stockfish's search, and is called when
368 /// the program receives the UCI 'go' command. It initializes various
369 /// search-related global variables, and calls root_search(). It returns false
370 /// when a quit command is received during the search.
372 bool think(const Position& pos, bool infinite, bool ponder, int side_to_move,
373 int time[], int increment[], int movesToGo, int maxDepth,
374 int maxNodes, int maxTime, Move searchMoves[]) {
376 // Initialize global search variables
377 StopOnPonderhit = AbortSearch = Quit = AspirationFailLow = false;
378 MaxSearchTime = AbsoluteMaxSearchTime = ExtraSearchTime = 0;
380 TM.resetNodeCounters();
381 SearchStartTime = get_system_time();
382 ExactMaxTime = maxTime;
385 InfiniteSearch = infinite;
386 PonderSearch = ponder;
387 UseTimeManagement = !ExactMaxTime && !MaxDepth && !MaxNodes && !InfiniteSearch;
389 // Look for a book move, only during games, not tests
390 if (UseTimeManagement && get_option_value_bool("OwnBook"))
392 if (get_option_value_string("Book File") != OpeningBook.file_name())
393 OpeningBook.open(get_option_value_string("Book File"));
395 Move bookMove = OpeningBook.get_move(pos);
396 if (bookMove != MOVE_NONE)
399 wait_for_stop_or_ponderhit();
401 cout << "bestmove " << bookMove << endl;
406 // Reset loseOnTime flag at the beginning of a new game
407 if (button_was_pressed("New Game"))
410 // Read UCI option values
411 TT.set_size(get_option_value_int("Hash"));
412 if (button_was_pressed("Clear Hash"))
415 CheckExtension[1] = Depth(get_option_value_int("Check Extension (PV nodes)"));
416 CheckExtension[0] = Depth(get_option_value_int("Check Extension (non-PV nodes)"));
417 SingleEvasionExtension[1] = Depth(get_option_value_int("Single Evasion Extension (PV nodes)"));
418 SingleEvasionExtension[0] = Depth(get_option_value_int("Single Evasion Extension (non-PV nodes)"));
419 PawnPushTo7thExtension[1] = Depth(get_option_value_int("Pawn Push to 7th Extension (PV nodes)"));
420 PawnPushTo7thExtension[0] = Depth(get_option_value_int("Pawn Push to 7th Extension (non-PV nodes)"));
421 PassedPawnExtension[1] = Depth(get_option_value_int("Passed Pawn Extension (PV nodes)"));
422 PassedPawnExtension[0] = Depth(get_option_value_int("Passed Pawn Extension (non-PV nodes)"));
423 PawnEndgameExtension[1] = Depth(get_option_value_int("Pawn Endgame Extension (PV nodes)"));
424 PawnEndgameExtension[0] = Depth(get_option_value_int("Pawn Endgame Extension (non-PV nodes)"));
425 MateThreatExtension[1] = Depth(get_option_value_int("Mate Threat Extension (PV nodes)"));
426 MateThreatExtension[0] = Depth(get_option_value_int("Mate Threat Extension (non-PV nodes)"));
428 MinimumSplitDepth = get_option_value_int("Minimum Split Depth") * OnePly;
429 MaxThreadsPerSplitPoint = get_option_value_int("Maximum Number of Threads per Split Point");
430 ShowCurrentLine = get_option_value_bool("UCI_ShowCurrLine");
431 MultiPV = get_option_value_int("MultiPV");
432 Chess960 = get_option_value_bool("UCI_Chess960");
433 UseLogFile = get_option_value_bool("Use Search Log");
436 LogFile.open(get_option_value_string("Search Log Filename").c_str(), std::ios::out | std::ios::app);
438 read_weights(pos.side_to_move());
440 // Set the number of active threads
441 int newActiveThreads = get_option_value_int("Threads");
442 if (newActiveThreads != TM.active_threads())
444 TM.set_active_threads(newActiveThreads);
445 init_eval(TM.active_threads());
448 // Wake up sleeping threads
449 TM.wake_sleeping_threads();
452 int myTime = time[side_to_move];
453 int myIncrement = increment[side_to_move];
454 if (UseTimeManagement)
456 if (!movesToGo) // Sudden death time control
460 MaxSearchTime = myTime / 30 + myIncrement;
461 AbsoluteMaxSearchTime = Max(myTime / 4, myIncrement - 100);
463 else // Blitz game without increment
465 MaxSearchTime = myTime / 30;
466 AbsoluteMaxSearchTime = myTime / 8;
469 else // (x moves) / (y minutes)
473 MaxSearchTime = myTime / 2;
474 AbsoluteMaxSearchTime = (myTime > 3000)? (myTime - 500) : ((myTime * 3) / 4);
478 MaxSearchTime = myTime / Min(movesToGo, 20);
479 AbsoluteMaxSearchTime = Min((4 * myTime) / movesToGo, myTime / 3);
483 if (get_option_value_bool("Ponder"))
485 MaxSearchTime += MaxSearchTime / 4;
486 MaxSearchTime = Min(MaxSearchTime, AbsoluteMaxSearchTime);
490 // Set best NodesBetweenPolls interval to avoid lagging under
491 // heavy time pressure.
493 NodesBetweenPolls = Min(MaxNodes, 30000);
494 else if (myTime && myTime < 1000)
495 NodesBetweenPolls = 1000;
496 else if (myTime && myTime < 5000)
497 NodesBetweenPolls = 5000;
499 NodesBetweenPolls = 30000;
501 // Write search information to log file
503 LogFile << "Searching: " << pos.to_fen() << endl
504 << "infinite: " << infinite
505 << " ponder: " << ponder
506 << " time: " << myTime
507 << " increment: " << myIncrement
508 << " moves to go: " << movesToGo << endl;
510 // LSN filtering. Used only for developing purposes, disabled by default
514 // Step 2. If after last move we decided to lose on time, do it now!
515 while (SearchStartTime + myTime + 1000 > get_system_time())
519 // We're ready to start thinking. Call the iterative deepening loop function
520 Value v = id_loop(pos, searchMoves);
524 // Step 1. If this is sudden death game and our position is hopeless,
525 // decide to lose on time.
526 if ( !loseOnTime // If we already lost on time, go to step 3.
536 // Step 3. Now after stepping over the time limit, reset flag for next match.
544 TM.put_threads_to_sleep();
549 // init_reduction_tables() is called by init_search() and initializes
550 // the tables used by LMR.
551 static void init_reduction_tables(int8_t pvTable[64][64], int8_t nonPvTable[64][64], int pvInhib, int nonPvInhib)
553 double pvBase = 1.001 - log(3.0) * log(16.0) / pvInhib;
554 double nonPvBase = 1.001 - log(3.0) * log(4.0) / nonPvInhib;
556 for (int i = 1; i < 64; i++) // i == depth (OnePly = 1)
557 for (int j = 1; j < 64; j++) // j == moveNumber
559 double pvRed = pvBase + log(double(i)) * log(double(j)) / pvInhib;
560 double nonPVRed = nonPvBase + log(double(i)) * log(double(j)) / nonPvInhib;
562 pvTable[i][j] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(OnePly)) : 0);
563 nonPvTable[i][j] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(OnePly)) : 0);
567 // init_search() is called during startup. It initializes various lookup tables
570 // Init reduction lookup tables
571 for (int i = 0; i < 8; i++)
572 init_reduction_tables(PVReductionMatrix[i], NonPVReductionMatrix[i], int(4 * pow(1.3, i)), int(2 * pow(1.3, i)));
574 // Init futility margins array
575 for (int i = 0; i < 16; i++) // i == depth (OnePly = 2)
576 for (int j = 0; j < 64; j++) // j == moveNumber
578 // FIXME: test using log instead of BSR
579 FutilityMarginsMatrix[i][j] = (i < 2 ? 0 : 112 * bitScanReverse32(i * i / 2)) - 8 * j;
582 // Init futility move count array
583 for (int i = 0; i < 32; i++) // i == depth (OnePly = 2)
584 FutilityMoveCountArray[i] = 3 + (1 << (3 * i / 8));
588 // SearchStack::init() initializes a search stack. Used at the beginning of a
589 // new search from the root.
590 void SearchStack::init(int ply) {
592 pv[ply] = pv[ply + 1] = MOVE_NONE;
593 currentMove = threatMove = MOVE_NONE;
594 reduction = Depth(0);
598 void SearchStack::initKillers() {
600 mateKiller = MOVE_NONE;
601 for (int i = 0; i < KILLER_MAX; i++)
602 killers[i] = MOVE_NONE;
607 // id_loop() is the main iterative deepening loop. It calls root_search
608 // repeatedly with increasing depth until the allocated thinking time has
609 // been consumed, the user stops the search, or the maximum search depth is
612 Value id_loop(const Position& pos, Move searchMoves[]) {
615 SearchStack ss[PLY_MAX_PLUS_2];
616 Move EasyMove = MOVE_NONE;
617 Value value, alpha = -VALUE_INFINITE, beta = VALUE_INFINITE;
619 // Moves to search are verified, copied, scored and sorted
620 RootMoveList rml(p, searchMoves);
622 // Handle special case of searching on a mate/stale position
623 if (rml.move_count() == 0)
626 wait_for_stop_or_ponderhit();
628 return pos.is_check() ? -VALUE_MATE : VALUE_DRAW;
631 // Print RootMoveList startup scoring to the standard output,
632 // so to output information also for iteration 1.
633 cout << "info depth " << 1
634 << "\ninfo depth " << 1
635 << " score " << value_to_string(rml.get_move_score(0))
636 << " time " << current_search_time()
637 << " nodes " << TM.nodes_searched()
639 << " pv " << rml.get_move(0) << "\n";
645 ValueByIteration[1] = rml.get_move_score(0);
648 // Is one move significantly better than others after initial scoring ?
649 if ( rml.move_count() == 1
650 || rml.get_move_score(0) > rml.get_move_score(1) + EasyMoveMargin)
651 EasyMove = rml.get_move(0);
653 // Iterative deepening loop
654 while (Iteration < PLY_MAX)
656 // Initialize iteration
658 BestMoveChangesByIteration[Iteration] = 0;
660 cout << "info depth " << Iteration << endl;
662 // Calculate dynamic aspiration window based on previous iterations
663 if (MultiPV == 1 && Iteration >= 6 && abs(ValueByIteration[Iteration - 1]) < VALUE_KNOWN_WIN)
665 int prevDelta1 = ValueByIteration[Iteration - 1] - ValueByIteration[Iteration - 2];
666 int prevDelta2 = ValueByIteration[Iteration - 2] - ValueByIteration[Iteration - 3];
668 AspirationDelta = Max(abs(prevDelta1) + abs(prevDelta2) / 2, 16);
669 AspirationDelta = (AspirationDelta + 7) / 8 * 8; // Round to match grainSize
671 alpha = Max(ValueByIteration[Iteration - 1] - AspirationDelta, -VALUE_INFINITE);
672 beta = Min(ValueByIteration[Iteration - 1] + AspirationDelta, VALUE_INFINITE);
675 // Choose optimum reduction level
678 if (UseTimeManagement)
680 int level = int(floor(log(float(MaxSearchTime) / current_search_time()) / log(2.0) + 1.0));
681 ReductionLevel = Min(Max(level, 0), 7);
688 // Search to the current depth, rml is updated and sorted, alpha and beta could change
689 value = root_search(p, ss, rml, &alpha, &beta);
691 // Write PV to transposition table, in case the relevant entries have
692 // been overwritten during the search.
693 TT.insert_pv(p, ss[0].pv);
696 break; // Value cannot be trusted. Break out immediately!
698 //Save info about search result
699 ValueByIteration[Iteration] = value;
701 // Drop the easy move if differs from the new best move
702 if (ss[0].pv[0] != EasyMove)
703 EasyMove = MOVE_NONE;
705 if (UseTimeManagement)
708 bool stopSearch = false;
710 // Stop search early if there is only a single legal move,
711 // we search up to Iteration 6 anyway to get a proper score.
712 if (Iteration >= 6 && rml.move_count() == 1)
715 // Stop search early when the last two iterations returned a mate score
717 && abs(ValueByIteration[Iteration]) >= abs(VALUE_MATE) - 100
718 && abs(ValueByIteration[Iteration-1]) >= abs(VALUE_MATE) - 100)
721 // Stop search early if one move seems to be much better than the others
722 int64_t nodes = TM.nodes_searched();
724 && EasyMove == ss[0].pv[0]
725 && ( ( rml.get_move_cumulative_nodes(0) > (nodes * 85) / 100
726 && current_search_time() > MaxSearchTime / 16)
727 ||( rml.get_move_cumulative_nodes(0) > (nodes * 98) / 100
728 && current_search_time() > MaxSearchTime / 32)))
731 // Add some extra time if the best move has changed during the last two iterations
732 if (Iteration > 5 && Iteration <= 50)
733 ExtraSearchTime = BestMoveChangesByIteration[Iteration] * (MaxSearchTime / 2)
734 + BestMoveChangesByIteration[Iteration-1] * (MaxSearchTime / 3);
736 // Stop search if most of MaxSearchTime is consumed at the end of the
737 // iteration. We probably don't have enough time to search the first
738 // move at the next iteration anyway.
739 if (current_search_time() > ((MaxSearchTime + ExtraSearchTime) * 80) / 128)
745 StopOnPonderhit = true;
751 if (MaxDepth && Iteration >= MaxDepth)
755 // If we are pondering or in infinite search, we shouldn't print the
756 // best move before we are told to do so.
757 if (!AbortSearch && (PonderSearch || InfiniteSearch))
758 wait_for_stop_or_ponderhit();
760 // Print final search statistics
761 cout << "info nodes " << TM.nodes_searched()
763 << " time " << current_search_time()
764 << " hashfull " << TT.full() << endl;
766 // Print the best move and the ponder move to the standard output
767 if (ss[0].pv[0] == MOVE_NONE)
769 ss[0].pv[0] = rml.get_move(0);
770 ss[0].pv[1] = MOVE_NONE;
773 assert(ss[0].pv[0] != MOVE_NONE);
775 cout << "bestmove " << ss[0].pv[0];
777 if (ss[0].pv[1] != MOVE_NONE)
778 cout << " ponder " << ss[0].pv[1];
785 dbg_print_mean(LogFile);
787 if (dbg_show_hit_rate)
788 dbg_print_hit_rate(LogFile);
790 LogFile << "\nNodes: " << TM.nodes_searched()
791 << "\nNodes/second: " << nps()
792 << "\nBest move: " << move_to_san(p, ss[0].pv[0]);
795 p.do_move(ss[0].pv[0], st);
796 LogFile << "\nPonder move: "
797 << move_to_san(p, ss[0].pv[1]) // Works also with MOVE_NONE
800 return rml.get_move_score(0);
804 // root_search() is the function which searches the root node. It is
805 // similar to search_pv except that it uses a different move ordering
806 // scheme, prints some information to the standard output and handles
807 // the fail low/high loops.
809 Value root_search(Position& pos, SearchStack ss[], RootMoveList& rml, Value* alphaPtr, Value* betaPtr) {
816 Depth depth, ext, newDepth;
817 Value value, alpha, beta;
818 bool isCheck, moveIsCheck, captureOrPromotion, dangerous;
819 int researchCountFH, researchCountFL;
821 researchCountFH = researchCountFL = 0;
824 isCheck = pos.is_check();
826 // Step 1. Initialize node and poll (omitted at root, but I can see no good reason for this, FIXME)
827 // Step 2. Check for aborted search (omitted at root, because we do not initialize root node)
828 // Step 3. Mate distance pruning (omitted at root)
829 // Step 4. Transposition table lookup (omitted at root)
831 // Step 5. Evaluate the position statically
832 // At root we do this only to get reference value for child nodes
834 ss[0].eval = evaluate(pos, ei, 0);
836 ss[0].eval = VALUE_NONE; // HACK because we do not initialize root node
838 // Step 6. Razoring (omitted at root)
839 // Step 7. Static null move pruning (omitted at root)
840 // Step 8. Null move search with verification search (omitted at root)
841 // Step 9. Internal iterative deepening (omitted at root)
843 // Step extra. Fail low loop
844 // We start with small aspiration window and in case of fail low, we research
845 // with bigger window until we are not failing low anymore.
848 // Sort the moves before to (re)search
851 // Step 10. Loop through all moves in the root move list
852 for (int i = 0; i < rml.move_count() && !AbortSearch; i++)
854 // This is used by time management
855 FirstRootMove = (i == 0);
857 // Save the current node count before the move is searched
858 nodes = TM.nodes_searched();
860 // Reset beta cut-off counters
861 TM.resetBetaCounters();
863 // Pick the next root move, and print the move and the move number to
864 // the standard output.
865 move = ss[0].currentMove = rml.get_move(i);
867 if (current_search_time() >= 1000)
868 cout << "info currmove " << move
869 << " currmovenumber " << i + 1 << endl;
871 moveIsCheck = pos.move_is_check(move);
872 captureOrPromotion = pos.move_is_capture_or_promotion(move);
874 // Step 11. Decide the new search depth
875 depth = (Iteration - 2) * OnePly + InitialDepth;
876 ext = extension(pos, move, true, captureOrPromotion, moveIsCheck, false, false, &dangerous);
877 newDepth = depth + ext;
879 // Step 12. Futility pruning (omitted at root)
881 // Step extra. Fail high loop
882 // If move fails high, we research with bigger window until we are not failing
884 value = - VALUE_INFINITE;
888 // Step 13. Make the move
889 pos.do_move(move, st, ci, moveIsCheck);
891 // Step extra. pv search
892 // We do pv search for first moves (i < MultiPV)
893 // and for fail high research (value > alpha)
894 if (i < MultiPV || value > alpha)
896 // Aspiration window is disabled in multi-pv case
898 alpha = -VALUE_INFINITE;
900 // Full depth PV search, done on first move or after a fail high
901 value = -search_pv(pos, ss, -beta, -alpha, newDepth, 1, 0);
905 // Step 14. Reduced search
906 // if the move fails high will be re-searched at full depth
907 bool doFullDepthSearch = true;
909 if ( depth >= 3 * OnePly
911 && !captureOrPromotion
912 && !move_is_castle(move))
914 ss[0].reduction = pv_reduction(depth, i - MultiPV + 2);
917 // Reduced depth non-pv search using alpha as upperbound
918 value = -search(pos, ss, -alpha, newDepth-ss[0].reduction, 1, true, 0);
919 doFullDepthSearch = (value > alpha);
923 // Step 15. Full depth search
924 if (doFullDepthSearch)
926 // Full depth non-pv search using alpha as upperbound
927 ss[0].reduction = Depth(0);
928 value = -search(pos, ss, -alpha, newDepth, 1, true, 0);
930 // If we are above alpha then research at same depth but as PV
931 // to get a correct score or eventually a fail high above beta.
933 value = -search_pv(pos, ss, -beta, -alpha, newDepth, 1, 0);
937 // Step 16. Undo move
940 // Can we exit fail high loop ?
941 if (AbortSearch || value < beta)
944 // We are failing high and going to do a research. It's important to update
945 // the score before research in case we run out of time while researching.
946 rml.set_move_score(i, value);
948 TT.extract_pv(pos, ss[0].pv, PLY_MAX);
949 rml.set_move_pv(i, ss[0].pv);
951 // Print information to the standard output
952 print_pv_info(pos, ss, alpha, beta, value);
954 // Prepare for a research after a fail high, each time with a wider window
955 *betaPtr = beta = Min(beta + AspirationDelta * (1 << researchCountFH), VALUE_INFINITE);
958 } // End of fail high loop
960 // Finished searching the move. If AbortSearch is true, the search
961 // was aborted because the user interrupted the search or because we
962 // ran out of time. In this case, the return value of the search cannot
963 // be trusted, and we break out of the loop without updating the best
968 // Remember beta-cutoff and searched nodes counts for this move. The
969 // info is used to sort the root moves for the next iteration.
971 TM.get_beta_counters(pos.side_to_move(), our, their);
972 rml.set_beta_counters(i, our, their);
973 rml.set_move_nodes(i, TM.nodes_searched() - nodes);
975 assert(value >= -VALUE_INFINITE && value <= VALUE_INFINITE);
976 assert(value < beta);
978 // Step 17. Check for new best move
979 if (value <= alpha && i >= MultiPV)
980 rml.set_move_score(i, -VALUE_INFINITE);
983 // PV move or new best move!
986 rml.set_move_score(i, value);
988 TT.extract_pv(pos, ss[0].pv, PLY_MAX);
989 rml.set_move_pv(i, ss[0].pv);
993 // We record how often the best move has been changed in each
994 // iteration. This information is used for time managment: When
995 // the best move changes frequently, we allocate some more time.
997 BestMoveChangesByIteration[Iteration]++;
999 // Print information to the standard output
1000 print_pv_info(pos, ss, alpha, beta, value);
1002 // Raise alpha to setup proper non-pv search upper bound
1008 rml.sort_multipv(i);
1009 for (int j = 0; j < Min(MultiPV, rml.move_count()); j++)
1011 cout << "info multipv " << j + 1
1012 << " score " << value_to_string(rml.get_move_score(j))
1013 << " depth " << (j <= i ? Iteration : Iteration - 1)
1014 << " time " << current_search_time()
1015 << " nodes " << TM.nodes_searched()
1019 for (int k = 0; rml.get_move_pv(j, k) != MOVE_NONE && k < PLY_MAX; k++)
1020 cout << rml.get_move_pv(j, k) << " ";
1024 alpha = rml.get_move_score(Min(i, MultiPV - 1));
1026 } // PV move or new best move
1028 assert(alpha >= *alphaPtr);
1030 AspirationFailLow = (alpha == *alphaPtr);
1032 if (AspirationFailLow && StopOnPonderhit)
1033 StopOnPonderhit = false;
1036 // Can we exit fail low loop ?
1037 if (AbortSearch || !AspirationFailLow)
1040 // Prepare for a research after a fail low, each time with a wider window
1041 *alphaPtr = alpha = Max(alpha - AspirationDelta * (1 << researchCountFL), -VALUE_INFINITE);
1046 // Sort the moves before to return
1053 // search_pv() is the main search function for PV nodes.
1055 Value search_pv(Position& pos, SearchStack ss[], Value alpha, Value beta,
1056 Depth depth, int ply, int threadID) {
1058 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1059 assert(beta > alpha && beta <= VALUE_INFINITE);
1060 assert(ply >= 0 && ply < PLY_MAX);
1061 assert(threadID >= 0 && threadID < TM.active_threads());
1063 Move movesSearched[256];
1068 Depth ext, newDepth;
1069 Value bestValue, value, oldAlpha;
1070 bool isCheck, singleEvasion, moveIsCheck, captureOrPromotion, dangerous;
1071 bool mateThreat = false;
1073 bestValue = value = -VALUE_INFINITE;
1076 return qsearch(pos, ss, alpha, beta, Depth(0), ply, threadID);
1078 // Step 1. Initialize node and poll
1079 // Polling can abort search.
1080 init_node(ss, ply, threadID);
1082 // Step 2. Check for aborted search and immediate draw
1083 if (AbortSearch || TM.thread_should_stop(threadID))
1086 if (pos.is_draw() || ply >= PLY_MAX - 1)
1089 // Step 3. Mate distance pruning
1091 alpha = Max(value_mated_in(ply), alpha);
1092 beta = Min(value_mate_in(ply+1), beta);
1096 // Step 4. Transposition table lookup
1097 // At PV nodes, we don't use the TT for pruning, but only for move ordering.
1098 // This is to avoid problems in the following areas:
1100 // * Repetition draw detection
1101 // * Fifty move rule detection
1102 // * Searching for a mate
1103 // * Printing of full PV line
1104 tte = TT.retrieve(pos.get_key());
1105 ttMove = (tte ? tte->move() : MOVE_NONE);
1107 // Step 5. Evaluate the position statically
1108 // At PV nodes we do this only to update gain statistics
1109 isCheck = pos.is_check();
1112 ss[ply].eval = evaluate(pos, ei, threadID);
1113 update_gains(pos, ss[ply - 1].currentMove, ss[ply - 1].eval, ss[ply].eval);
1116 // Step 6. Razoring (is omitted in PV nodes)
1117 // Step 7. Static null move pruning (is omitted in PV nodes)
1118 // Step 8. Null move search with verification search (is omitted in PV nodes)
1120 // Step 9. Internal iterative deepening
1121 if ( depth >= IIDDepthAtPVNodes
1122 && ttMove == MOVE_NONE)
1124 search_pv(pos, ss, alpha, beta, depth-2*OnePly, ply, threadID);
1125 ttMove = ss[ply].pv[ply];
1126 tte = TT.retrieve(pos.get_key());
1129 // Initialize a MovePicker object for the current position
1130 mateThreat = pos.has_mate_threat(opposite_color(pos.side_to_move()));
1131 MovePicker mp = MovePicker(pos, ttMove, depth, H, &ss[ply]);
1134 // Step 10. Loop through moves
1135 // Loop through all legal moves until no moves remain or a beta cutoff occurs
1136 while ( alpha < beta
1137 && (move = mp.get_next_move()) != MOVE_NONE
1138 && !TM.thread_should_stop(threadID))
1140 assert(move_is_ok(move));
1142 singleEvasion = (isCheck && mp.number_of_evasions() == 1);
1143 moveIsCheck = pos.move_is_check(move, ci);
1144 captureOrPromotion = pos.move_is_capture_or_promotion(move);
1146 // Step 11. Decide the new search depth
1147 ext = extension(pos, move, true, captureOrPromotion, moveIsCheck, singleEvasion, mateThreat, &dangerous);
1149 // Singular extension search. We extend the TT move if its value is much better than
1150 // its siblings. To verify this we do a reduced search on all the other moves but the
1151 // ttMove, if result is lower then ttValue minus a margin then we extend ttMove.
1152 if ( depth >= SingularExtensionDepthAtPVNodes
1154 && move == tte->move()
1156 && is_lower_bound(tte->type())
1157 && tte->depth() >= depth - 3 * OnePly)
1159 Value ttValue = value_from_tt(tte->value(), ply);
1161 if (abs(ttValue) < VALUE_KNOWN_WIN)
1163 Value excValue = search(pos, ss, ttValue - SingularExtensionMargin, depth / 2, ply, false, threadID, move);
1165 if (excValue < ttValue - SingularExtensionMargin)
1170 newDepth = depth - OnePly + ext;
1172 // Update current move (this must be done after singular extension search)
1173 movesSearched[moveCount++] = ss[ply].currentMove = move;
1175 // Step 12. Futility pruning (is omitted in PV nodes)
1177 // Step 13. Make the move
1178 pos.do_move(move, st, ci, moveIsCheck);
1180 // Step extra. pv search (only in PV nodes)
1181 // The first move in list is the expected PV
1183 value = -search_pv(pos, ss, -beta, -alpha, newDepth, ply+1, threadID);
1186 // Step 14. Reduced search
1187 // if the move fails high will be re-searched at full depth.
1188 bool doFullDepthSearch = true;
1190 if ( depth >= 3 * OnePly
1192 && !captureOrPromotion
1193 && !move_is_castle(move)
1194 && !move_is_killer(move, ss[ply]))
1196 ss[ply].reduction = pv_reduction(depth, moveCount);
1197 if (ss[ply].reduction)
1199 value = -search(pos, ss, -alpha, newDepth-ss[ply].reduction, ply+1, true, threadID);
1200 doFullDepthSearch = (value > alpha);
1204 // Step 15. Full depth search
1205 if (doFullDepthSearch)
1207 ss[ply].reduction = Depth(0);
1208 value = -search(pos, ss, -alpha, newDepth, ply+1, true, threadID);
1210 // Step extra. pv search (only in PV nodes)
1211 if (value > alpha && value < beta)
1212 value = -search_pv(pos, ss, -beta, -alpha, newDepth, ply+1, threadID);
1216 // Step 16. Undo move
1217 pos.undo_move(move);
1219 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1221 // Step 17. Check for new best move
1222 if (value > bestValue)
1229 if (value == value_mate_in(ply + 1))
1230 ss[ply].mateKiller = move;
1234 // Step 18. Check for split
1235 if ( TM.active_threads() > 1
1237 && depth >= MinimumSplitDepth
1239 && TM.available_thread_exists(threadID)
1241 && !TM.thread_should_stop(threadID)
1242 && TM.split(pos, ss, ply, &alpha, beta, &bestValue,
1243 depth, mateThreat, &moveCount, &mp, threadID, true))
1247 // Step 19. Check for mate and stalemate
1248 // All legal moves have been searched and if there were
1249 // no legal moves, it must be mate or stalemate.
1251 return (isCheck ? value_mated_in(ply) : VALUE_DRAW);
1253 // Step 20. Update tables
1254 // If the search is not aborted, update the transposition table,
1255 // history counters, and killer moves.
1256 if (AbortSearch || TM.thread_should_stop(threadID))
1259 if (bestValue <= oldAlpha)
1260 TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_UPPER, depth, MOVE_NONE);
1262 else if (bestValue >= beta)
1264 TM.incrementBetaCounter(pos.side_to_move(), depth, threadID);
1265 move = ss[ply].pv[ply];
1266 if (!pos.move_is_capture_or_promotion(move))
1268 update_history(pos, move, depth, movesSearched, moveCount);
1269 update_killers(move, ss[ply]);
1271 TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_LOWER, depth, move);
1274 TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_EXACT, depth, ss[ply].pv[ply]);
1280 // search() is the search function for zero-width nodes.
1282 Value search(Position& pos, SearchStack ss[], Value beta, Depth depth,
1283 int ply, bool allowNullmove, int threadID, Move excludedMove) {
1285 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1286 assert(ply >= 0 && ply < PLY_MAX);
1287 assert(threadID >= 0 && threadID < TM.active_threads());
1289 Move movesSearched[256];
1294 Depth ext, newDepth;
1295 Value bestValue, refinedValue, nullValue, value, futilityValueScaled;
1296 bool isCheck, singleEvasion, moveIsCheck, captureOrPromotion, dangerous;
1297 bool mateThreat = false;
1299 refinedValue = bestValue = value = -VALUE_INFINITE;
1302 return qsearch(pos, ss, beta-1, beta, Depth(0), ply, threadID);
1304 // Step 1. Initialize node and poll
1305 // Polling can abort search.
1306 init_node(ss, ply, threadID);
1308 // Step 2. Check for aborted search and immediate draw
1309 if (AbortSearch || TM.thread_should_stop(threadID))
1312 if (pos.is_draw() || ply >= PLY_MAX - 1)
1315 // Step 3. Mate distance pruning
1316 if (value_mated_in(ply) >= beta)
1319 if (value_mate_in(ply + 1) < beta)
1322 // Step 4. Transposition table lookup
1324 // We don't want the score of a partial search to overwrite a previous full search
1325 // TT value, so we use a different position key in case of an excluded move exists.
1326 Key posKey = excludedMove ? pos.get_exclusion_key() : pos.get_key();
1328 tte = TT.retrieve(posKey);
1329 ttMove = (tte ? tte->move() : MOVE_NONE);
1331 if (tte && ok_to_use_TT(tte, depth, beta, ply))
1333 ss[ply].currentMove = ttMove; // Can be MOVE_NONE
1334 return value_from_tt(tte->value(), ply);
1337 // Step 5. Evaluate the position statically
1338 isCheck = pos.is_check();
1342 if (tte && (tte->type() & VALUE_TYPE_EVAL))
1343 ss[ply].eval = value_from_tt(tte->value(), ply);
1345 ss[ply].eval = evaluate(pos, ei, threadID);
1347 refinedValue = refine_eval(tte, ss[ply].eval, ply); // Enhance accuracy with TT value if possible
1348 update_gains(pos, ss[ply - 1].currentMove, ss[ply - 1].eval, ss[ply].eval);
1352 if ( refinedValue < beta - razor_margin(depth)
1353 && ttMove == MOVE_NONE
1354 && ss[ply - 1].currentMove != MOVE_NULL
1355 && depth < RazorDepth
1357 && !value_is_mate(beta)
1358 && !pos.has_pawn_on_7th(pos.side_to_move()))
1360 Value rbeta = beta - razor_margin(depth);
1361 Value v = qsearch(pos, ss, rbeta-1, rbeta, Depth(0), ply, threadID);
1363 // Logically we should return (v + razor_margin(depth)), but
1364 // surprisingly this did slightly weaker in tests.
1368 // Step 7. Static null move pruning
1369 // We're betting that the opponent doesn't have a move that will reduce
1370 // the score by more than futility_margin(depth) if we do a null move.
1372 && depth < RazorDepth
1374 && !value_is_mate(beta)
1375 && ok_to_do_nullmove(pos)
1376 && refinedValue >= beta + futility_margin(depth, 0))
1377 return refinedValue - futility_margin(depth, 0);
1379 // Step 8. Null move search with verification search
1380 // When we jump directly to qsearch() we do a null move only if static value is
1381 // at least beta. Otherwise we do a null move if static value is not more than
1382 // NullMoveMargin under beta.
1386 && !value_is_mate(beta)
1387 && ok_to_do_nullmove(pos)
1388 && refinedValue >= beta - (depth >= 4 * OnePly ? NullMoveMargin : 0))
1390 ss[ply].currentMove = MOVE_NULL;
1392 // Null move dynamic reduction based on depth
1393 int R = 3 + (depth >= 5 * OnePly ? depth / 8 : 0);
1395 // Null move dynamic reduction based on value
1396 if (refinedValue - beta > PawnValueMidgame)
1399 pos.do_null_move(st);
1401 nullValue = -search(pos, ss, -(beta-1), depth-R*OnePly, ply+1, false, threadID);
1403 pos.undo_null_move();
1405 if (nullValue >= beta)
1407 // Do not return unproven mate scores
1408 if (nullValue >= value_mate_in(PLY_MAX))
1411 if (depth < 6 * OnePly)
1414 // Do zugzwang verification search
1415 Value v = search(pos, ss, beta, depth-5*OnePly, ply, false, threadID);
1419 // The null move failed low, which means that we may be faced with
1420 // some kind of threat. If the previous move was reduced, check if
1421 // the move that refuted the null move was somehow connected to the
1422 // move which was reduced. If a connection is found, return a fail
1423 // low score (which will cause the reduced move to fail high in the
1424 // parent node, which will trigger a re-search with full depth).
1425 if (nullValue == value_mated_in(ply + 2))
1428 ss[ply].threatMove = ss[ply + 1].currentMove;
1429 if ( depth < ThreatDepth
1430 && ss[ply - 1].reduction
1431 && connected_moves(pos, ss[ply - 1].currentMove, ss[ply].threatMove))
1436 // Step 9. Internal iterative deepening
1437 if ( depth >= IIDDepthAtNonPVNodes
1438 && ttMove == MOVE_NONE
1440 && ss[ply].eval >= beta - IIDMargin)
1442 search(pos, ss, beta, depth/2, ply, false, threadID);
1443 ttMove = ss[ply].pv[ply];
1444 tte = TT.retrieve(posKey);
1447 // Initialize a MovePicker object for the current position
1448 MovePicker mp = MovePicker(pos, ttMove, depth, H, &ss[ply], beta);
1451 // Step 10. Loop through moves
1452 // Loop through all legal moves until no moves remain or a beta cutoff occurs
1453 while ( bestValue < beta
1454 && (move = mp.get_next_move()) != MOVE_NONE
1455 && !TM.thread_should_stop(threadID))
1457 assert(move_is_ok(move));
1459 if (move == excludedMove)
1462 moveIsCheck = pos.move_is_check(move, ci);
1463 singleEvasion = (isCheck && mp.number_of_evasions() == 1);
1464 captureOrPromotion = pos.move_is_capture_or_promotion(move);
1466 // Step 11. Decide the new search depth
1467 ext = extension(pos, move, false, captureOrPromotion, moveIsCheck, singleEvasion, mateThreat, &dangerous);
1469 // Singular extension search. We extend the TT move if its value is much better than
1470 // its siblings. To verify this we do a reduced search on all the other moves but the
1471 // ttMove, if result is lower then ttValue minus a margin then we extend ttMove.
1472 if ( depth >= SingularExtensionDepthAtNonPVNodes
1474 && move == tte->move()
1475 && !excludedMove // Do not allow recursive singular extension search
1477 && is_lower_bound(tte->type())
1478 && tte->depth() >= depth - 3 * OnePly)
1480 Value ttValue = value_from_tt(tte->value(), ply);
1482 if (abs(ttValue) < VALUE_KNOWN_WIN)
1484 Value excValue = search(pos, ss, ttValue - SingularExtensionMargin, depth / 2, ply, false, threadID, move);
1486 if (excValue < ttValue - SingularExtensionMargin)
1491 newDepth = depth - OnePly + ext;
1493 // Update current move (this must be done after singular extension search)
1494 movesSearched[moveCount++] = ss[ply].currentMove = move;
1496 // Step 12. Futility pruning
1499 && !captureOrPromotion
1500 && !move_is_castle(move)
1503 // Move count based pruning
1504 if ( moveCount >= futility_move_count(depth)
1505 && ok_to_prune(pos, move, ss[ply].threatMove)
1506 && bestValue > value_mated_in(PLY_MAX))
1509 // Value based pruning
1510 Depth predictedDepth = newDepth - nonpv_reduction(depth, moveCount); // We illogically ignore reduction condition depth >= 3*OnePly
1511 futilityValueScaled = ss[ply].eval + futility_margin(predictedDepth, moveCount)
1512 + H.gain(pos.piece_on(move_from(move)), move_to(move)) + 45;
1514 if (futilityValueScaled < beta)
1516 if (futilityValueScaled > bestValue)
1517 bestValue = futilityValueScaled;
1522 // Step 13. Make the move
1523 pos.do_move(move, st, ci, moveIsCheck);
1525 // Step 14. Reduced search, if the move fails high
1526 // will be re-searched at full depth.
1527 bool doFullDepthSearch = true;
1529 if ( depth >= 3*OnePly
1531 && !captureOrPromotion
1532 && !move_is_castle(move)
1533 && !move_is_killer(move, ss[ply]))
1535 ss[ply].reduction = nonpv_reduction(depth, moveCount);
1536 if (ss[ply].reduction)
1538 value = -search(pos, ss, -(beta-1), newDepth-ss[ply].reduction, ply+1, true, threadID);
1539 doFullDepthSearch = (value >= beta);
1543 // Step 15. Full depth search
1544 if (doFullDepthSearch)
1546 ss[ply].reduction = Depth(0);
1547 value = -search(pos, ss, -(beta-1), newDepth, ply+1, true, threadID);
1550 // Step 16. Undo move
1551 pos.undo_move(move);
1553 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1555 // Step 17. Check for new best move
1556 if (value > bestValue)
1562 if (value == value_mate_in(ply + 1))
1563 ss[ply].mateKiller = move;
1566 // Step 18. Check for split
1567 if ( TM.active_threads() > 1
1569 && depth >= MinimumSplitDepth
1571 && TM.available_thread_exists(threadID)
1573 && !TM.thread_should_stop(threadID)
1574 && TM.split(pos, ss, ply, NULL, beta, &bestValue,
1575 depth, mateThreat, &moveCount, &mp, threadID, false))
1579 // Step 19. Check for mate and stalemate
1580 // All legal moves have been searched and if there are
1581 // no legal moves, it must be mate or stalemate.
1582 // If one move was excluded return fail low score.
1584 return excludedMove ? beta - 1 : (isCheck ? value_mated_in(ply) : VALUE_DRAW);
1586 // Step 20. Update tables
1587 // If the search is not aborted, update the transposition table,
1588 // history counters, and killer moves.
1589 if (AbortSearch || TM.thread_should_stop(threadID))
1592 if (bestValue < beta)
1593 TT.store(posKey, value_to_tt(bestValue, ply), VALUE_TYPE_UPPER, depth, MOVE_NONE);
1596 TM.incrementBetaCounter(pos.side_to_move(), depth, threadID);
1597 move = ss[ply].pv[ply];
1598 TT.store(posKey, value_to_tt(bestValue, ply), VALUE_TYPE_LOWER, depth, move);
1599 if (!pos.move_is_capture_or_promotion(move))
1601 update_history(pos, move, depth, movesSearched, moveCount);
1602 update_killers(move, ss[ply]);
1607 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1613 // qsearch() is the quiescence search function, which is called by the main
1614 // search function when the remaining depth is zero (or, to be more precise,
1615 // less than OnePly).
1617 Value qsearch(Position& pos, SearchStack ss[], Value alpha, Value beta,
1618 Depth depth, int ply, int threadID) {
1620 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1621 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1623 assert(ply >= 0 && ply < PLY_MAX);
1624 assert(threadID >= 0 && threadID < TM.active_threads());
1629 Value staticValue, bestValue, value, futilityBase, futilityValue;
1630 bool isCheck, enoughMaterial, moveIsCheck, evasionPrunable;
1631 const TTEntry* tte = NULL;
1633 bool pvNode = (beta - alpha != 1);
1634 Value oldAlpha = alpha;
1636 // Initialize, and make an early exit in case of an aborted search,
1637 // an instant draw, maximum ply reached, etc.
1638 init_node(ss, ply, threadID);
1640 // After init_node() that calls poll()
1641 if (AbortSearch || TM.thread_should_stop(threadID))
1644 if (pos.is_draw() || ply >= PLY_MAX - 1)
1647 // Transposition table lookup. At PV nodes, we don't use the TT for
1648 // pruning, but only for move ordering.
1649 tte = TT.retrieve(pos.get_key());
1650 ttMove = (tte ? tte->move() : MOVE_NONE);
1652 if (!pvNode && tte && ok_to_use_TT(tte, depth, beta, ply))
1654 assert(tte->type() != VALUE_TYPE_EVAL);
1656 ss[ply].currentMove = ttMove; // Can be MOVE_NONE
1657 return value_from_tt(tte->value(), ply);
1660 isCheck = pos.is_check();
1662 // Evaluate the position statically
1664 staticValue = -VALUE_INFINITE;
1665 else if (tte && (tte->type() & VALUE_TYPE_EVAL))
1666 staticValue = value_from_tt(tte->value(), ply);
1668 staticValue = evaluate(pos, ei, threadID);
1672 ss[ply].eval = staticValue;
1673 update_gains(pos, ss[ply - 1].currentMove, ss[ply - 1].eval, ss[ply].eval);
1676 // Initialize "stand pat score", and return it immediately if it is
1678 bestValue = staticValue;
1680 if (bestValue >= beta)
1682 // Store the score to avoid a future costly evaluation() call
1683 if (!isCheck && !tte && ei.futilityMargin[pos.side_to_move()] == 0)
1684 TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_EV_LO, Depth(-127*OnePly), MOVE_NONE);
1689 if (bestValue > alpha)
1692 // If we are near beta then try to get a cutoff pushing checks a bit further
1693 bool deepChecks = depth == -OnePly && staticValue >= beta - PawnValueMidgame / 8;
1695 // Initialize a MovePicker object for the current position, and prepare
1696 // to search the moves. Because the depth is <= 0 here, only captures,
1697 // queen promotions and checks (only if depth == 0 or depth == -OnePly
1698 // and we are near beta) will be generated.
1699 MovePicker mp = MovePicker(pos, ttMove, deepChecks ? Depth(0) : depth, H);
1701 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1702 futilityBase = staticValue + FutilityMarginQS + ei.futilityMargin[pos.side_to_move()];
1704 // Loop through the moves until no moves remain or a beta cutoff occurs
1705 while ( alpha < beta
1706 && (move = mp.get_next_move()) != MOVE_NONE)
1708 assert(move_is_ok(move));
1710 moveIsCheck = pos.move_is_check(move, ci);
1712 // Update current move
1714 ss[ply].currentMove = move;
1722 && !move_is_promotion(move)
1723 && !pos.move_is_passed_pawn_push(move))
1725 futilityValue = futilityBase
1726 + pos.endgame_value_of_piece_on(move_to(move))
1727 + (move_is_ep(move) ? PawnValueEndgame : Value(0));
1729 if (futilityValue < alpha)
1731 if (futilityValue > bestValue)
1732 bestValue = futilityValue;
1737 // Detect blocking evasions that are candidate to be pruned
1738 evasionPrunable = isCheck
1739 && bestValue != -VALUE_INFINITE
1740 && !pos.move_is_capture(move)
1741 && pos.type_of_piece_on(move_from(move)) != KING
1742 && !pos.can_castle(pos.side_to_move());
1744 // Don't search moves with negative SEE values
1745 if ( (!isCheck || evasionPrunable)
1748 && !move_is_promotion(move)
1749 && pos.see_sign(move) < 0)
1752 // Make and search the move
1753 pos.do_move(move, st, ci, moveIsCheck);
1754 value = -qsearch(pos, ss, -beta, -alpha, depth-OnePly, ply+1, threadID);
1755 pos.undo_move(move);
1757 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1760 if (value > bestValue)
1771 // All legal moves have been searched. A special case: If we're in check
1772 // and no legal moves were found, it is checkmate.
1773 if (!moveCount && isCheck) // Mate!
1774 return value_mated_in(ply);
1776 // Update transposition table
1777 Depth d = (depth == Depth(0) ? Depth(0) : Depth(-1));
1778 if (bestValue <= oldAlpha)
1780 // If bestValue isn't changed it means it is still the static evaluation
1781 // of the node, so keep this info to avoid a future evaluation() call.
1782 ValueType type = (bestValue == staticValue && !ei.futilityMargin[pos.side_to_move()] ? VALUE_TYPE_EV_UP : VALUE_TYPE_UPPER);
1783 TT.store(pos.get_key(), value_to_tt(bestValue, ply), type, d, MOVE_NONE);
1785 else if (bestValue >= beta)
1787 move = ss[ply].pv[ply];
1788 TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_LOWER, d, move);
1790 // Update killers only for good checking moves
1791 if (!pos.move_is_capture_or_promotion(move))
1792 update_killers(move, ss[ply]);
1795 TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_EXACT, d, ss[ply].pv[ply]);
1797 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1803 // sp_search() is used to search from a split point. This function is called
1804 // by each thread working at the split point. It is similar to the normal
1805 // search() function, but simpler. Because we have already probed the hash
1806 // table, done a null move search, and searched the first move before
1807 // splitting, we don't have to repeat all this work in sp_search(). We
1808 // also don't need to store anything to the hash table here: This is taken
1809 // care of after we return from the split point.
1811 void sp_search(SplitPoint* sp, int threadID) {
1813 assert(threadID >= 0 && threadID < TM.active_threads());
1814 assert(TM.active_threads() > 1);
1818 Depth ext, newDepth;
1819 Value value, futilityValueScaled;
1820 bool isCheck, moveIsCheck, captureOrPromotion, dangerous;
1822 value = -VALUE_INFINITE;
1824 Position pos(*sp->pos);
1826 SearchStack* ss = sp->sstack[threadID];
1827 isCheck = pos.is_check();
1829 // Step 10. Loop through moves
1830 // Loop through all legal moves until no moves remain or a beta cutoff occurs
1831 lock_grab(&(sp->lock));
1833 while ( sp->bestValue < sp->beta
1834 && !TM.thread_should_stop(threadID)
1835 && (move = sp->mp->get_next_move()) != MOVE_NONE)
1837 moveCount = ++sp->moves;
1838 lock_release(&(sp->lock));
1840 assert(move_is_ok(move));
1842 moveIsCheck = pos.move_is_check(move, ci);
1843 captureOrPromotion = pos.move_is_capture_or_promotion(move);
1845 // Step 11. Decide the new search depth
1846 ext = extension(pos, move, false, captureOrPromotion, moveIsCheck, false, sp->mateThreat, &dangerous);
1847 newDepth = sp->depth - OnePly + ext;
1849 // Update current move
1850 ss[sp->ply].currentMove = move;
1852 // Step 12. Futility pruning
1855 && !captureOrPromotion
1856 && !move_is_castle(move))
1858 // Move count based pruning
1859 if ( moveCount >= futility_move_count(sp->depth)
1860 && ok_to_prune(pos, move, ss[sp->ply].threatMove)
1861 && sp->bestValue > value_mated_in(PLY_MAX))
1863 lock_grab(&(sp->lock));
1867 // Value based pruning
1868 Depth predictedDepth = newDepth - nonpv_reduction(sp->depth, moveCount);
1869 futilityValueScaled = ss[sp->ply].eval + futility_margin(predictedDepth, moveCount)
1870 + H.gain(pos.piece_on(move_from(move)), move_to(move)) + 45;
1872 if (futilityValueScaled < sp->beta)
1874 lock_grab(&(sp->lock));
1876 if (futilityValueScaled > sp->bestValue)
1877 sp->bestValue = futilityValueScaled;
1882 // Step 13. Make the move
1883 pos.do_move(move, st, ci, moveIsCheck);
1885 // Step 14. Reduced search
1886 // if the move fails high will be re-searched at full depth.
1887 bool doFullDepthSearch = true;
1890 && !captureOrPromotion
1891 && !move_is_castle(move)
1892 && !move_is_killer(move, ss[sp->ply]))
1894 ss[sp->ply].reduction = nonpv_reduction(sp->depth, moveCount);
1895 if (ss[sp->ply].reduction)
1897 value = -search(pos, ss, -(sp->beta-1), newDepth-ss[sp->ply].reduction, sp->ply+1, true, threadID);
1898 doFullDepthSearch = (value >= sp->beta && !TM.thread_should_stop(threadID));
1902 // Step 15. Full depth search
1903 if (doFullDepthSearch)
1905 ss[sp->ply].reduction = Depth(0);
1906 value = -search(pos, ss, -(sp->beta - 1), newDepth, sp->ply+1, true, threadID);
1909 // Step 16. Undo move
1910 pos.undo_move(move);
1912 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1914 // Step 17. Check for new best move
1915 lock_grab(&(sp->lock));
1917 if (value > sp->bestValue && !TM.thread_should_stop(threadID))
1919 sp->bestValue = value;
1920 if (sp->bestValue >= sp->beta)
1922 sp->stopRequest = true;
1923 sp_update_pv(sp->parentSstack, ss, sp->ply);
1928 /* Here we have the lock still grabbed */
1930 sp->slaves[threadID] = 0;
1933 lock_release(&(sp->lock));
1937 // sp_search_pv() is used to search from a PV split point. This function
1938 // is called by each thread working at the split point. It is similar to
1939 // the normal search_pv() function, but simpler. Because we have already
1940 // probed the hash table and searched the first move before splitting, we
1941 // don't have to repeat all this work in sp_search_pv(). We also don't
1942 // need to store anything to the hash table here: This is taken care of
1943 // after we return from the split point.
1945 void sp_search_pv(SplitPoint* sp, int threadID) {
1947 assert(threadID >= 0 && threadID < TM.active_threads());
1948 assert(TM.active_threads() > 1);
1952 Depth ext, newDepth;
1954 bool moveIsCheck, captureOrPromotion, dangerous;
1956 value = -VALUE_INFINITE;
1958 Position pos(*sp->pos);
1960 SearchStack* ss = sp->sstack[threadID];
1962 // Step 10. Loop through moves
1963 // Loop through all legal moves until no moves remain or a beta cutoff occurs
1964 lock_grab(&(sp->lock));
1966 while ( sp->alpha < sp->beta
1967 && !TM.thread_should_stop(threadID)
1968 && (move = sp->mp->get_next_move()) != MOVE_NONE)
1970 moveCount = ++sp->moves;
1971 lock_release(&(sp->lock));
1973 assert(move_is_ok(move));
1975 moveIsCheck = pos.move_is_check(move, ci);
1976 captureOrPromotion = pos.move_is_capture_or_promotion(move);
1978 // Step 11. Decide the new search depth
1979 ext = extension(pos, move, true, captureOrPromotion, moveIsCheck, false, sp->mateThreat, &dangerous);
1980 newDepth = sp->depth - OnePly + ext;
1982 // Update current move
1983 ss[sp->ply].currentMove = move;
1985 // Step 12. Futility pruning (is omitted in PV nodes)
1987 // Step 13. Make the move
1988 pos.do_move(move, st, ci, moveIsCheck);
1990 // Step 14. Reduced search
1991 // if the move fails high will be re-searched at full depth.
1992 bool doFullDepthSearch = true;
1995 && !captureOrPromotion
1996 && !move_is_castle(move)
1997 && !move_is_killer(move, ss[sp->ply]))
1999 ss[sp->ply].reduction = pv_reduction(sp->depth, moveCount);
2000 if (ss[sp->ply].reduction)
2002 Value localAlpha = sp->alpha;
2003 value = -search(pos, ss, -localAlpha, newDepth-ss[sp->ply].reduction, sp->ply+1, true, threadID);
2004 doFullDepthSearch = (value > localAlpha && !TM.thread_should_stop(threadID));
2008 // Step 15. Full depth search
2009 if (doFullDepthSearch)
2011 Value localAlpha = sp->alpha;
2012 ss[sp->ply].reduction = Depth(0);
2013 value = -search(pos, ss, -localAlpha, newDepth, sp->ply+1, true, threadID);
2015 if (value > localAlpha && value < sp->beta && !TM.thread_should_stop(threadID))
2017 // If another thread has failed high then sp->alpha has been increased
2018 // to be higher or equal then beta, if so, avoid to start a PV search.
2019 localAlpha = sp->alpha;
2020 if (localAlpha < sp->beta)
2021 value = -search_pv(pos, ss, -sp->beta, -localAlpha, newDepth, sp->ply+1, threadID);
2025 // Step 16. Undo move
2026 pos.undo_move(move);
2028 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
2030 // Step 17. Check for new best move
2031 lock_grab(&(sp->lock));
2033 if (value > sp->bestValue && !TM.thread_should_stop(threadID))
2035 sp->bestValue = value;
2036 if (value > sp->alpha)
2038 // Ask threads to stop before to modify sp->alpha
2039 if (value >= sp->beta)
2040 sp->stopRequest = true;
2044 sp_update_pv(sp->parentSstack, ss, sp->ply);
2045 if (value == value_mate_in(sp->ply + 1))
2046 ss[sp->ply].mateKiller = move;
2051 /* Here we have the lock still grabbed */
2053 sp->slaves[threadID] = 0;
2056 lock_release(&(sp->lock));
2060 // init_node() is called at the beginning of all the search functions
2061 // (search(), search_pv(), qsearch(), and so on) and initializes the
2062 // search stack object corresponding to the current node. Once every
2063 // NodesBetweenPolls nodes, init_node() also calls poll(), which polls
2064 // for user input and checks whether it is time to stop the search.
2066 void init_node(SearchStack ss[], int ply, int threadID) {
2068 assert(ply >= 0 && ply < PLY_MAX);
2069 assert(threadID >= 0 && threadID < TM.active_threads());
2071 TM.incrementNodeCounter(threadID);
2076 if (NodesSincePoll >= NodesBetweenPolls)
2083 ss[ply + 2].initKillers();
2087 // update_pv() is called whenever a search returns a value > alpha.
2088 // It updates the PV in the SearchStack object corresponding to the
2091 void update_pv(SearchStack ss[], int ply) {
2093 assert(ply >= 0 && ply < PLY_MAX);
2097 ss[ply].pv[ply] = ss[ply].currentMove;
2099 for (p = ply + 1; ss[ply + 1].pv[p] != MOVE_NONE; p++)
2100 ss[ply].pv[p] = ss[ply + 1].pv[p];
2102 ss[ply].pv[p] = MOVE_NONE;
2106 // sp_update_pv() is a variant of update_pv for use at split points. The
2107 // difference between the two functions is that sp_update_pv also updates
2108 // the PV at the parent node.
2110 void sp_update_pv(SearchStack* pss, SearchStack ss[], int ply) {
2112 assert(ply >= 0 && ply < PLY_MAX);
2116 ss[ply].pv[ply] = pss[ply].pv[ply] = ss[ply].currentMove;
2118 for (p = ply + 1; ss[ply + 1].pv[p] != MOVE_NONE; p++)
2119 ss[ply].pv[p] = pss[ply].pv[p] = ss[ply + 1].pv[p];
2121 ss[ply].pv[p] = pss[ply].pv[p] = MOVE_NONE;
2125 // connected_moves() tests whether two moves are 'connected' in the sense
2126 // that the first move somehow made the second move possible (for instance
2127 // if the moving piece is the same in both moves). The first move is assumed
2128 // to be the move that was made to reach the current position, while the
2129 // second move is assumed to be a move from the current position.
2131 bool connected_moves(const Position& pos, Move m1, Move m2) {
2133 Square f1, t1, f2, t2;
2136 assert(move_is_ok(m1));
2137 assert(move_is_ok(m2));
2139 if (m2 == MOVE_NONE)
2142 // Case 1: The moving piece is the same in both moves
2148 // Case 2: The destination square for m2 was vacated by m1
2154 // Case 3: Moving through the vacated square
2155 if ( piece_is_slider(pos.piece_on(f2))
2156 && bit_is_set(squares_between(f2, t2), f1))
2159 // Case 4: The destination square for m2 is defended by the moving piece in m1
2160 p = pos.piece_on(t1);
2161 if (bit_is_set(pos.attacks_from(p, t1), t2))
2164 // Case 5: Discovered check, checking piece is the piece moved in m1
2165 if ( piece_is_slider(p)
2166 && bit_is_set(squares_between(t1, pos.king_square(pos.side_to_move())), f2)
2167 && !bit_is_set(squares_between(t1, pos.king_square(pos.side_to_move())), t2))
2169 // discovered_check_candidates() works also if the Position's side to
2170 // move is the opposite of the checking piece.
2171 Color them = opposite_color(pos.side_to_move());
2172 Bitboard dcCandidates = pos.discovered_check_candidates(them);
2174 if (bit_is_set(dcCandidates, f2))
2181 // value_is_mate() checks if the given value is a mate one
2182 // eventually compensated for the ply.
2184 bool value_is_mate(Value value) {
2186 assert(abs(value) <= VALUE_INFINITE);
2188 return value <= value_mated_in(PLY_MAX)
2189 || value >= value_mate_in(PLY_MAX);
2193 // move_is_killer() checks if the given move is among the
2194 // killer moves of that ply.
2196 bool move_is_killer(Move m, const SearchStack& ss) {
2198 const Move* k = ss.killers;
2199 for (int i = 0; i < KILLER_MAX; i++, k++)
2207 // extension() decides whether a move should be searched with normal depth,
2208 // or with extended depth. Certain classes of moves (checking moves, in
2209 // particular) are searched with bigger depth than ordinary moves and in
2210 // any case are marked as 'dangerous'. Note that also if a move is not
2211 // extended, as example because the corresponding UCI option is set to zero,
2212 // the move is marked as 'dangerous' so, at least, we avoid to prune it.
2214 Depth extension(const Position& pos, Move m, bool pvNode, bool captureOrPromotion,
2215 bool moveIsCheck, bool singleEvasion, bool mateThreat, bool* dangerous) {
2217 assert(m != MOVE_NONE);
2219 Depth result = Depth(0);
2220 *dangerous = moveIsCheck | singleEvasion | mateThreat;
2225 result += CheckExtension[pvNode];
2228 result += SingleEvasionExtension[pvNode];
2231 result += MateThreatExtension[pvNode];
2234 if (pos.type_of_piece_on(move_from(m)) == PAWN)
2236 Color c = pos.side_to_move();
2237 if (relative_rank(c, move_to(m)) == RANK_7)
2239 result += PawnPushTo7thExtension[pvNode];
2242 if (pos.pawn_is_passed(c, move_to(m)))
2244 result += PassedPawnExtension[pvNode];
2249 if ( captureOrPromotion
2250 && pos.type_of_piece_on(move_to(m)) != PAWN
2251 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
2252 - pos.midgame_value_of_piece_on(move_to(m)) == Value(0))
2253 && !move_is_promotion(m)
2256 result += PawnEndgameExtension[pvNode];
2261 && captureOrPromotion
2262 && pos.type_of_piece_on(move_to(m)) != PAWN
2263 && pos.see_sign(m) >= 0)
2269 return Min(result, OnePly);
2273 // ok_to_do_nullmove() looks at the current position and decides whether
2274 // doing a 'null move' should be allowed. In order to avoid zugzwang
2275 // problems, null moves are not allowed when the side to move has very
2276 // little material left. Currently, the test is a bit too simple: Null
2277 // moves are avoided only when the side to move has only pawns left.
2278 // It's probably a good idea to avoid null moves in at least some more
2279 // complicated endgames, e.g. KQ vs KR. FIXME
2281 bool ok_to_do_nullmove(const Position& pos) {
2283 return pos.non_pawn_material(pos.side_to_move()) != Value(0);
2287 // ok_to_prune() tests whether it is safe to forward prune a move. Only
2288 // non-tactical moves late in the move list close to the leaves are
2289 // candidates for pruning.
2291 bool ok_to_prune(const Position& pos, Move m, Move threat) {
2293 assert(move_is_ok(m));
2294 assert(threat == MOVE_NONE || move_is_ok(threat));
2295 assert(!pos.move_is_check(m));
2296 assert(!pos.move_is_capture_or_promotion(m));
2297 assert(!pos.move_is_passed_pawn_push(m));
2299 Square mfrom, mto, tfrom, tto;
2301 // Prune if there isn't any threat move
2302 if (threat == MOVE_NONE)
2305 mfrom = move_from(m);
2307 tfrom = move_from(threat);
2308 tto = move_to(threat);
2310 // Case 1: Don't prune moves which move the threatened piece
2314 // Case 2: If the threatened piece has value less than or equal to the
2315 // value of the threatening piece, don't prune move which defend it.
2316 if ( pos.move_is_capture(threat)
2317 && ( pos.midgame_value_of_piece_on(tfrom) >= pos.midgame_value_of_piece_on(tto)
2318 || pos.type_of_piece_on(tfrom) == KING)
2319 && pos.move_attacks_square(m, tto))
2322 // Case 3: If the moving piece in the threatened move is a slider, don't
2323 // prune safe moves which block its ray.
2324 if ( piece_is_slider(pos.piece_on(tfrom))
2325 && bit_is_set(squares_between(tfrom, tto), mto)
2326 && pos.see_sign(m) >= 0)
2333 // ok_to_use_TT() returns true if a transposition table score
2334 // can be used at a given point in search.
2336 bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply) {
2338 Value v = value_from_tt(tte->value(), ply);
2340 return ( tte->depth() >= depth
2341 || v >= Max(value_mate_in(PLY_MAX), beta)
2342 || v < Min(value_mated_in(PLY_MAX), beta))
2344 && ( (is_lower_bound(tte->type()) && v >= beta)
2345 || (is_upper_bound(tte->type()) && v < beta));
2349 // refine_eval() returns the transposition table score if
2350 // possible otherwise falls back on static position evaluation.
2352 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
2357 Value v = value_from_tt(tte->value(), ply);
2359 if ( (is_lower_bound(tte->type()) && v >= defaultEval)
2360 || (is_upper_bound(tte->type()) && v < defaultEval))
2367 // update_history() registers a good move that produced a beta-cutoff
2368 // in history and marks as failures all the other moves of that ply.
2370 void update_history(const Position& pos, Move move, Depth depth,
2371 Move movesSearched[], int moveCount) {
2375 H.success(pos.piece_on(move_from(move)), move_to(move), depth);
2377 for (int i = 0; i < moveCount - 1; i++)
2379 m = movesSearched[i];
2383 if (!pos.move_is_capture_or_promotion(m))
2384 H.failure(pos.piece_on(move_from(m)), move_to(m), depth);
2389 // update_killers() add a good move that produced a beta-cutoff
2390 // among the killer moves of that ply.
2392 void update_killers(Move m, SearchStack& ss) {
2394 if (m == ss.killers[0])
2397 for (int i = KILLER_MAX - 1; i > 0; i--)
2398 ss.killers[i] = ss.killers[i - 1];
2404 // update_gains() updates the gains table of a non-capture move given
2405 // the static position evaluation before and after the move.
2407 void update_gains(const Position& pos, Move m, Value before, Value after) {
2410 && before != VALUE_NONE
2411 && after != VALUE_NONE
2412 && pos.captured_piece() == NO_PIECE_TYPE
2413 && !move_is_castle(m)
2414 && !move_is_promotion(m))
2415 H.set_gain(pos.piece_on(move_to(m)), move_to(m), -(before + after));
2419 // current_search_time() returns the number of milliseconds which have passed
2420 // since the beginning of the current search.
2422 int current_search_time() {
2424 return get_system_time() - SearchStartTime;
2428 // nps() computes the current nodes/second count.
2432 int t = current_search_time();
2433 return (t > 0 ? int((TM.nodes_searched() * 1000) / t) : 0);
2437 // poll() performs two different functions: It polls for user input, and it
2438 // looks at the time consumed so far and decides if it's time to abort the
2441 void poll(SearchStack ss[], int ply) {
2443 static int lastInfoTime;
2444 int t = current_search_time();
2449 // We are line oriented, don't read single chars
2450 std::string command;
2452 if (!std::getline(std::cin, command))
2455 if (command == "quit")
2458 PonderSearch = false;
2462 else if (command == "stop")
2465 PonderSearch = false;
2467 else if (command == "ponderhit")
2471 // Print search information
2475 else if (lastInfoTime > t)
2476 // HACK: Must be a new search where we searched less than
2477 // NodesBetweenPolls nodes during the first second of search.
2480 else if (t - lastInfoTime >= 1000)
2487 if (dbg_show_hit_rate)
2488 dbg_print_hit_rate();
2490 cout << "info nodes " << TM.nodes_searched() << " nps " << nps()
2491 << " time " << t << " hashfull " << TT.full() << endl;
2493 // We only support current line printing in single thread mode
2494 if (ShowCurrentLine && TM.active_threads() == 1)
2496 cout << "info currline";
2497 for (int p = 0; p < ply; p++)
2498 cout << " " << ss[p].currentMove;
2504 // Should we stop the search?
2508 bool stillAtFirstMove = FirstRootMove
2509 && !AspirationFailLow
2510 && t > MaxSearchTime + ExtraSearchTime;
2512 bool noMoreTime = t > AbsoluteMaxSearchTime
2513 || stillAtFirstMove;
2515 if ( (Iteration >= 3 && UseTimeManagement && noMoreTime)
2516 || (ExactMaxTime && t >= ExactMaxTime)
2517 || (Iteration >= 3 && MaxNodes && TM.nodes_searched() >= MaxNodes))
2522 // ponderhit() is called when the program is pondering (i.e. thinking while
2523 // it's the opponent's turn to move) in order to let the engine know that
2524 // it correctly predicted the opponent's move.
2528 int t = current_search_time();
2529 PonderSearch = false;
2531 bool stillAtFirstMove = FirstRootMove
2532 && !AspirationFailLow
2533 && t > MaxSearchTime + ExtraSearchTime;
2535 bool noMoreTime = t > AbsoluteMaxSearchTime
2536 || stillAtFirstMove;
2538 if (Iteration >= 3 && UseTimeManagement && (noMoreTime || StopOnPonderhit))
2543 // init_ss_array() does a fast reset of the first entries of a SearchStack array
2545 void init_ss_array(SearchStack ss[]) {
2547 for (int i = 0; i < 3; i++)
2550 ss[i].initKillers();
2555 // wait_for_stop_or_ponderhit() is called when the maximum depth is reached
2556 // while the program is pondering. The point is to work around a wrinkle in
2557 // the UCI protocol: When pondering, the engine is not allowed to give a
2558 // "bestmove" before the GUI sends it a "stop" or "ponderhit" command.
2559 // We simply wait here until one of these commands is sent, and return,
2560 // after which the bestmove and pondermove will be printed (in id_loop()).
2562 void wait_for_stop_or_ponderhit() {
2564 std::string command;
2568 if (!std::getline(std::cin, command))
2571 if (command == "quit")
2576 else if (command == "ponderhit" || command == "stop")
2582 // print_pv_info() prints to standard output and eventually to log file information on
2583 // the current PV line. It is called at each iteration or after a new pv is found.
2585 void print_pv_info(const Position& pos, SearchStack ss[], Value alpha, Value beta, Value value) {
2587 cout << "info depth " << Iteration
2588 << " score " << value_to_string(value)
2589 << ((value >= beta) ? " lowerbound" :
2590 ((value <= alpha)? " upperbound" : ""))
2591 << " time " << current_search_time()
2592 << " nodes " << TM.nodes_searched()
2596 for (int j = 0; ss[0].pv[j] != MOVE_NONE && j < PLY_MAX; j++)
2597 cout << ss[0].pv[j] << " ";
2603 ValueType type = (value >= beta ? VALUE_TYPE_LOWER
2604 : (value <= alpha ? VALUE_TYPE_UPPER : VALUE_TYPE_EXACT));
2606 LogFile << pretty_pv(pos, current_search_time(), Iteration,
2607 TM.nodes_searched(), value, type, ss[0].pv) << endl;
2612 // init_thread() is the function which is called when a new thread is
2613 // launched. It simply calls the idle_loop() function with the supplied
2614 // threadID. There are two versions of this function; one for POSIX
2615 // threads and one for Windows threads.
2617 #if !defined(_MSC_VER)
2619 void* init_thread(void *threadID) {
2621 TM.idle_loop(*(int*)threadID, NULL);
2627 DWORD WINAPI init_thread(LPVOID threadID) {
2629 TM.idle_loop(*(int*)threadID, NULL);
2636 /// The ThreadsManager class
2638 // resetNodeCounters(), resetBetaCounters(), searched_nodes() and
2639 // get_beta_counters() are getters/setters for the per thread
2640 // counters used to sort the moves at root.
2642 void ThreadsManager::resetNodeCounters() {
2644 for (int i = 0; i < MAX_THREADS; i++)
2645 threads[i].nodes = 0ULL;
2648 void ThreadsManager::resetBetaCounters() {
2650 for (int i = 0; i < MAX_THREADS; i++)
2651 threads[i].betaCutOffs[WHITE] = threads[i].betaCutOffs[BLACK] = 0ULL;
2654 int64_t ThreadsManager::nodes_searched() const {
2656 int64_t result = 0ULL;
2657 for (int i = 0; i < ActiveThreads; i++)
2658 result += threads[i].nodes;
2663 void ThreadsManager::get_beta_counters(Color us, int64_t& our, int64_t& their) const {
2666 for (int i = 0; i < MAX_THREADS; i++)
2668 our += threads[i].betaCutOffs[us];
2669 their += threads[i].betaCutOffs[opposite_color(us)];
2674 // idle_loop() is where the threads are parked when they have no work to do.
2675 // The parameter "waitSp", if non-NULL, is a pointer to an active SplitPoint
2676 // object for which the current thread is the master.
2678 void ThreadsManager::idle_loop(int threadID, SplitPoint* waitSp) {
2680 assert(threadID >= 0 && threadID < MAX_THREADS);
2684 // Slave threads can exit as soon as AllThreadsShouldExit raises,
2685 // master should exit as last one.
2686 if (AllThreadsShouldExit)
2689 threads[threadID].state = THREAD_TERMINATED;
2693 // If we are not thinking, wait for a condition to be signaled
2694 // instead of wasting CPU time polling for work.
2695 while (AllThreadsShouldSleep || threadID >= ActiveThreads)
2698 assert(threadID != 0);
2699 threads[threadID].state = THREAD_SLEEPING;
2701 #if !defined(_MSC_VER)
2702 lock_grab(&WaitLock);
2703 if (AllThreadsShouldSleep || threadID >= ActiveThreads)
2704 pthread_cond_wait(&WaitCond, &WaitLock);
2705 lock_release(&WaitLock);
2707 WaitForSingleObject(SitIdleEvent[threadID], INFINITE);
2711 // If thread has just woken up, mark it as available
2712 if (threads[threadID].state == THREAD_SLEEPING)
2713 threads[threadID].state = THREAD_AVAILABLE;
2715 // If this thread has been assigned work, launch a search
2716 if (threads[threadID].state == THREAD_WORKISWAITING)
2718 assert(!AllThreadsShouldExit && !AllThreadsShouldSleep);
2720 threads[threadID].state = THREAD_SEARCHING;
2722 if (threads[threadID].splitPoint->pvNode)
2723 sp_search_pv(threads[threadID].splitPoint, threadID);
2725 sp_search(threads[threadID].splitPoint, threadID);
2727 assert(threads[threadID].state == THREAD_SEARCHING);
2729 threads[threadID].state = THREAD_AVAILABLE;
2732 // If this thread is the master of a split point and all threads have
2733 // finished their work at this split point, return from the idle loop.
2734 if (waitSp != NULL && waitSp->cpus == 0)
2736 assert(threads[threadID].state == THREAD_AVAILABLE);
2738 threads[threadID].state = THREAD_SEARCHING;
2745 // init_threads() is called during startup. It launches all helper threads,
2746 // and initializes the split point stack and the global locks and condition
2749 void ThreadsManager::init_threads() {
2754 #if !defined(_MSC_VER)
2755 pthread_t pthread[1];
2758 // Initialize global locks
2759 lock_init(&MPLock, NULL);
2760 lock_init(&WaitLock, NULL);
2762 #if !defined(_MSC_VER)
2763 pthread_cond_init(&WaitCond, NULL);
2765 for (i = 0; i < MAX_THREADS; i++)
2766 SitIdleEvent[i] = CreateEvent(0, FALSE, FALSE, 0);
2769 // Initialize SplitPointStack locks
2770 for (i = 0; i < MAX_THREADS; i++)
2771 for (int j = 0; j < ACTIVE_SPLIT_POINTS_MAX; j++)
2773 SplitPointStack[i][j].parent = NULL;
2774 lock_init(&(SplitPointStack[i][j].lock), NULL);
2777 // Will be set just before program exits to properly end the threads
2778 AllThreadsShouldExit = false;
2780 // Threads will be put to sleep as soon as created
2781 AllThreadsShouldSleep = true;
2783 // All threads except the main thread should be initialized to THREAD_AVAILABLE
2785 threads[0].state = THREAD_SEARCHING;
2786 for (i = 1; i < MAX_THREADS; i++)
2787 threads[i].state = THREAD_AVAILABLE;
2789 // Launch the helper threads
2790 for (i = 1; i < MAX_THREADS; i++)
2793 #if !defined(_MSC_VER)
2794 ok = (pthread_create(pthread, NULL, init_thread, (void*)(&i)) == 0);
2796 ok = (CreateThread(NULL, 0, init_thread, (LPVOID)(&i), 0, NULL) != NULL);
2801 cout << "Failed to create thread number " << i << endl;
2802 Application::exit_with_failure();
2805 // Wait until the thread has finished launching and is gone to sleep
2806 while (threads[i].state != THREAD_SLEEPING);
2811 // exit_threads() is called when the program exits. It makes all the
2812 // helper threads exit cleanly.
2814 void ThreadsManager::exit_threads() {
2816 ActiveThreads = MAX_THREADS; // HACK
2817 AllThreadsShouldSleep = true; // HACK
2818 wake_sleeping_threads();
2820 // This makes the threads to exit idle_loop()
2821 AllThreadsShouldExit = true;
2823 // Wait for thread termination
2824 for (int i = 1; i < MAX_THREADS; i++)
2825 while (threads[i].state != THREAD_TERMINATED);
2827 // Now we can safely destroy the locks
2828 for (int i = 0; i < MAX_THREADS; i++)
2829 for (int j = 0; j < ACTIVE_SPLIT_POINTS_MAX; j++)
2830 lock_destroy(&(SplitPointStack[i][j].lock));
2832 lock_destroy(&WaitLock);
2833 lock_destroy(&MPLock);
2837 // thread_should_stop() checks whether the thread should stop its search.
2838 // This can happen if a beta cutoff has occurred in the thread's currently
2839 // active split point, or in some ancestor of the current split point.
2841 bool ThreadsManager::thread_should_stop(int threadID) const {
2843 assert(threadID >= 0 && threadID < ActiveThreads);
2847 for (sp = threads[threadID].splitPoint; sp && !sp->stopRequest; sp = sp->parent);
2852 // thread_is_available() checks whether the thread with threadID "slave" is
2853 // available to help the thread with threadID "master" at a split point. An
2854 // obvious requirement is that "slave" must be idle. With more than two
2855 // threads, this is not by itself sufficient: If "slave" is the master of
2856 // some active split point, it is only available as a slave to the other
2857 // threads which are busy searching the split point at the top of "slave"'s
2858 // split point stack (the "helpful master concept" in YBWC terminology).
2860 bool ThreadsManager::thread_is_available(int slave, int master) const {
2862 assert(slave >= 0 && slave < ActiveThreads);
2863 assert(master >= 0 && master < ActiveThreads);
2864 assert(ActiveThreads > 1);
2866 if (threads[slave].state != THREAD_AVAILABLE || slave == master)
2869 // Make a local copy to be sure doesn't change under our feet
2870 int localActiveSplitPoints = threads[slave].activeSplitPoints;
2872 if (localActiveSplitPoints == 0)
2873 // No active split points means that the thread is available as
2874 // a slave for any other thread.
2877 if (ActiveThreads == 2)
2880 // Apply the "helpful master" concept if possible. Use localActiveSplitPoints
2881 // that is known to be > 0, instead of threads[slave].activeSplitPoints that
2882 // could have been set to 0 by another thread leading to an out of bound access.
2883 if (SplitPointStack[slave][localActiveSplitPoints - 1].slaves[master])
2890 // available_thread_exists() tries to find an idle thread which is available as
2891 // a slave for the thread with threadID "master".
2893 bool ThreadsManager::available_thread_exists(int master) const {
2895 assert(master >= 0 && master < ActiveThreads);
2896 assert(ActiveThreads > 1);
2898 for (int i = 0; i < ActiveThreads; i++)
2899 if (thread_is_available(i, master))
2906 // split() does the actual work of distributing the work at a node between
2907 // several threads at PV nodes. If it does not succeed in splitting the
2908 // node (because no idle threads are available, or because we have no unused
2909 // split point objects), the function immediately returns false. If
2910 // splitting is possible, a SplitPoint object is initialized with all the
2911 // data that must be copied to the helper threads (the current position and
2912 // search stack, alpha, beta, the search depth, etc.), and we tell our
2913 // helper threads that they have been assigned work. This will cause them
2914 // to instantly leave their idle loops and call sp_search_pv(). When all
2915 // threads have returned from sp_search_pv (or, equivalently, when
2916 // splitPoint->cpus becomes 0), split() returns true.
2918 bool ThreadsManager::split(const Position& p, SearchStack* sstck, int ply,
2919 Value* alpha, const Value beta, Value* bestValue,
2920 Depth depth, bool mateThreat, int* moves, MovePicker* mp, int master, bool pvNode) {
2923 assert(sstck != NULL);
2924 assert(ply >= 0 && ply < PLY_MAX);
2925 assert(*bestValue >= -VALUE_INFINITE);
2926 assert( ( pvNode && *bestValue <= *alpha)
2927 || (!pvNode && *bestValue < beta ));
2928 assert(!pvNode || *alpha < beta);
2929 assert(beta <= VALUE_INFINITE);
2930 assert(depth > Depth(0));
2931 assert(master >= 0 && master < ActiveThreads);
2932 assert(ActiveThreads > 1);
2934 SplitPoint* splitPoint;
2938 // If no other thread is available to help us, or if we have too many
2939 // active split points, don't split.
2940 if ( !available_thread_exists(master)
2941 || threads[master].activeSplitPoints >= ACTIVE_SPLIT_POINTS_MAX)
2943 lock_release(&MPLock);
2947 // Pick the next available split point object from the split point stack
2948 splitPoint = &SplitPointStack[master][threads[master].activeSplitPoints];
2950 // Initialize the split point object
2951 splitPoint->parent = threads[master].splitPoint;
2952 splitPoint->stopRequest = false;
2953 splitPoint->ply = ply;
2954 splitPoint->depth = depth;
2955 splitPoint->mateThreat = mateThreat;
2956 splitPoint->alpha = pvNode ? *alpha : beta - 1;
2957 splitPoint->beta = beta;
2958 splitPoint->pvNode = pvNode;
2959 splitPoint->bestValue = *bestValue;
2960 splitPoint->master = master;
2961 splitPoint->mp = mp;
2962 splitPoint->moves = *moves;
2963 splitPoint->cpus = 1;
2964 splitPoint->pos = &p;
2965 splitPoint->parentSstack = sstck;
2966 for (int i = 0; i < ActiveThreads; i++)
2967 splitPoint->slaves[i] = 0;
2969 threads[master].splitPoint = splitPoint;
2970 threads[master].activeSplitPoints++;
2972 // If we are here it means we are not available
2973 assert(threads[master].state != THREAD_AVAILABLE);
2975 // Allocate available threads setting state to THREAD_BOOKED
2976 for (int i = 0; i < ActiveThreads && splitPoint->cpus < MaxThreadsPerSplitPoint; i++)
2977 if (thread_is_available(i, master))
2979 threads[i].state = THREAD_BOOKED;
2980 threads[i].splitPoint = splitPoint;
2981 splitPoint->slaves[i] = 1;
2985 assert(splitPoint->cpus > 1);
2987 // We can release the lock because slave threads are already booked and master is not available
2988 lock_release(&MPLock);
2990 // Tell the threads that they have work to do. This will make them leave
2991 // their idle loop. But before copy search stack tail for each thread.
2992 for (int i = 0; i < ActiveThreads; i++)
2993 if (i == master || splitPoint->slaves[i])
2995 memcpy(splitPoint->sstack[i] + ply - 1, sstck + ply - 1, 4 * sizeof(SearchStack));
2997 assert(i == master || threads[i].state == THREAD_BOOKED);
2999 threads[i].state = THREAD_WORKISWAITING; // This makes the slave to exit from idle_loop()
3002 // Everything is set up. The master thread enters the idle loop, from
3003 // which it will instantly launch a search, because its state is
3004 // THREAD_WORKISWAITING. We send the split point as a second parameter to the
3005 // idle loop, which means that the main thread will return from the idle
3006 // loop when all threads have finished their work at this split point
3007 // (i.e. when splitPoint->cpus == 0).
3008 idle_loop(master, splitPoint);
3010 // We have returned from the idle loop, which means that all threads are
3011 // finished. Update alpha, beta and bestValue, and return.
3015 *alpha = splitPoint->alpha;
3017 *bestValue = splitPoint->bestValue;
3018 threads[master].activeSplitPoints--;
3019 threads[master].splitPoint = splitPoint->parent;
3021 lock_release(&MPLock);
3026 // wake_sleeping_threads() wakes up all sleeping threads when it is time
3027 // to start a new search from the root.
3029 void ThreadsManager::wake_sleeping_threads() {
3031 assert(AllThreadsShouldSleep);
3032 assert(ActiveThreads > 0);
3034 AllThreadsShouldSleep = false;
3036 if (ActiveThreads == 1)
3039 #if !defined(_MSC_VER)
3040 pthread_mutex_lock(&WaitLock);
3041 pthread_cond_broadcast(&WaitCond);
3042 pthread_mutex_unlock(&WaitLock);
3044 for (int i = 1; i < MAX_THREADS; i++)
3045 SetEvent(SitIdleEvent[i]);
3051 // put_threads_to_sleep() makes all the threads go to sleep just before
3052 // to leave think(), at the end of the search. Threads should have already
3053 // finished the job and should be idle.
3055 void ThreadsManager::put_threads_to_sleep() {
3057 assert(!AllThreadsShouldSleep);
3059 // This makes the threads to go to sleep
3060 AllThreadsShouldSleep = true;
3063 /// The RootMoveList class
3065 // RootMoveList c'tor
3067 RootMoveList::RootMoveList(Position& pos, Move searchMoves[]) : count(0) {
3069 SearchStack ss[PLY_MAX_PLUS_2];
3070 MoveStack mlist[MaxRootMoves];
3072 bool includeAllMoves = (searchMoves[0] == MOVE_NONE);
3074 // Generate all legal moves
3075 MoveStack* last = generate_moves(pos, mlist);
3077 // Add each move to the moves[] array
3078 for (MoveStack* cur = mlist; cur != last; cur++)
3080 bool includeMove = includeAllMoves;
3082 for (int k = 0; !includeMove && searchMoves[k] != MOVE_NONE; k++)
3083 includeMove = (searchMoves[k] == cur->move);
3088 // Find a quick score for the move
3090 pos.do_move(cur->move, st);
3091 moves[count].move = cur->move;
3092 moves[count].score = -qsearch(pos, ss, -VALUE_INFINITE, VALUE_INFINITE, Depth(0), 1, 0);
3093 moves[count].pv[0] = cur->move;
3094 moves[count].pv[1] = MOVE_NONE;
3095 pos.undo_move(cur->move);
3102 // RootMoveList simple methods definitions
3104 void RootMoveList::set_move_nodes(int moveNum, int64_t nodes) {
3106 moves[moveNum].nodes = nodes;
3107 moves[moveNum].cumulativeNodes += nodes;
3110 void RootMoveList::set_beta_counters(int moveNum, int64_t our, int64_t their) {
3112 moves[moveNum].ourBeta = our;
3113 moves[moveNum].theirBeta = their;
3116 void RootMoveList::set_move_pv(int moveNum, const Move pv[]) {
3120 for (j = 0; pv[j] != MOVE_NONE; j++)
3121 moves[moveNum].pv[j] = pv[j];
3123 moves[moveNum].pv[j] = MOVE_NONE;
3127 // RootMoveList::sort() sorts the root move list at the beginning of a new
3130 void RootMoveList::sort() {
3132 sort_multipv(count - 1); // Sort all items
3136 // RootMoveList::sort_multipv() sorts the first few moves in the root move
3137 // list by their scores and depths. It is used to order the different PVs
3138 // correctly in MultiPV mode.
3140 void RootMoveList::sort_multipv(int n) {
3144 for (i = 1; i <= n; i++)
3146 RootMove rm = moves[i];
3147 for (j = i; j > 0 && moves[j - 1] < rm; j--)
3148 moves[j] = moves[j - 1];