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 = 2; // 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 // Step 10. Loop through moves
1130 // Loop through all legal moves until no moves remain or a beta cutoff occurs
1132 // Initialize a MovePicker object for the current position
1133 mateThreat = pos.has_mate_threat(opposite_color(pos.side_to_move()));
1134 MovePicker mp = MovePicker(pos, ttMove, depth, H, &ss[ply]);
1137 while ( alpha < beta
1138 && (move = mp.get_next_move()) != MOVE_NONE
1139 && !TM.thread_should_stop(threadID))
1141 assert(move_is_ok(move));
1143 singleEvasion = (isCheck && mp.number_of_evasions() == 1);
1144 moveIsCheck = pos.move_is_check(move, ci);
1145 captureOrPromotion = pos.move_is_capture_or_promotion(move);
1147 // Step 11. Decide the new search depth
1148 ext = extension(pos, move, true, captureOrPromotion, moveIsCheck, singleEvasion, mateThreat, &dangerous);
1150 // Singular extension search. We extend the TT move if its value is much better than
1151 // its siblings. To verify this we do a reduced search on all the other moves but the
1152 // ttMove, if result is lower then ttValue minus a margin then we extend ttMove.
1153 if ( depth >= SingularExtensionDepthAtPVNodes
1155 && move == tte->move()
1157 && is_lower_bound(tte->type())
1158 && tte->depth() >= depth - 3 * OnePly)
1160 Value ttValue = value_from_tt(tte->value(), ply);
1162 if (abs(ttValue) < VALUE_KNOWN_WIN)
1164 Value excValue = search(pos, ss, ttValue - SingularExtensionMargin, depth / 2, ply, false, threadID, move);
1166 if (excValue < ttValue - SingularExtensionMargin)
1171 newDepth = depth - OnePly + ext;
1173 // Update current move (this must be done after singular extension search)
1174 movesSearched[moveCount++] = ss[ply].currentMove = move;
1176 // Step 12. Futility pruning (is omitted in PV nodes)
1178 // Step 13. Make the move
1179 pos.do_move(move, st, ci, moveIsCheck);
1181 // Step extra. pv search (only in PV nodes)
1182 // The first move in list is the expected PV
1184 value = -search_pv(pos, ss, -beta, -alpha, newDepth, ply+1, threadID);
1187 // Step 14. Reduced search
1188 // if the move fails high will be re-searched at full depth.
1189 bool doFullDepthSearch = true;
1191 if ( depth >= 3 * OnePly
1193 && !captureOrPromotion
1194 && !move_is_castle(move)
1195 && !move_is_killer(move, ss[ply]))
1197 ss[ply].reduction = pv_reduction(depth, moveCount);
1198 if (ss[ply].reduction)
1200 value = -search(pos, ss, -alpha, newDepth-ss[ply].reduction, ply+1, true, threadID);
1201 doFullDepthSearch = (value > alpha);
1205 // Step 15. Full depth search
1206 if (doFullDepthSearch)
1208 ss[ply].reduction = Depth(0);
1209 value = -search(pos, ss, -alpha, newDepth, ply+1, true, threadID);
1211 // Step extra. pv search (only in PV nodes)
1212 if (value > alpha && value < beta)
1213 value = -search_pv(pos, ss, -beta, -alpha, newDepth, ply+1, threadID);
1217 // Step 16. Undo move
1218 pos.undo_move(move);
1220 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1222 // Step 17. Check for new best move
1223 if (value > bestValue)
1230 if (value == value_mate_in(ply + 1))
1231 ss[ply].mateKiller = move;
1235 // Step 18. Check for split
1236 if ( TM.active_threads() > 1
1238 && depth >= MinimumSplitDepth
1240 && TM.available_thread_exists(threadID)
1242 && !TM.thread_should_stop(threadID)
1243 && TM.split(pos, ss, ply, &alpha, beta, &bestValue,
1244 depth, mateThreat, &moveCount, &mp, threadID, true))
1248 // Step 19. Check for mate and stalemate
1249 // All legal moves have been searched and if there were
1250 // no legal moves, it must be mate or stalemate.
1252 return (isCheck ? value_mated_in(ply) : VALUE_DRAW);
1254 // Step 20. Update tables
1255 // If the search is not aborted, update the transposition table,
1256 // history counters, and killer moves.
1257 if (AbortSearch || TM.thread_should_stop(threadID))
1260 if (bestValue <= oldAlpha)
1261 TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_UPPER, depth, MOVE_NONE);
1263 else if (bestValue >= beta)
1265 TM.incrementBetaCounter(pos.side_to_move(), depth, threadID);
1266 move = ss[ply].pv[ply];
1267 if (!pos.move_is_capture_or_promotion(move))
1269 update_history(pos, move, depth, movesSearched, moveCount);
1270 update_killers(move, ss[ply]);
1272 TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_LOWER, depth, move);
1275 TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_EXACT, depth, ss[ply].pv[ply]);
1281 // search() is the search function for zero-width nodes.
1283 Value search(Position& pos, SearchStack ss[], Value beta, Depth depth,
1284 int ply, bool allowNullmove, int threadID, Move excludedMove) {
1286 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1287 assert(ply >= 0 && ply < PLY_MAX);
1288 assert(threadID >= 0 && threadID < TM.active_threads());
1290 Move movesSearched[256];
1295 Depth ext, newDepth;
1296 Value bestValue, refinedValue, nullValue, value, futilityValueScaled;
1297 bool isCheck, singleEvasion, moveIsCheck, captureOrPromotion, dangerous;
1298 bool mateThreat = false;
1300 refinedValue = bestValue = value = -VALUE_INFINITE;
1303 return qsearch(pos, ss, beta-1, beta, Depth(0), ply, threadID);
1305 // Step 1. Initialize node and poll
1306 // Polling can abort search.
1307 init_node(ss, ply, threadID);
1309 // Step 2. Check for aborted search and immediate draw
1310 if (AbortSearch || TM.thread_should_stop(threadID))
1313 if (pos.is_draw() || ply >= PLY_MAX - 1)
1316 // Step 3. Mate distance pruning
1317 if (value_mated_in(ply) >= beta)
1320 if (value_mate_in(ply + 1) < beta)
1323 // Step 4. Transposition table lookup
1325 // We don't want the score of a partial search to overwrite a previous full search
1326 // TT value, so we use a different position key in case of an excluded move exists.
1327 Key posKey = excludedMove ? pos.get_exclusion_key() : pos.get_key();
1329 tte = TT.retrieve(posKey);
1330 ttMove = (tte ? tte->move() : MOVE_NONE);
1332 if (tte && ok_to_use_TT(tte, depth, beta, ply))
1334 ss[ply].currentMove = ttMove; // Can be MOVE_NONE
1335 return value_from_tt(tte->value(), ply);
1338 // Step 5. Evaluate the position statically
1339 isCheck = pos.is_check();
1343 if (tte && (tte->type() & VALUE_TYPE_EVAL))
1344 ss[ply].eval = value_from_tt(tte->value(), ply);
1346 ss[ply].eval = evaluate(pos, ei, threadID);
1348 refinedValue = refine_eval(tte, ss[ply].eval, ply); // Enhance accuracy with TT value if possible
1349 update_gains(pos, ss[ply - 1].currentMove, ss[ply - 1].eval, ss[ply].eval);
1353 if ( !value_is_mate(beta)
1355 && depth < RazorDepth
1356 && refinedValue < beta - razor_margin(depth)
1357 && ss[ply - 1].currentMove != MOVE_NULL
1358 && ttMove == MOVE_NONE
1359 && !pos.has_pawn_on_7th(pos.side_to_move()))
1361 Value rbeta = beta - razor_margin(depth);
1362 Value v = qsearch(pos, ss, rbeta-1, rbeta, Depth(0), ply, threadID);
1364 // Logically we should return (v + razor_margin(depth)), but
1365 // surprisingly this did slightly weaker in tests.
1369 // Step 7. Static null move pruning
1370 // We're betting that the opponent doesn't have a move that will reduce
1371 // the score by more than futility_margin(depth) if we do a null move.
1373 && depth < RazorDepth
1375 && !value_is_mate(beta)
1376 && ok_to_do_nullmove(pos)
1377 && refinedValue >= beta + futility_margin(depth, 0))
1378 return refinedValue - futility_margin(depth, 0);
1380 // Step 8. Null move search with verification search
1381 // When we jump directly to qsearch() we do a null move only if static value is
1382 // at least beta. Otherwise we do a null move if static value is not more than
1383 // NullMoveMargin under beta.
1387 && !value_is_mate(beta)
1388 && ok_to_do_nullmove(pos)
1389 && refinedValue >= beta - (depth >= 4 * OnePly ? NullMoveMargin : 0))
1391 ss[ply].currentMove = MOVE_NULL;
1393 // Null move dynamic reduction based on depth
1394 int R = 3 + (depth >= 5 * OnePly ? depth / 8 : 0);
1396 // Null move dynamic reduction based on value
1397 if (refinedValue - beta > PawnValueMidgame)
1400 pos.do_null_move(st);
1402 nullValue = -search(pos, ss, -(beta-1), depth-R*OnePly, ply+1, false, threadID);
1404 pos.undo_null_move();
1406 if (nullValue >= beta)
1408 // Do not return unproven mate scores
1409 if (nullValue >= value_mate_in(PLY_MAX))
1412 if (depth < 6 * OnePly)
1415 // Do zugzwang verification search
1416 Value v = search(pos, ss, beta, depth-5*OnePly, ply, false, threadID);
1420 // The null move failed low, which means that we may be faced with
1421 // some kind of threat. If the previous move was reduced, check if
1422 // the move that refuted the null move was somehow connected to the
1423 // move which was reduced. If a connection is found, return a fail
1424 // low score (which will cause the reduced move to fail high in the
1425 // parent node, which will trigger a re-search with full depth).
1426 if (nullValue == value_mated_in(ply + 2))
1429 ss[ply].threatMove = ss[ply + 1].currentMove;
1430 if ( depth < ThreatDepth
1431 && ss[ply - 1].reduction
1432 && connected_moves(pos, ss[ply - 1].currentMove, ss[ply].threatMove))
1437 // Step 9. Internal iterative deepening
1438 if ( depth >= IIDDepthAtNonPVNodes
1439 && ttMove == MOVE_NONE
1441 && ss[ply].eval >= beta - IIDMargin)
1443 search(pos, ss, beta, depth/2, ply, false, threadID);
1444 ttMove = ss[ply].pv[ply];
1445 tte = TT.retrieve(posKey);
1448 // Step 10. Loop through moves
1449 // Loop through all legal moves until no moves remain or a beta cutoff occurs
1451 // Initialize a MovePicker object for the current position
1452 MovePicker mp = MovePicker(pos, ttMove, depth, H, &ss[ply], beta);
1455 while ( bestValue < beta
1456 && (move = mp.get_next_move()) != MOVE_NONE
1457 && !TM.thread_should_stop(threadID))
1459 assert(move_is_ok(move));
1461 if (move == excludedMove)
1464 moveIsCheck = pos.move_is_check(move, ci);
1465 singleEvasion = (isCheck && mp.number_of_evasions() == 1);
1466 captureOrPromotion = pos.move_is_capture_or_promotion(move);
1468 // Step 11. Decide the new search depth
1469 ext = extension(pos, move, false, captureOrPromotion, moveIsCheck, singleEvasion, mateThreat, &dangerous);
1471 // Singular extension search. We extend the TT move if its value is much better than
1472 // its siblings. To verify this we do a reduced search on all the other moves but the
1473 // ttMove, if result is lower then ttValue minus a margin then we extend ttMove.
1474 if ( depth >= SingularExtensionDepthAtNonPVNodes
1476 && move == tte->move()
1477 && !excludedMove // Do not allow recursive single-reply search
1479 && is_lower_bound(tte->type())
1480 && tte->depth() >= depth - 3 * OnePly)
1482 Value ttValue = value_from_tt(tte->value(), ply);
1484 if (abs(ttValue) < VALUE_KNOWN_WIN)
1486 Value excValue = search(pos, ss, ttValue - SingularExtensionMargin, depth / 2, ply, false, threadID, move);
1488 if (excValue < ttValue - SingularExtensionMargin)
1493 newDepth = depth - OnePly + ext;
1495 // Update current move (this must be done after singular extension search)
1496 movesSearched[moveCount++] = ss[ply].currentMove = move;
1498 // Step 12. Futility pruning
1501 && !captureOrPromotion
1502 && !move_is_castle(move)
1505 // Move count based pruning
1506 if ( moveCount >= futility_move_count(depth)
1507 && ok_to_prune(pos, move, ss[ply].threatMove)
1508 && bestValue > value_mated_in(PLY_MAX))
1511 // Value based pruning
1512 Depth predictedDepth = newDepth - nonpv_reduction(depth, moveCount); // We illogically ignore reduction condition depth >= 3*OnePly
1513 futilityValueScaled = ss[ply].eval + futility_margin(predictedDepth, moveCount)
1514 + H.gain(pos.piece_on(move_from(move)), move_to(move)) + 45;
1516 if (futilityValueScaled < beta)
1518 if (futilityValueScaled > bestValue)
1519 bestValue = futilityValueScaled;
1524 // Step 13. Make the move
1525 pos.do_move(move, st, ci, moveIsCheck);
1527 // Step 14. Reduced search
1528 // if the move fails high will be re-searched at full depth.
1529 bool doFullDepthSearch = true;
1531 if ( depth >= 3*OnePly
1533 && !captureOrPromotion
1534 && !move_is_castle(move)
1535 && !move_is_killer(move, ss[ply]))
1537 ss[ply].reduction = nonpv_reduction(depth, moveCount);
1538 if (ss[ply].reduction)
1540 value = -search(pos, ss, -(beta-1), newDepth-ss[ply].reduction, ply+1, true, threadID);
1541 doFullDepthSearch = (value >= beta);
1545 // Step 15. Full depth search
1546 if (doFullDepthSearch)
1548 ss[ply].reduction = Depth(0);
1549 value = -search(pos, ss, -(beta-1), newDepth, ply+1, true, threadID);
1552 // Step 16. Undo move
1553 pos.undo_move(move);
1555 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1557 // Step 17. Check for new best move
1558 if (value > bestValue)
1564 if (value == value_mate_in(ply + 1))
1565 ss[ply].mateKiller = move;
1568 // Step 18. Check for split
1569 if ( TM.active_threads() > 1
1571 && depth >= MinimumSplitDepth
1573 && TM.available_thread_exists(threadID)
1575 && !TM.thread_should_stop(threadID)
1576 && TM.split(pos, ss, ply, NULL, beta, &bestValue,
1577 depth, mateThreat, &moveCount, &mp, threadID, false))
1581 // Step 19. Check for mate and stalemate
1582 // All legal moves have been searched and if there were
1583 // no legal moves, it must be mate or stalemate.
1584 // If one move was excluded return fail low.
1586 return excludedMove ? beta - 1 : (pos.is_check() ? value_mated_in(ply) : VALUE_DRAW);
1588 // Step 20. Update tables
1589 // If the search is not aborted, update the transposition table,
1590 // history counters, and killer moves.
1591 if (AbortSearch || TM.thread_should_stop(threadID))
1594 if (bestValue < beta)
1595 TT.store(posKey, value_to_tt(bestValue, ply), VALUE_TYPE_UPPER, depth, MOVE_NONE);
1598 TM.incrementBetaCounter(pos.side_to_move(), depth, threadID);
1599 move = ss[ply].pv[ply];
1600 TT.store(posKey, value_to_tt(bestValue, ply), VALUE_TYPE_LOWER, depth, move);
1601 if (!pos.move_is_capture_or_promotion(move))
1603 update_history(pos, move, depth, movesSearched, moveCount);
1604 update_killers(move, ss[ply]);
1609 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1615 // qsearch() is the quiescence search function, which is called by the main
1616 // search function when the remaining depth is zero (or, to be more precise,
1617 // less than OnePly).
1619 Value qsearch(Position& pos, SearchStack ss[], Value alpha, Value beta,
1620 Depth depth, int ply, int threadID) {
1622 assert(alpha >= -VALUE_INFINITE && alpha <= VALUE_INFINITE);
1623 assert(beta >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
1625 assert(ply >= 0 && ply < PLY_MAX);
1626 assert(threadID >= 0 && threadID < TM.active_threads());
1631 Value staticValue, bestValue, value, futilityBase, futilityValue;
1632 bool isCheck, enoughMaterial, moveIsCheck, evasionPrunable;
1633 const TTEntry* tte = NULL;
1635 bool pvNode = (beta - alpha != 1);
1636 Value oldAlpha = alpha;
1638 // Initialize, and make an early exit in case of an aborted search,
1639 // an instant draw, maximum ply reached, etc.
1640 init_node(ss, ply, threadID);
1642 // After init_node() that calls poll()
1643 if (AbortSearch || TM.thread_should_stop(threadID))
1646 if (pos.is_draw() || ply >= PLY_MAX - 1)
1649 // Transposition table lookup. At PV nodes, we don't use the TT for
1650 // pruning, but only for move ordering.
1651 tte = TT.retrieve(pos.get_key());
1652 ttMove = (tte ? tte->move() : MOVE_NONE);
1654 if (!pvNode && tte && ok_to_use_TT(tte, depth, beta, ply))
1656 assert(tte->type() != VALUE_TYPE_EVAL);
1658 ss[ply].currentMove = ttMove; // Can be MOVE_NONE
1659 return value_from_tt(tte->value(), ply);
1662 isCheck = pos.is_check();
1664 // Evaluate the position statically
1666 staticValue = -VALUE_INFINITE;
1667 else if (tte && (tte->type() & VALUE_TYPE_EVAL))
1668 staticValue = value_from_tt(tte->value(), ply);
1670 staticValue = evaluate(pos, ei, threadID);
1674 ss[ply].eval = staticValue;
1675 update_gains(pos, ss[ply - 1].currentMove, ss[ply - 1].eval, ss[ply].eval);
1678 // Initialize "stand pat score", and return it immediately if it is
1680 bestValue = staticValue;
1682 if (bestValue >= beta)
1684 // Store the score to avoid a future costly evaluation() call
1685 if (!isCheck && !tte && ei.futilityMargin[pos.side_to_move()] == 0)
1686 TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_EV_LO, Depth(-127*OnePly), MOVE_NONE);
1691 if (bestValue > alpha)
1694 // If we are near beta then try to get a cutoff pushing checks a bit further
1695 bool deepChecks = depth == -OnePly && staticValue >= beta - PawnValueMidgame / 8;
1697 // Initialize a MovePicker object for the current position, and prepare
1698 // to search the moves. Because the depth is <= 0 here, only captures,
1699 // queen promotions and checks (only if depth == 0 or depth == -OnePly
1700 // and we are near beta) will be generated.
1701 MovePicker mp = MovePicker(pos, ttMove, deepChecks ? Depth(0) : depth, H);
1703 enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame;
1704 futilityBase = staticValue + FutilityMarginQS + ei.futilityMargin[pos.side_to_move()];
1706 // Loop through the moves until no moves remain or a beta cutoff
1708 while ( alpha < beta
1709 && (move = mp.get_next_move()) != MOVE_NONE)
1711 assert(move_is_ok(move));
1713 moveIsCheck = pos.move_is_check(move, ci);
1715 // Update current move
1717 ss[ply].currentMove = move;
1725 && !move_is_promotion(move)
1726 && !pos.move_is_passed_pawn_push(move))
1728 futilityValue = futilityBase
1729 + pos.endgame_value_of_piece_on(move_to(move))
1730 + (move_is_ep(move) ? PawnValueEndgame : Value(0));
1732 if (futilityValue < alpha)
1734 if (futilityValue > bestValue)
1735 bestValue = futilityValue;
1740 // Detect blocking evasions that are candidate to be pruned
1741 evasionPrunable = isCheck
1742 && bestValue != -VALUE_INFINITE
1743 && !pos.move_is_capture(move)
1744 && pos.type_of_piece_on(move_from(move)) != KING
1745 && !pos.can_castle(pos.side_to_move());
1747 // Don't search moves with negative SEE values
1748 if ( (!isCheck || evasionPrunable)
1751 && !move_is_promotion(move)
1752 && pos.see_sign(move) < 0)
1755 // Make and search the move
1756 pos.do_move(move, st, ci, moveIsCheck);
1757 value = -qsearch(pos, ss, -beta, -alpha, depth-OnePly, ply+1, threadID);
1758 pos.undo_move(move);
1760 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1763 if (value > bestValue)
1774 // All legal moves have been searched. A special case: If we're in check
1775 // and no legal moves were found, it is checkmate.
1776 if (!moveCount && pos.is_check()) // Mate!
1777 return value_mated_in(ply);
1779 // Update transposition table
1780 Depth d = (depth == Depth(0) ? Depth(0) : Depth(-1));
1781 if (bestValue <= oldAlpha)
1783 // If bestValue isn't changed it means it is still the static evaluation
1784 // of the node, so keep this info to avoid a future evaluation() call.
1785 ValueType type = (bestValue == staticValue && !ei.futilityMargin[pos.side_to_move()] ? VALUE_TYPE_EV_UP : VALUE_TYPE_UPPER);
1786 TT.store(pos.get_key(), value_to_tt(bestValue, ply), type, d, MOVE_NONE);
1788 else if (bestValue >= beta)
1790 move = ss[ply].pv[ply];
1791 TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_LOWER, d, move);
1793 // Update killers only for good checking moves
1794 if (!pos.move_is_capture_or_promotion(move))
1795 update_killers(move, ss[ply]);
1798 TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_EXACT, d, ss[ply].pv[ply]);
1800 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1806 // sp_search() is used to search from a split point. This function is called
1807 // by each thread working at the split point. It is similar to the normal
1808 // search() function, but simpler. Because we have already probed the hash
1809 // table, done a null move search, and searched the first move before
1810 // splitting, we don't have to repeat all this work in sp_search(). We
1811 // also don't need to store anything to the hash table here: This is taken
1812 // care of after we return from the split point.
1814 void sp_search(SplitPoint* sp, int threadID) {
1816 assert(threadID >= 0 && threadID < TM.active_threads());
1817 assert(TM.active_threads() > 1);
1821 Depth ext, newDepth;
1822 Value value, futilityValueScaled;
1823 bool isCheck, moveIsCheck, captureOrPromotion, dangerous;
1825 value = -VALUE_INFINITE;
1827 Position pos(*sp->pos);
1829 SearchStack* ss = sp->sstack[threadID];
1830 isCheck = pos.is_check();
1832 // Step 10. Loop through moves
1833 // Loop through all legal moves until no moves remain or a beta cutoff occurs
1834 lock_grab(&(sp->lock));
1836 while ( sp->bestValue < sp->beta
1837 && !TM.thread_should_stop(threadID)
1838 && (move = sp->mp->get_next_move()) != MOVE_NONE)
1840 moveCount = ++sp->moves;
1841 lock_release(&(sp->lock));
1843 assert(move_is_ok(move));
1845 moveIsCheck = pos.move_is_check(move, ci);
1846 captureOrPromotion = pos.move_is_capture_or_promotion(move);
1848 // Step 11. Decide the new search depth
1849 ext = extension(pos, move, false, captureOrPromotion, moveIsCheck, false, sp->mateThreat, &dangerous);
1850 newDepth = sp->depth - OnePly + ext;
1852 // Update current move
1853 ss[sp->ply].currentMove = move;
1855 // Step 12. Futility pruning
1858 && !captureOrPromotion
1859 && !move_is_castle(move))
1861 // Move count based pruning
1862 if ( moveCount >= futility_move_count(sp->depth)
1863 && ok_to_prune(pos, move, ss[sp->ply].threatMove)
1864 && sp->bestValue > value_mated_in(PLY_MAX))
1866 lock_grab(&(sp->lock));
1870 // Value based pruning
1871 Depth predictedDepth = newDepth - nonpv_reduction(sp->depth, moveCount);
1872 futilityValueScaled = ss[sp->ply].eval + futility_margin(predictedDepth, moveCount)
1873 + H.gain(pos.piece_on(move_from(move)), move_to(move)) + 45;
1875 if (futilityValueScaled < sp->beta)
1877 lock_grab(&(sp->lock));
1879 if (futilityValueScaled > sp->bestValue)
1880 sp->bestValue = futilityValueScaled;
1885 // Step 13. Make the move
1886 pos.do_move(move, st, ci, moveIsCheck);
1888 // Step 14. Reduced search
1889 // if the move fails high will be re-searched at full depth.
1890 bool doFullDepthSearch = true;
1893 && !captureOrPromotion
1894 && !move_is_castle(move)
1895 && !move_is_killer(move, ss[sp->ply]))
1897 ss[sp->ply].reduction = nonpv_reduction(sp->depth, moveCount);
1898 if (ss[sp->ply].reduction)
1900 value = -search(pos, ss, -(sp->beta-1), newDepth-ss[sp->ply].reduction, sp->ply+1, true, threadID);
1901 doFullDepthSearch = (value >= sp->beta && !TM.thread_should_stop(threadID));
1905 // Step 15. Full depth search
1906 if (doFullDepthSearch)
1908 ss[sp->ply].reduction = Depth(0);
1909 value = -search(pos, ss, -(sp->beta - 1), newDepth, sp->ply+1, true, threadID);
1912 // Step 16. Undo move
1913 pos.undo_move(move);
1915 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1917 // Step 17. Check for new best move
1918 lock_grab(&(sp->lock));
1920 if (value > sp->bestValue && !TM.thread_should_stop(threadID))
1922 sp->bestValue = value;
1923 if (sp->bestValue >= sp->beta)
1925 sp->stopRequest = true;
1926 sp_update_pv(sp->parentSstack, ss, sp->ply);
1931 /* Here we have the lock still grabbed */
1933 sp->slaves[threadID] = 0;
1936 lock_release(&(sp->lock));
1940 // sp_search_pv() is used to search from a PV split point. This function
1941 // is called by each thread working at the split point. It is similar to
1942 // the normal search_pv() function, but simpler. Because we have already
1943 // probed the hash table and searched the first move before splitting, we
1944 // don't have to repeat all this work in sp_search_pv(). We also don't
1945 // need to store anything to the hash table here: This is taken care of
1946 // after we return from the split point.
1948 void sp_search_pv(SplitPoint* sp, int threadID) {
1950 assert(threadID >= 0 && threadID < TM.active_threads());
1951 assert(TM.active_threads() > 1);
1955 Depth ext, newDepth;
1957 bool moveIsCheck, captureOrPromotion, dangerous;
1959 value = -VALUE_INFINITE;
1961 Position pos(*sp->pos);
1963 SearchStack* ss = sp->sstack[threadID];
1965 // Step 10. Loop through moves
1966 // Loop through all legal moves until no moves remain or a beta cutoff occurs
1967 lock_grab(&(sp->lock));
1969 while ( sp->alpha < sp->beta
1970 && !TM.thread_should_stop(threadID)
1971 && (move = sp->mp->get_next_move()) != MOVE_NONE)
1973 moveCount = ++sp->moves;
1974 lock_release(&(sp->lock));
1976 assert(move_is_ok(move));
1978 moveIsCheck = pos.move_is_check(move, ci);
1979 captureOrPromotion = pos.move_is_capture_or_promotion(move);
1981 // Step 11. Decide the new search depth
1982 ext = extension(pos, move, true, captureOrPromotion, moveIsCheck, false, sp->mateThreat, &dangerous);
1983 newDepth = sp->depth - OnePly + ext;
1985 // Update current move
1986 ss[sp->ply].currentMove = move;
1988 // Step 12. Futility pruning (is omitted in PV nodes)
1990 // Step 13. Make the move
1991 pos.do_move(move, st, ci, moveIsCheck);
1993 // Step 14. Reduced search
1994 // if the move fails high will be re-searched at full depth.
1995 bool doFullDepthSearch = true;
1998 && !captureOrPromotion
1999 && !move_is_castle(move)
2000 && !move_is_killer(move, ss[sp->ply]))
2002 ss[sp->ply].reduction = pv_reduction(sp->depth, moveCount);
2003 if (ss[sp->ply].reduction)
2005 Value localAlpha = sp->alpha;
2006 value = -search(pos, ss, -localAlpha, newDepth-ss[sp->ply].reduction, sp->ply+1, true, threadID);
2007 doFullDepthSearch = (value > localAlpha && !TM.thread_should_stop(threadID));
2011 // Step 15. Full depth search
2012 if (doFullDepthSearch)
2014 Value localAlpha = sp->alpha;
2015 ss[sp->ply].reduction = Depth(0);
2016 value = -search(pos, ss, -localAlpha, newDepth, sp->ply+1, true, threadID);
2018 if (value > localAlpha && value < sp->beta && !TM.thread_should_stop(threadID))
2020 // If another thread has failed high then sp->alpha has been increased
2021 // to be higher or equal then beta, if so, avoid to start a PV search.
2022 localAlpha = sp->alpha;
2023 if (localAlpha < sp->beta)
2024 value = -search_pv(pos, ss, -sp->beta, -localAlpha, newDepth, sp->ply+1, threadID);
2028 // Step 16. Undo move
2029 pos.undo_move(move);
2031 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
2033 // Step 17. Check for new best move
2034 lock_grab(&(sp->lock));
2036 if (value > sp->bestValue && !TM.thread_should_stop(threadID))
2038 sp->bestValue = value;
2039 if (value > sp->alpha)
2041 // Ask threads to stop before to modify sp->alpha
2042 if (value >= sp->beta)
2043 sp->stopRequest = true;
2047 sp_update_pv(sp->parentSstack, ss, sp->ply);
2048 if (value == value_mate_in(sp->ply + 1))
2049 ss[sp->ply].mateKiller = move;
2054 /* Here we have the lock still grabbed */
2056 sp->slaves[threadID] = 0;
2059 lock_release(&(sp->lock));
2063 // init_node() is called at the beginning of all the search functions
2064 // (search(), search_pv(), qsearch(), and so on) and initializes the
2065 // search stack object corresponding to the current node. Once every
2066 // NodesBetweenPolls nodes, init_node() also calls poll(), which polls
2067 // for user input and checks whether it is time to stop the search.
2069 void init_node(SearchStack ss[], int ply, int threadID) {
2071 assert(ply >= 0 && ply < PLY_MAX);
2072 assert(threadID >= 0 && threadID < TM.active_threads());
2074 TM.incrementNodeCounter(threadID);
2079 if (NodesSincePoll >= NodesBetweenPolls)
2086 ss[ply + 2].initKillers();
2090 // update_pv() is called whenever a search returns a value > alpha.
2091 // It updates the PV in the SearchStack object corresponding to the
2094 void update_pv(SearchStack ss[], int ply) {
2096 assert(ply >= 0 && ply < PLY_MAX);
2100 ss[ply].pv[ply] = ss[ply].currentMove;
2102 for (p = ply + 1; ss[ply + 1].pv[p] != MOVE_NONE; p++)
2103 ss[ply].pv[p] = ss[ply + 1].pv[p];
2105 ss[ply].pv[p] = MOVE_NONE;
2109 // sp_update_pv() is a variant of update_pv for use at split points. The
2110 // difference between the two functions is that sp_update_pv also updates
2111 // the PV at the parent node.
2113 void sp_update_pv(SearchStack* pss, SearchStack ss[], int ply) {
2115 assert(ply >= 0 && ply < PLY_MAX);
2119 ss[ply].pv[ply] = pss[ply].pv[ply] = ss[ply].currentMove;
2121 for (p = ply + 1; ss[ply + 1].pv[p] != MOVE_NONE; p++)
2122 ss[ply].pv[p] = pss[ply].pv[p] = ss[ply + 1].pv[p];
2124 ss[ply].pv[p] = pss[ply].pv[p] = MOVE_NONE;
2128 // connected_moves() tests whether two moves are 'connected' in the sense
2129 // that the first move somehow made the second move possible (for instance
2130 // if the moving piece is the same in both moves). The first move is assumed
2131 // to be the move that was made to reach the current position, while the
2132 // second move is assumed to be a move from the current position.
2134 bool connected_moves(const Position& pos, Move m1, Move m2) {
2136 Square f1, t1, f2, t2;
2139 assert(move_is_ok(m1));
2140 assert(move_is_ok(m2));
2142 if (m2 == MOVE_NONE)
2145 // Case 1: The moving piece is the same in both moves
2151 // Case 2: The destination square for m2 was vacated by m1
2157 // Case 3: Moving through the vacated square
2158 if ( piece_is_slider(pos.piece_on(f2))
2159 && bit_is_set(squares_between(f2, t2), f1))
2162 // Case 4: The destination square for m2 is defended by the moving piece in m1
2163 p = pos.piece_on(t1);
2164 if (bit_is_set(pos.attacks_from(p, t1), t2))
2167 // Case 5: Discovered check, checking piece is the piece moved in m1
2168 if ( piece_is_slider(p)
2169 && bit_is_set(squares_between(t1, pos.king_square(pos.side_to_move())), f2)
2170 && !bit_is_set(squares_between(t1, pos.king_square(pos.side_to_move())), t2))
2172 // discovered_check_candidates() works also if the Position's side to
2173 // move is the opposite of the checking piece.
2174 Color them = opposite_color(pos.side_to_move());
2175 Bitboard dcCandidates = pos.discovered_check_candidates(them);
2177 if (bit_is_set(dcCandidates, f2))
2184 // value_is_mate() checks if the given value is a mate one
2185 // eventually compensated for the ply.
2187 bool value_is_mate(Value value) {
2189 assert(abs(value) <= VALUE_INFINITE);
2191 return value <= value_mated_in(PLY_MAX)
2192 || value >= value_mate_in(PLY_MAX);
2196 // move_is_killer() checks if the given move is among the
2197 // killer moves of that ply.
2199 bool move_is_killer(Move m, const SearchStack& ss) {
2201 const Move* k = ss.killers;
2202 for (int i = 0; i < KILLER_MAX; i++, k++)
2210 // extension() decides whether a move should be searched with normal depth,
2211 // or with extended depth. Certain classes of moves (checking moves, in
2212 // particular) are searched with bigger depth than ordinary moves and in
2213 // any case are marked as 'dangerous'. Note that also if a move is not
2214 // extended, as example because the corresponding UCI option is set to zero,
2215 // the move is marked as 'dangerous' so, at least, we avoid to prune it.
2217 Depth extension(const Position& pos, Move m, bool pvNode, bool captureOrPromotion,
2218 bool moveIsCheck, bool singleEvasion, bool mateThreat, bool* dangerous) {
2220 assert(m != MOVE_NONE);
2222 Depth result = Depth(0);
2223 *dangerous = moveIsCheck | singleEvasion | mateThreat;
2228 result += CheckExtension[pvNode];
2231 result += SingleEvasionExtension[pvNode];
2234 result += MateThreatExtension[pvNode];
2237 if (pos.type_of_piece_on(move_from(m)) == PAWN)
2239 Color c = pos.side_to_move();
2240 if (relative_rank(c, move_to(m)) == RANK_7)
2242 result += PawnPushTo7thExtension[pvNode];
2245 if (pos.pawn_is_passed(c, move_to(m)))
2247 result += PassedPawnExtension[pvNode];
2252 if ( captureOrPromotion
2253 && pos.type_of_piece_on(move_to(m)) != PAWN
2254 && ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
2255 - pos.midgame_value_of_piece_on(move_to(m)) == Value(0))
2256 && !move_is_promotion(m)
2259 result += PawnEndgameExtension[pvNode];
2264 && captureOrPromotion
2265 && pos.type_of_piece_on(move_to(m)) != PAWN
2266 && pos.see_sign(m) >= 0)
2272 return Min(result, OnePly);
2276 // ok_to_do_nullmove() looks at the current position and decides whether
2277 // doing a 'null move' should be allowed. In order to avoid zugzwang
2278 // problems, null moves are not allowed when the side to move has very
2279 // little material left. Currently, the test is a bit too simple: Null
2280 // moves are avoided only when the side to move has only pawns left.
2281 // It's probably a good idea to avoid null moves in at least some more
2282 // complicated endgames, e.g. KQ vs KR. FIXME
2284 bool ok_to_do_nullmove(const Position& pos) {
2286 return pos.non_pawn_material(pos.side_to_move()) != Value(0);
2290 // ok_to_prune() tests whether it is safe to forward prune a move. Only
2291 // non-tactical moves late in the move list close to the leaves are
2292 // candidates for pruning.
2294 bool ok_to_prune(const Position& pos, Move m, Move threat) {
2296 assert(move_is_ok(m));
2297 assert(threat == MOVE_NONE || move_is_ok(threat));
2298 assert(!pos.move_is_check(m));
2299 assert(!pos.move_is_capture_or_promotion(m));
2300 assert(!pos.move_is_passed_pawn_push(m));
2302 Square mfrom, mto, tfrom, tto;
2304 // Prune if there isn't any threat move
2305 if (threat == MOVE_NONE)
2308 mfrom = move_from(m);
2310 tfrom = move_from(threat);
2311 tto = move_to(threat);
2313 // Case 1: Don't prune moves which move the threatened piece
2317 // Case 2: If the threatened piece has value less than or equal to the
2318 // value of the threatening piece, don't prune move which defend it.
2319 if ( pos.move_is_capture(threat)
2320 && ( pos.midgame_value_of_piece_on(tfrom) >= pos.midgame_value_of_piece_on(tto)
2321 || pos.type_of_piece_on(tfrom) == KING)
2322 && pos.move_attacks_square(m, tto))
2325 // Case 3: If the moving piece in the threatened move is a slider, don't
2326 // prune safe moves which block its ray.
2327 if ( piece_is_slider(pos.piece_on(tfrom))
2328 && bit_is_set(squares_between(tfrom, tto), mto)
2329 && pos.see_sign(m) >= 0)
2336 // ok_to_use_TT() returns true if a transposition table score
2337 // can be used at a given point in search.
2339 bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply) {
2341 Value v = value_from_tt(tte->value(), ply);
2343 return ( tte->depth() >= depth
2344 || v >= Max(value_mate_in(PLY_MAX), beta)
2345 || v < Min(value_mated_in(PLY_MAX), beta))
2347 && ( (is_lower_bound(tte->type()) && v >= beta)
2348 || (is_upper_bound(tte->type()) && v < beta));
2352 // refine_eval() returns the transposition table score if
2353 // possible otherwise falls back on static position evaluation.
2355 Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
2360 Value v = value_from_tt(tte->value(), ply);
2362 if ( (is_lower_bound(tte->type()) && v >= defaultEval)
2363 || (is_upper_bound(tte->type()) && v < defaultEval))
2370 // update_history() registers a good move that produced a beta-cutoff
2371 // in history and marks as failures all the other moves of that ply.
2373 void update_history(const Position& pos, Move move, Depth depth,
2374 Move movesSearched[], int moveCount) {
2378 H.success(pos.piece_on(move_from(move)), move_to(move), depth);
2380 for (int i = 0; i < moveCount - 1; i++)
2382 m = movesSearched[i];
2386 if (!pos.move_is_capture_or_promotion(m))
2387 H.failure(pos.piece_on(move_from(m)), move_to(m), depth);
2392 // update_killers() add a good move that produced a beta-cutoff
2393 // among the killer moves of that ply.
2395 void update_killers(Move m, SearchStack& ss) {
2397 if (m == ss.killers[0])
2400 for (int i = KILLER_MAX - 1; i > 0; i--)
2401 ss.killers[i] = ss.killers[i - 1];
2407 // update_gains() updates the gains table of a non-capture move given
2408 // the static position evaluation before and after the move.
2410 void update_gains(const Position& pos, Move m, Value before, Value after) {
2413 && before != VALUE_NONE
2414 && after != VALUE_NONE
2415 && pos.captured_piece() == NO_PIECE_TYPE
2416 && !move_is_castle(m)
2417 && !move_is_promotion(m))
2418 H.set_gain(pos.piece_on(move_to(m)), move_to(m), -(before + after));
2422 // current_search_time() returns the number of milliseconds which have passed
2423 // since the beginning of the current search.
2425 int current_search_time() {
2427 return get_system_time() - SearchStartTime;
2431 // nps() computes the current nodes/second count.
2435 int t = current_search_time();
2436 return (t > 0 ? int((TM.nodes_searched() * 1000) / t) : 0);
2440 // poll() performs two different functions: It polls for user input, and it
2441 // looks at the time consumed so far and decides if it's time to abort the
2444 void poll(SearchStack ss[], int ply) {
2446 static int lastInfoTime;
2447 int t = current_search_time();
2452 // We are line oriented, don't read single chars
2453 std::string command;
2455 if (!std::getline(std::cin, command))
2458 if (command == "quit")
2461 PonderSearch = false;
2465 else if (command == "stop")
2468 PonderSearch = false;
2470 else if (command == "ponderhit")
2474 // Print search information
2478 else if (lastInfoTime > t)
2479 // HACK: Must be a new search where we searched less than
2480 // NodesBetweenPolls nodes during the first second of search.
2483 else if (t - lastInfoTime >= 1000)
2490 if (dbg_show_hit_rate)
2491 dbg_print_hit_rate();
2493 cout << "info nodes " << TM.nodes_searched() << " nps " << nps()
2494 << " time " << t << " hashfull " << TT.full() << endl;
2496 // We only support current line printing in single thread mode
2497 if (ShowCurrentLine && TM.active_threads() == 1)
2499 cout << "info currline";
2500 for (int p = 0; p < ply; p++)
2501 cout << " " << ss[p].currentMove;
2507 // Should we stop the search?
2511 bool stillAtFirstMove = FirstRootMove
2512 && !AspirationFailLow
2513 && t > MaxSearchTime + ExtraSearchTime;
2515 bool noMoreTime = t > AbsoluteMaxSearchTime
2516 || stillAtFirstMove;
2518 if ( (Iteration >= 3 && UseTimeManagement && noMoreTime)
2519 || (ExactMaxTime && t >= ExactMaxTime)
2520 || (Iteration >= 3 && MaxNodes && TM.nodes_searched() >= MaxNodes))
2525 // ponderhit() is called when the program is pondering (i.e. thinking while
2526 // it's the opponent's turn to move) in order to let the engine know that
2527 // it correctly predicted the opponent's move.
2531 int t = current_search_time();
2532 PonderSearch = false;
2534 bool stillAtFirstMove = FirstRootMove
2535 && !AspirationFailLow
2536 && t > MaxSearchTime + ExtraSearchTime;
2538 bool noMoreTime = t > AbsoluteMaxSearchTime
2539 || stillAtFirstMove;
2541 if (Iteration >= 3 && UseTimeManagement && (noMoreTime || StopOnPonderhit))
2546 // init_ss_array() does a fast reset of the first entries of a SearchStack array
2548 void init_ss_array(SearchStack ss[]) {
2550 for (int i = 0; i < 3; i++)
2553 ss[i].initKillers();
2558 // wait_for_stop_or_ponderhit() is called when the maximum depth is reached
2559 // while the program is pondering. The point is to work around a wrinkle in
2560 // the UCI protocol: When pondering, the engine is not allowed to give a
2561 // "bestmove" before the GUI sends it a "stop" or "ponderhit" command.
2562 // We simply wait here until one of these commands is sent, and return,
2563 // after which the bestmove and pondermove will be printed (in id_loop()).
2565 void wait_for_stop_or_ponderhit() {
2567 std::string command;
2571 if (!std::getline(std::cin, command))
2574 if (command == "quit")
2579 else if (command == "ponderhit" || command == "stop")
2585 // print_pv_info() prints to standard output and eventually to log file information on
2586 // the current PV line. It is called at each iteration or after a new pv is found.
2588 void print_pv_info(const Position& pos, SearchStack ss[], Value alpha, Value beta, Value value) {
2590 cout << "info depth " << Iteration
2591 << " score " << value_to_string(value)
2592 << ((value >= beta) ? " lowerbound" :
2593 ((value <= alpha)? " upperbound" : ""))
2594 << " time " << current_search_time()
2595 << " nodes " << TM.nodes_searched()
2599 for (int j = 0; ss[0].pv[j] != MOVE_NONE && j < PLY_MAX; j++)
2600 cout << ss[0].pv[j] << " ";
2606 ValueType type = (value >= beta ? VALUE_TYPE_LOWER
2607 : (value <= alpha ? VALUE_TYPE_UPPER : VALUE_TYPE_EXACT));
2609 LogFile << pretty_pv(pos, current_search_time(), Iteration,
2610 TM.nodes_searched(), value, type, ss[0].pv) << endl;
2615 // init_thread() is the function which is called when a new thread is
2616 // launched. It simply calls the idle_loop() function with the supplied
2617 // threadID. There are two versions of this function; one for POSIX
2618 // threads and one for Windows threads.
2620 #if !defined(_MSC_VER)
2622 void* init_thread(void *threadID) {
2624 TM.idle_loop(*(int*)threadID, NULL);
2630 DWORD WINAPI init_thread(LPVOID threadID) {
2632 TM.idle_loop(*(int*)threadID, NULL);
2639 /// The ThreadsManager class
2641 // resetNodeCounters(), resetBetaCounters(), searched_nodes() and
2642 // get_beta_counters() are getters/setters for the per thread
2643 // counters used to sort the moves at root.
2645 void ThreadsManager::resetNodeCounters() {
2647 for (int i = 0; i < MAX_THREADS; i++)
2648 threads[i].nodes = 0ULL;
2651 void ThreadsManager::resetBetaCounters() {
2653 for (int i = 0; i < MAX_THREADS; i++)
2654 threads[i].betaCutOffs[WHITE] = threads[i].betaCutOffs[BLACK] = 0ULL;
2657 int64_t ThreadsManager::nodes_searched() const {
2659 int64_t result = 0ULL;
2660 for (int i = 0; i < ActiveThreads; i++)
2661 result += threads[i].nodes;
2666 void ThreadsManager::get_beta_counters(Color us, int64_t& our, int64_t& their) const {
2669 for (int i = 0; i < MAX_THREADS; i++)
2671 our += threads[i].betaCutOffs[us];
2672 their += threads[i].betaCutOffs[opposite_color(us)];
2677 // idle_loop() is where the threads are parked when they have no work to do.
2678 // The parameter "waitSp", if non-NULL, is a pointer to an active SplitPoint
2679 // object for which the current thread is the master.
2681 void ThreadsManager::idle_loop(int threadID, SplitPoint* waitSp) {
2683 assert(threadID >= 0 && threadID < MAX_THREADS);
2687 // Slave threads can exit as soon as AllThreadsShouldExit raises,
2688 // master should exit as last one.
2689 if (AllThreadsShouldExit)
2692 threads[threadID].state = THREAD_TERMINATED;
2696 // If we are not thinking, wait for a condition to be signaled
2697 // instead of wasting CPU time polling for work.
2698 while (AllThreadsShouldSleep || threadID >= ActiveThreads)
2701 assert(threadID != 0);
2702 threads[threadID].state = THREAD_SLEEPING;
2704 #if !defined(_MSC_VER)
2705 lock_grab(&WaitLock);
2706 if (AllThreadsShouldSleep || threadID >= ActiveThreads)
2707 pthread_cond_wait(&WaitCond, &WaitLock);
2708 lock_release(&WaitLock);
2710 WaitForSingleObject(SitIdleEvent[threadID], INFINITE);
2714 // If thread has just woken up, mark it as available
2715 if (threads[threadID].state == THREAD_SLEEPING)
2716 threads[threadID].state = THREAD_AVAILABLE;
2718 // If this thread has been assigned work, launch a search
2719 if (threads[threadID].state == THREAD_WORKISWAITING)
2721 assert(!AllThreadsShouldExit && !AllThreadsShouldSleep);
2723 threads[threadID].state = THREAD_SEARCHING;
2725 if (threads[threadID].splitPoint->pvNode)
2726 sp_search_pv(threads[threadID].splitPoint, threadID);
2728 sp_search(threads[threadID].splitPoint, threadID);
2730 assert(threads[threadID].state == THREAD_SEARCHING);
2732 threads[threadID].state = THREAD_AVAILABLE;
2735 // If this thread is the master of a split point and all threads have
2736 // finished their work at this split point, return from the idle loop.
2737 if (waitSp != NULL && waitSp->cpus == 0)
2739 assert(threads[threadID].state == THREAD_AVAILABLE);
2741 threads[threadID].state = THREAD_SEARCHING;
2748 // init_threads() is called during startup. It launches all helper threads,
2749 // and initializes the split point stack and the global locks and condition
2752 void ThreadsManager::init_threads() {
2757 #if !defined(_MSC_VER)
2758 pthread_t pthread[1];
2761 // Initialize global locks
2762 lock_init(&MPLock, NULL);
2763 lock_init(&WaitLock, NULL);
2765 #if !defined(_MSC_VER)
2766 pthread_cond_init(&WaitCond, NULL);
2768 for (i = 0; i < MAX_THREADS; i++)
2769 SitIdleEvent[i] = CreateEvent(0, FALSE, FALSE, 0);
2772 // Initialize SplitPointStack locks
2773 for (i = 0; i < MAX_THREADS; i++)
2774 for (int j = 0; j < ACTIVE_SPLIT_POINTS_MAX; j++)
2776 SplitPointStack[i][j].parent = NULL;
2777 lock_init(&(SplitPointStack[i][j].lock), NULL);
2780 // Will be set just before program exits to properly end the threads
2781 AllThreadsShouldExit = false;
2783 // Threads will be put to sleep as soon as created
2784 AllThreadsShouldSleep = true;
2786 // All threads except the main thread should be initialized to THREAD_AVAILABLE
2788 threads[0].state = THREAD_SEARCHING;
2789 for (i = 1; i < MAX_THREADS; i++)
2790 threads[i].state = THREAD_AVAILABLE;
2792 // Launch the helper threads
2793 for (i = 1; i < MAX_THREADS; i++)
2796 #if !defined(_MSC_VER)
2797 ok = (pthread_create(pthread, NULL, init_thread, (void*)(&i)) == 0);
2799 ok = (CreateThread(NULL, 0, init_thread, (LPVOID)(&i), 0, NULL) != NULL);
2804 cout << "Failed to create thread number " << i << endl;
2805 Application::exit_with_failure();
2808 // Wait until the thread has finished launching and is gone to sleep
2809 while (threads[i].state != THREAD_SLEEPING);
2814 // exit_threads() is called when the program exits. It makes all the
2815 // helper threads exit cleanly.
2817 void ThreadsManager::exit_threads() {
2819 ActiveThreads = MAX_THREADS; // HACK
2820 AllThreadsShouldSleep = true; // HACK
2821 wake_sleeping_threads();
2823 // This makes the threads to exit idle_loop()
2824 AllThreadsShouldExit = true;
2826 // Wait for thread termination
2827 for (int i = 1; i < MAX_THREADS; i++)
2828 while (threads[i].state != THREAD_TERMINATED);
2830 // Now we can safely destroy the locks
2831 for (int i = 0; i < MAX_THREADS; i++)
2832 for (int j = 0; j < ACTIVE_SPLIT_POINTS_MAX; j++)
2833 lock_destroy(&(SplitPointStack[i][j].lock));
2835 lock_destroy(&WaitLock);
2836 lock_destroy(&MPLock);
2840 // thread_should_stop() checks whether the thread should stop its search.
2841 // This can happen if a beta cutoff has occurred in the thread's currently
2842 // active split point, or in some ancestor of the current split point.
2844 bool ThreadsManager::thread_should_stop(int threadID) const {
2846 assert(threadID >= 0 && threadID < ActiveThreads);
2850 for (sp = threads[threadID].splitPoint; sp && !sp->stopRequest; sp = sp->parent);
2855 // thread_is_available() checks whether the thread with threadID "slave" is
2856 // available to help the thread with threadID "master" at a split point. An
2857 // obvious requirement is that "slave" must be idle. With more than two
2858 // threads, this is not by itself sufficient: If "slave" is the master of
2859 // some active split point, it is only available as a slave to the other
2860 // threads which are busy searching the split point at the top of "slave"'s
2861 // split point stack (the "helpful master concept" in YBWC terminology).
2863 bool ThreadsManager::thread_is_available(int slave, int master) const {
2865 assert(slave >= 0 && slave < ActiveThreads);
2866 assert(master >= 0 && master < ActiveThreads);
2867 assert(ActiveThreads > 1);
2869 if (threads[slave].state != THREAD_AVAILABLE || slave == master)
2872 // Make a local copy to be sure doesn't change under our feet
2873 int localActiveSplitPoints = threads[slave].activeSplitPoints;
2875 if (localActiveSplitPoints == 0)
2876 // No active split points means that the thread is available as
2877 // a slave for any other thread.
2880 if (ActiveThreads == 2)
2883 // Apply the "helpful master" concept if possible. Use localActiveSplitPoints
2884 // that is known to be > 0, instead of threads[slave].activeSplitPoints that
2885 // could have been set to 0 by another thread leading to an out of bound access.
2886 if (SplitPointStack[slave][localActiveSplitPoints - 1].slaves[master])
2893 // available_thread_exists() tries to find an idle thread which is available as
2894 // a slave for the thread with threadID "master".
2896 bool ThreadsManager::available_thread_exists(int master) const {
2898 assert(master >= 0 && master < ActiveThreads);
2899 assert(ActiveThreads > 1);
2901 for (int i = 0; i < ActiveThreads; i++)
2902 if (thread_is_available(i, master))
2909 // split() does the actual work of distributing the work at a node between
2910 // several threads at PV nodes. If it does not succeed in splitting the
2911 // node (because no idle threads are available, or because we have no unused
2912 // split point objects), the function immediately returns false. If
2913 // splitting is possible, a SplitPoint object is initialized with all the
2914 // data that must be copied to the helper threads (the current position and
2915 // search stack, alpha, beta, the search depth, etc.), and we tell our
2916 // helper threads that they have been assigned work. This will cause them
2917 // to instantly leave their idle loops and call sp_search_pv(). When all
2918 // threads have returned from sp_search_pv (or, equivalently, when
2919 // splitPoint->cpus becomes 0), split() returns true.
2921 bool ThreadsManager::split(const Position& p, SearchStack* sstck, int ply,
2922 Value* alpha, const Value beta, Value* bestValue,
2923 Depth depth, bool mateThreat, int* moves, MovePicker* mp, int master, bool pvNode) {
2926 assert(sstck != NULL);
2927 assert(ply >= 0 && ply < PLY_MAX);
2928 assert(*bestValue >= -VALUE_INFINITE);
2929 assert( ( pvNode && *bestValue <= *alpha)
2930 || (!pvNode && *bestValue < beta ));
2931 assert(!pvNode || *alpha < beta);
2932 assert(beta <= VALUE_INFINITE);
2933 assert(depth > Depth(0));
2934 assert(master >= 0 && master < ActiveThreads);
2935 assert(ActiveThreads > 1);
2937 SplitPoint* splitPoint;
2941 // If no other thread is available to help us, or if we have too many
2942 // active split points, don't split.
2943 if ( !available_thread_exists(master)
2944 || threads[master].activeSplitPoints >= ACTIVE_SPLIT_POINTS_MAX)
2946 lock_release(&MPLock);
2950 // Pick the next available split point object from the split point stack
2951 splitPoint = &SplitPointStack[master][threads[master].activeSplitPoints];
2953 // Initialize the split point object
2954 splitPoint->parent = threads[master].splitPoint;
2955 splitPoint->stopRequest = false;
2956 splitPoint->ply = ply;
2957 splitPoint->depth = depth;
2958 splitPoint->mateThreat = mateThreat;
2959 splitPoint->alpha = pvNode ? *alpha : beta - 1;
2960 splitPoint->beta = beta;
2961 splitPoint->pvNode = pvNode;
2962 splitPoint->bestValue = *bestValue;
2963 splitPoint->master = master;
2964 splitPoint->mp = mp;
2965 splitPoint->moves = *moves;
2966 splitPoint->cpus = 1;
2967 splitPoint->pos = &p;
2968 splitPoint->parentSstack = sstck;
2969 for (int i = 0; i < ActiveThreads; i++)
2970 splitPoint->slaves[i] = 0;
2972 threads[master].splitPoint = splitPoint;
2973 threads[master].activeSplitPoints++;
2975 // If we are here it means we are not available
2976 assert(threads[master].state != THREAD_AVAILABLE);
2978 // Allocate available threads setting state to THREAD_BOOKED
2979 for (int i = 0; i < ActiveThreads && splitPoint->cpus < MaxThreadsPerSplitPoint; i++)
2980 if (thread_is_available(i, master))
2982 threads[i].state = THREAD_BOOKED;
2983 threads[i].splitPoint = splitPoint;
2984 splitPoint->slaves[i] = 1;
2988 assert(splitPoint->cpus > 1);
2990 // We can release the lock because slave threads are already booked and master is not available
2991 lock_release(&MPLock);
2993 // Tell the threads that they have work to do. This will make them leave
2994 // their idle loop. But before copy search stack tail for each thread.
2995 for (int i = 0; i < ActiveThreads; i++)
2996 if (i == master || splitPoint->slaves[i])
2998 memcpy(splitPoint->sstack[i] + ply - 1, sstck + ply - 1, 4 * sizeof(SearchStack));
3000 assert(i == master || threads[i].state == THREAD_BOOKED);
3002 threads[i].state = THREAD_WORKISWAITING; // This makes the slave to exit from idle_loop()
3005 // Everything is set up. The master thread enters the idle loop, from
3006 // which it will instantly launch a search, because its state is
3007 // THREAD_WORKISWAITING. We send the split point as a second parameter to the
3008 // idle loop, which means that the main thread will return from the idle
3009 // loop when all threads have finished their work at this split point
3010 // (i.e. when splitPoint->cpus == 0).
3011 idle_loop(master, splitPoint);
3013 // We have returned from the idle loop, which means that all threads are
3014 // finished. Update alpha, beta and bestValue, and return.
3018 *alpha = splitPoint->alpha;
3020 *bestValue = splitPoint->bestValue;
3021 threads[master].activeSplitPoints--;
3022 threads[master].splitPoint = splitPoint->parent;
3024 lock_release(&MPLock);
3029 // wake_sleeping_threads() wakes up all sleeping threads when it is time
3030 // to start a new search from the root.
3032 void ThreadsManager::wake_sleeping_threads() {
3034 assert(AllThreadsShouldSleep);
3035 assert(ActiveThreads > 0);
3037 AllThreadsShouldSleep = false;
3039 if (ActiveThreads == 1)
3042 #if !defined(_MSC_VER)
3043 pthread_mutex_lock(&WaitLock);
3044 pthread_cond_broadcast(&WaitCond);
3045 pthread_mutex_unlock(&WaitLock);
3047 for (int i = 1; i < MAX_THREADS; i++)
3048 SetEvent(SitIdleEvent[i]);
3054 // put_threads_to_sleep() makes all the threads go to sleep just before
3055 // to leave think(), at the end of the search. Threads should have already
3056 // finished the job and should be idle.
3058 void ThreadsManager::put_threads_to_sleep() {
3060 assert(!AllThreadsShouldSleep);
3062 // This makes the threads to go to sleep
3063 AllThreadsShouldSleep = true;
3066 /// The RootMoveList class
3068 // RootMoveList c'tor
3070 RootMoveList::RootMoveList(Position& pos, Move searchMoves[]) : count(0) {
3072 SearchStack ss[PLY_MAX_PLUS_2];
3073 MoveStack mlist[MaxRootMoves];
3075 bool includeAllMoves = (searchMoves[0] == MOVE_NONE);
3077 // Generate all legal moves
3078 MoveStack* last = generate_moves(pos, mlist);
3080 // Add each move to the moves[] array
3081 for (MoveStack* cur = mlist; cur != last; cur++)
3083 bool includeMove = includeAllMoves;
3085 for (int k = 0; !includeMove && searchMoves[k] != MOVE_NONE; k++)
3086 includeMove = (searchMoves[k] == cur->move);
3091 // Find a quick score for the move
3093 pos.do_move(cur->move, st);
3094 moves[count].move = cur->move;
3095 moves[count].score = -qsearch(pos, ss, -VALUE_INFINITE, VALUE_INFINITE, Depth(0), 1, 0);
3096 moves[count].pv[0] = cur->move;
3097 moves[count].pv[1] = MOVE_NONE;
3098 pos.undo_move(cur->move);
3105 // RootMoveList simple methods definitions
3107 void RootMoveList::set_move_nodes(int moveNum, int64_t nodes) {
3109 moves[moveNum].nodes = nodes;
3110 moves[moveNum].cumulativeNodes += nodes;
3113 void RootMoveList::set_beta_counters(int moveNum, int64_t our, int64_t their) {
3115 moves[moveNum].ourBeta = our;
3116 moves[moveNum].theirBeta = their;
3119 void RootMoveList::set_move_pv(int moveNum, const Move pv[]) {
3123 for (j = 0; pv[j] != MOVE_NONE; j++)
3124 moves[moveNum].pv[j] = pv[j];
3126 moves[moveNum].pv[j] = MOVE_NONE;
3130 // RootMoveList::sort() sorts the root move list at the beginning of a new
3133 void RootMoveList::sort() {
3135 sort_multipv(count - 1); // Sort all items
3139 // RootMoveList::sort_multipv() sorts the first few moves in the root move
3140 // list by their scores and depths. It is used to order the different PVs
3141 // correctly in MultiPV mode.
3143 void RootMoveList::sort_multipv(int n) {
3147 for (i = 1; i <= n; i++)
3149 RootMove rm = moves[i];
3150 for (j = i; j > 0 && moves[j - 1] < rm; j--)
3151 moves[j] = moves[j - 1];