1/*
2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2019 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
6
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
11
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
16
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
19*/
20
21#include <algorithm>
22#include <cassert>
23#include <cmath>
24#include <cstring> // For std::memset
25#include <iostream>
26#include <sstream>
27
28#include "evaluate.h"
29#include "misc.h"
30#include "movegen.h"
31#include "movepick.h"
32#include "position.h"
33#include "search.h"
34#include "thread.h"
35#include "timeman.h"
36#include "tt.h"
37#include "uci.h"
38#include "syzygy/tbprobe.h"
39
40namespace Search {
41
42 LimitsType Limits;
43}
44
45namespace Tablebases {
46
47 int Cardinality;
48 bool RootInTB;
49 bool UseRule50;
50 Depth ProbeDepth;
51}
52
53namespace TB = Tablebases;
54
55using std::string;
56using Eval::evaluate;
57using namespace Search;
58
59namespace {
60
61 // Different node types, used as a template parameter
62 enum NodeType { NonPV, PV };
63
64 // Razor and futility margins
65 constexpr int RazorMargin = 661;
66 Value futility_margin(Depth d, bool improving) {
67 return Value((168 - 51 * improving) * d / ONE_PLY);
68 }
69
70 // Reductions lookup table, initialized at startup
71 int Reductions[MAX_MOVES]; // [depth or moveNumber]
72
73 Depth reduction(bool i, Depth d, int mn) {
74 int r = Reductions[d / ONE_PLY] * Reductions[mn];
75 return ((r + 520) / 1024 + (!i && r > 999)) * ONE_PLY;
76 }
77
78 constexpr int futility_move_count(bool improving, int depth) {
79 return (5 + depth * depth) * (1 + improving) / 2;
80 }
81
82 // History and stats update bonus, based on depth
83 int stat_bonus(Depth depth) {
84 int d = depth / ONE_PLY;
85 return d > 17 ? -8 : 22 * d * d + 151 * d - 140;
86 }
87
88 // Add a small random component to draw evaluations to avoid 3fold-blindness
89 Value value_draw(Depth depth, Thread* thisThread) {
90 return depth < 4 * ONE_PLY ? VALUE_DRAW
91 : VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
92 }
93
94 // Skill structure is used to implement strength limit
95 struct Skill {
96 explicit Skill(int l) : level(l) {}
97 bool enabled() const { return level < 20; }
98 bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
99 Move pick_best(size_t multiPV);
100
101 int level;
102 Move best = MOVE_NONE;
103 };
104
105 // Breadcrumbs are used to mark nodes as being searched by a given thread
106 struct Breadcrumb {
107 std::atomic<Thread*> thread;
108 std::atomic<Key> key;
109 };
110 std::array<Breadcrumb, 1024> breadcrumbs;
111
112 // ThreadHolding structure keeps track of which thread left breadcrumbs at the given
113 // node for potential reductions. A free node will be marked upon entering the moves
114 // loop by the constructor, and unmarked upon leaving that loop by the destructor.
115 struct ThreadHolding {
116 explicit ThreadHolding(Thread* thisThread, Key posKey, int ply) {
117 location = ply < 8 ? &breadcrumbs[posKey & (breadcrumbs.size() - 1)] : nullptr;
118 otherThread = false;
119 owning = false;
120 if (location)
121 {
122 // See if another already marked this location, if not, mark it ourselves
123 Thread* tmp = (*location).thread.load(std::memory_order_relaxed);
124 if (tmp == nullptr)
125 {
126 (*location).thread.store(thisThread, std::memory_order_relaxed);
127 (*location).key.store(posKey, std::memory_order_relaxed);
128 owning = true;
129 }
130 else if ( tmp != thisThread
131 && (*location).key.load(std::memory_order_relaxed) == posKey)
132 otherThread = true;
133 }
134 }
135
136 ~ThreadHolding() {
137 if (owning) // Free the marked location
138 (*location).thread.store(nullptr, std::memory_order_relaxed);
139 }
140
141 bool marked() { return otherThread; }
142
143 private:
144 Breadcrumb* location;
145 bool otherThread, owning;
146 };
147
148 template <NodeType NT>
149 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
150
151 template <NodeType NT>
152 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
153
154 Value value_to_tt(Value v, int ply);
155 Value value_from_tt(Value v, int ply);
156 void update_pv(Move* pv, Move move, Move* childPv);
157 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
158 void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietCount, int bonus);
159 void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCount, int bonus);
160
161 // perft() is our utility to verify move generation. All the leaf nodes up
162 // to the given depth are generated and counted, and the sum is returned.
163 template<bool Root>
164 uint64_t perft(Position& pos, Depth depth) {
165
166 StateInfo st;
167 uint64_t cnt, nodes = 0;
168 const bool leaf = (depth == 2 * ONE_PLY);
169
170 for (const auto& m : MoveList<LEGAL>(pos))
171 {
172 if (Root && depth <= ONE_PLY)
173 cnt = 1, nodes++;
174 else
175 {
176 pos.do_move(m, st);
177 cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
178 nodes += cnt;
179 pos.undo_move(m);
180 }
181 if (Root)
182 sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
183 }
184 return nodes;
185 }
186
187} // namespace
188
189
190/// Search::init() is called at startup to initialize various lookup tables
191
192void Search::init() {
193
194 for (int i = 1; i < MAX_MOVES; ++i)
195 Reductions[i] = int(23.4 * std::log(i));
196}
197
198
199/// Search::clear() resets search state to its initial value
200
201void Search::clear() {
202
203 Threads.main()->wait_for_search_finished();
204
205 Time.availableNodes = 0;
206 TT.clear();
207 Threads.clear();
208 Tablebases::init(Options["SyzygyPath"]); // Free mapped files
209}
210
211
212/// MainThread::search() is started when the program receives the UCI 'go'
213/// command. It searches from the root position and outputs the "bestmove".
214
215void MainThread::search() {
216
217 if (Limits.perft)
218 {
219 nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
220 sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
221 return;
222 }
223
224 Color us = rootPos.side_to_move();
225 Time.init(Limits, us, rootPos.game_ply());
226 TT.new_search();
227
228 if (rootMoves.empty())
229 {
230 rootMoves.emplace_back(MOVE_NONE);
231 sync_cout << "info depth 0 score "
232 << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
233 << sync_endl;
234 }
235 else
236 {
237 for (Thread* th : Threads)
238 {
239 th->bestMoveChanges = 0;
240 if (th != this)
241 th->start_searching();
242 }
243
244 Thread::search(); // Let's start searching!
245 }
246
247 // When we reach the maximum depth, we can arrive here without a raise of
248 // Threads.stop. However, if we are pondering or in an infinite search,
249 // the UCI protocol states that we shouldn't print the best move before the
250 // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
251 // until the GUI sends one of those commands.
252
253 while (!Threads.stop && (ponder || Limits.infinite))
254 {} // Busy wait for a stop or a ponder reset
255
256 // Stop the threads if not already stopped (also raise the stop if
257 // "ponderhit" just reset Threads.ponder).
258 Threads.stop = true;
259
260 // Wait until all threads have finished
261 for (Thread* th : Threads)
262 if (th != this)
263 th->wait_for_search_finished();
264
265 // When playing in 'nodes as time' mode, subtract the searched nodes from
266 // the available ones before exiting.
267 if (Limits.npmsec)
268 Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
269
270 Thread* bestThread = this;
271
272 // Check if there are threads with a better score than main thread
273 if ( Options["MultiPV"] == 1
274 && !Limits.depth
275 && !(Skill(Options["Skill Level"]).enabled() || Options["UCI_LimitStrength"])
276 && rootMoves[0].pv[0] != MOVE_NONE)
277 {
278 std::map<Move, int64_t> votes;
279 Value minScore = this->rootMoves[0].score;
280
281 // Find out minimum score
282 for (Thread* th: Threads)
283 minScore = std::min(minScore, th->rootMoves[0].score);
284
285 // Vote according to score and depth, and select the best thread
286 for (Thread* th : Threads)
287 {
288 votes[th->rootMoves[0].pv[0]] +=
289 (th->rootMoves[0].score - minScore + 14) * int(th->completedDepth);
290
291 if (bestThread->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY)
292 {
293 // Make sure we pick the shortest mate
294 if (th->rootMoves[0].score > bestThread->rootMoves[0].score)
295 bestThread = th;
296 }
297 else if ( th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY
298 || votes[th->rootMoves[0].pv[0]] > votes[bestThread->rootMoves[0].pv[0]])
299 bestThread = th;
300 }
301 }
302
303 previousScore = bestThread->rootMoves[0].score;
304
305 // Send again PV info if we have a new best thread
306 if (bestThread != this)
307 sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
308
309 sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
310
311 if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
312 std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
313
314 std::cout << sync_endl;
315}
316
317
318/// Thread::search() is the main iterative deepening loop. It calls search()
319/// repeatedly with increasing depth until the allocated thinking time has been
320/// consumed, the user stops the search, or the maximum search depth is reached.
321
322void Thread::search() {
323
324 // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
325 // The former is needed to allow update_continuation_histories(ss-1, ...),
326 // which accesses its argument at ss-6, also near the root.
327 // The latter is needed for statScores and killer initialization.
328 Stack stack[MAX_PLY+10], *ss = stack+7;
329 Move pv[MAX_PLY+1];
330 Value bestValue, alpha, beta, delta;
331 Move lastBestMove = MOVE_NONE;
332 Depth lastBestMoveDepth = DEPTH_ZERO;
333 MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
334 double timeReduction = 1, totBestMoveChanges = 0;
335 Color us = rootPos.side_to_move();
336
337 std::memset(ss-7, 0, 10 * sizeof(Stack));
338 for (int i = 7; i > 0; i--)
339 (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
340 ss->pv = pv;
341
342 bestValue = delta = alpha = -VALUE_INFINITE;
343 beta = VALUE_INFINITE;
344
345 size_t multiPV = Options["MultiPV"];
346
347 // Pick integer skill levels, but non-deterministically round up or down
348 // such that the average integer skill corresponds to the input floating point one.
349 // UCI_Elo is converted to a suitable fractional skill level, using anchoring
350 // to CCRL Elo (goldfish 1.13 = 2000) and a fit through Ordo derived Elo
351 // for match (TC 60+0.6) results spanning a wide range of k values.
352 PRNG rng(now());
353 double floatLevel = Options["UCI_LimitStrength"] ?
354 clamp(std::pow((Options["UCI_Elo"] - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0) :
355 double(Options["Skill Level"]);
356 int intLevel = int(floatLevel) +
357 ((floatLevel - int(floatLevel)) * 1024 > rng.rand<unsigned>() % 1024 ? 1 : 0);
358 Skill skill(intLevel);
359
360 // When playing with strength handicap enable MultiPV search that we will
361 // use behind the scenes to retrieve a set of possible moves.
362 if (skill.enabled())
363 multiPV = std::max(multiPV, (size_t)4);
364
365 multiPV = std::min(multiPV, rootMoves.size());
366
367 int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
368
369 // In analysis mode, adjust contempt in accordance with user preference
370 if (Limits.infinite || Options["UCI_AnalyseMode"])
371 ct = Options["Analysis Contempt"] == "Off" ? 0
372 : Options["Analysis Contempt"] == "Both" ? ct
373 : Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
374 : Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
375 : ct;
376
377 // Evaluation score is from the white point of view
378 contempt = (us == WHITE ? make_score(ct, ct / 2)
379 : -make_score(ct, ct / 2));
380
381 // Iterative deepening loop until requested to stop or the target depth is reached
382 while ( (rootDepth += ONE_PLY) < DEPTH_MAX
383 && !Threads.stop
384 && !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
385 {
386 // Age out PV variability metric
387 if (mainThread)
388 totBestMoveChanges /= 2;
389
390 // Save the last iteration's scores before first PV line is searched and
391 // all the move scores except the (new) PV are set to -VALUE_INFINITE.
392 for (RootMove& rm : rootMoves)
393 rm.previousScore = rm.score;
394
395 size_t pvFirst = 0;
396 pvLast = 0;
397
398 // MultiPV loop. We perform a full root search for each PV line
399 for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
400 {
401 if (pvIdx == pvLast)
402 {
403 pvFirst = pvLast;
404 for (pvLast++; pvLast < rootMoves.size(); pvLast++)
405 if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
406 break;
407 }
408
409 // Reset UCI info selDepth for each depth and each PV line
410 selDepth = 0;
411
412 // Reset aspiration window starting size
413 if (rootDepth >= 4 * ONE_PLY)
414 {
415 Value previousScore = rootMoves[pvIdx].previousScore;
416 delta = Value(23);
417 alpha = std::max(previousScore - delta,-VALUE_INFINITE);
418 beta = std::min(previousScore + delta, VALUE_INFINITE);
419
420 // Adjust contempt based on root move's previousScore (dynamic contempt)
421 int dct = ct + 86 * previousScore / (abs(previousScore) + 176);
422
423 contempt = (us == WHITE ? make_score(dct, dct / 2)
424 : -make_score(dct, dct / 2));
425 }
426
427 // Start with a small aspiration window and, in the case of a fail
428 // high/low, re-search with a bigger window until we don't fail
429 // high/low anymore.
430 int failedHighCnt = 0;
431 while (true)
432 {
433 Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
434 bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
435
436 // Bring the best move to the front. It is critical that sorting
437 // is done with a stable algorithm because all the values but the
438 // first and eventually the new best one are set to -VALUE_INFINITE
439 // and we want to keep the same order for all the moves except the
440 // new PV that goes to the front. Note that in case of MultiPV
441 // search the already searched PV lines are preserved.
442 std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
443
444 // If search has been stopped, we break immediately. Sorting is
445 // safe because RootMoves is still valid, although it refers to
446 // the previous iteration.
447 if (Threads.stop)
448 break;
449
450 // When failing high/low give some update (without cluttering
451 // the UI) before a re-search.
452 if ( mainThread
453 && multiPV == 1
454 && (bestValue <= alpha || bestValue >= beta)
455 && Time.elapsed() > 3000)
456 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
457
458 // In case of failing low/high increase aspiration window and
459 // re-search, otherwise exit the loop.
460 if (bestValue <= alpha)
461 {
462 beta = (alpha + beta) / 2;
463 alpha = std::max(bestValue - delta, -VALUE_INFINITE);
464
465 failedHighCnt = 0;
466 if (mainThread)
467 mainThread->stopOnPonderhit = false;
468 }
469 else if (bestValue >= beta)
470 {
471 beta = std::min(bestValue + delta, VALUE_INFINITE);
472 ++failedHighCnt;
473 }
474 else
475 break;
476
477 delta += delta / 4 + 5;
478
479 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
480 }
481
482 // Sort the PV lines searched so far and update the GUI
483 std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
484
485 if ( mainThread
486 && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
487 sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
488 }
489
490 if (!Threads.stop)
491 completedDepth = rootDepth;
492
493 if (rootMoves[0].pv[0] != lastBestMove) {
494 lastBestMove = rootMoves[0].pv[0];
495 lastBestMoveDepth = rootDepth;
496 }
497
498 // Have we found a "mate in x"?
499 if ( Limits.mate
500 && bestValue >= VALUE_MATE_IN_MAX_PLY
501 && VALUE_MATE - bestValue <= 2 * Limits.mate)
502 Threads.stop = true;
503
504 if (!mainThread)
505 continue;
506
507 // If skill level is enabled and time is up, pick a sub-optimal best move
508 if (skill.enabled() && skill.time_to_pick(rootDepth))
509 skill.pick_best(multiPV);
510
511 // Do we have time for the next iteration? Can we stop searching now?
512 if ( Limits.use_time_management()
513 && !Threads.stop
514 && !mainThread->stopOnPonderhit)
515 {
516 double fallingEval = (354 + 10 * (mainThread->previousScore - bestValue)) / 692.0;
517 fallingEval = clamp(fallingEval, 0.5, 1.5);
518
519 // If the bestMove is stable over several iterations, reduce time accordingly
520 timeReduction = lastBestMoveDepth + 9 * ONE_PLY < completedDepth ? 1.97 : 0.98;
521 double reduction = (1.36 + mainThread->previousTimeReduction) / (2.29 * timeReduction);
522
523 // Use part of the gained time from a previous stable move for the current move
524 for (Thread* th : Threads)
525 {
526 totBestMoveChanges += th->bestMoveChanges;
527 th->bestMoveChanges = 0;
528 }
529 double bestMoveInstability = 1 + totBestMoveChanges / Threads.size();
530
531 // Stop the search if we have only one legal move, or if available time elapsed
532 if ( rootMoves.size() == 1
533 || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
534 {
535 // If we are allowed to ponder do not stop the search now but
536 // keep pondering until the GUI sends "ponderhit" or "stop".
537 if (mainThread->ponder)
538 mainThread->stopOnPonderhit = true;
539 else
540 Threads.stop = true;
541 }
542 }
543 }
544
545 if (!mainThread)
546 return;
547
548 mainThread->previousTimeReduction = timeReduction;
549
550 // If skill level is enabled, swap best PV line with the sub-optimal one
551 if (skill.enabled())
552 std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
553 skill.best ? skill.best : skill.pick_best(multiPV)));
554}
555
556
557namespace {
558
559 // search<>() is the main search function for both PV and non-PV nodes
560
561 template <NodeType NT>
562 Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
563
564 constexpr bool PvNode = NT == PV;
565 const bool rootNode = PvNode && ss->ply == 0;
566
567 // Check if we have an upcoming move which draws by repetition, or
568 // if the opponent had an alternative move earlier to this position.
569 if ( pos.rule50_count() >= 3
570 && alpha < VALUE_DRAW
571 && !rootNode
572 && pos.has_game_cycle(ss->ply))
573 {
574 alpha = value_draw(depth, pos.this_thread());
575 if (alpha >= beta)
576 return alpha;
577 }
578
579 // Dive into quiescence search when the depth reaches zero
580 if (depth < ONE_PLY)
581 return qsearch<NT>(pos, ss, alpha, beta);
582
583 assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
584 assert(PvNode || (alpha == beta - 1));
585 assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
586 assert(!(PvNode && cutNode));
587 assert(depth / ONE_PLY * ONE_PLY == depth);
588
589 Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
590 StateInfo st;
591 TTEntry* tte;
592 Key posKey;
593 Move ttMove, move, excludedMove, bestMove;
594 Depth extension, newDepth;
595 Value bestValue, value, ttValue, eval, maxValue;
596 bool ttHit, ttPv, inCheck, givesCheck, improving, doLMR;
597 bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
598 Piece movedPiece;
599 int moveCount, captureCount, quietCount, singularLMR;
600
601 // Step 1. Initialize node
602 Thread* thisThread = pos.this_thread();
603 inCheck = pos.checkers();
604 Color us = pos.side_to_move();
605 moveCount = captureCount = quietCount = singularLMR = ss->moveCount = 0;
606 bestValue = -VALUE_INFINITE;
607 maxValue = VALUE_INFINITE;
608
609 // Check for the available remaining time
610 if (thisThread == Threads.main())
611 static_cast<MainThread*>(thisThread)->check_time();
612
613 // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
614 if (PvNode && thisThread->selDepth < ss->ply + 1)
615 thisThread->selDepth = ss->ply + 1;
616
617 if (!rootNode)
618 {
619 // Step 2. Check for aborted search and immediate draw
620 if ( Threads.stop.load(std::memory_order_relaxed)
621 || pos.is_draw(ss->ply)
622 || ss->ply >= MAX_PLY)
623 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
624 : value_draw(depth, pos.this_thread());
625
626 // Step 3. Mate distance pruning. Even if we mate at the next move our score
627 // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
628 // a shorter mate was found upward in the tree then there is no need to search
629 // because we will never beat the current alpha. Same logic but with reversed
630 // signs applies also in the opposite condition of being mated instead of giving
631 // mate. In this case return a fail-high score.
632 alpha = std::max(mated_in(ss->ply), alpha);
633 beta = std::min(mate_in(ss->ply+1), beta);
634 if (alpha >= beta)
635 return alpha;
636 }
637
638 assert(0 <= ss->ply && ss->ply < MAX_PLY);
639
640 (ss+1)->ply = ss->ply + 1;
641 (ss+1)->excludedMove = bestMove = MOVE_NONE;
642 (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
643 Square prevSq = to_sq((ss-1)->currentMove);
644
645 // Initialize statScore to zero for the grandchildren of the current position.
646 // So statScore is shared between all grandchildren and only the first grandchild
647 // starts with statScore = 0. Later grandchildren start with the last calculated
648 // statScore of the previous grandchild. This influences the reduction rules in
649 // LMR which are based on the statScore of parent position.
650 if (rootNode)
651 (ss+4)->statScore = 0;
652 else
653 (ss+2)->statScore = 0;
654
655 // Step 4. Transposition table lookup. We don't want the score of a partial
656 // search to overwrite a previous full search TT value, so we use a different
657 // position key in case of an excluded move.
658 excludedMove = ss->excludedMove;
659 posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
660 tte = TT.probe(posKey, ttHit);
661 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
662 ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
663 : ttHit ? tte->move() : MOVE_NONE;
664 ttPv = PvNode || (ttHit && tte->is_pv());
665
666 // At non-PV nodes we check for an early TT cutoff
667 if ( !PvNode
668 && ttHit
669 && tte->depth() >= depth
670 && ttValue != VALUE_NONE // Possible in case of TT access race
671 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
672 : (tte->bound() & BOUND_UPPER)))
673 {
674 // If ttMove is quiet, update move sorting heuristics on TT hit
675 if (ttMove)
676 {
677 if (ttValue >= beta)
678 {
679 if (!pos.capture_or_promotion(ttMove))
680 update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
681
682 // Extra penalty for early quiet moves of the previous ply
683 if ((ss-1)->moveCount <= 2 && !pos.captured_piece())
684 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
685 }
686 // Penalty for a quiet ttMove that fails low
687 else if (!pos.capture_or_promotion(ttMove))
688 {
689 int penalty = -stat_bonus(depth);
690 thisThread->mainHistory[us][from_to(ttMove)] << penalty;
691 update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
692 }
693 }
694 return ttValue;
695 }
696
697 // Step 5. Tablebases probe
698 if (!rootNode && TB::Cardinality)
699 {
700 int piecesCount = pos.count<ALL_PIECES>();
701
702 if ( piecesCount <= TB::Cardinality
703 && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
704 && pos.rule50_count() == 0
705 && !pos.can_castle(ANY_CASTLING))
706 {
707 TB::ProbeState err;
708 TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
709
710 // Force check of time on the next occasion
711 if (thisThread == Threads.main())
712 static_cast<MainThread*>(thisThread)->callsCnt = 0;
713
714 if (err != TB::ProbeState::FAIL)
715 {
716 thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
717
718 int drawScore = TB::UseRule50 ? 1 : 0;
719
720 value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
721 : wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
722 : VALUE_DRAW + 2 * wdl * drawScore;
723
724 Bound b = wdl < -drawScore ? BOUND_UPPER
725 : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
726
727 if ( b == BOUND_EXACT
728 || (b == BOUND_LOWER ? value >= beta : value <= alpha))
729 {
730 tte->save(posKey, value_to_tt(value, ss->ply), ttPv, b,
731 std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
732 MOVE_NONE, VALUE_NONE);
733
734 return value;
735 }
736
737 if (PvNode)
738 {
739 if (b == BOUND_LOWER)
740 bestValue = value, alpha = std::max(alpha, bestValue);
741 else
742 maxValue = value;
743 }
744 }
745 }
746 }
747
748 // Step 6. Static evaluation of the position
749 if (inCheck)
750 {
751 ss->staticEval = eval = VALUE_NONE;
752 improving = false;
753 goto moves_loop; // Skip early pruning when in check
754 }
755 else if (ttHit)
756 {
757 // Never assume anything about values stored in TT
758 ss->staticEval = eval = tte->eval();
759 if (eval == VALUE_NONE)
760 ss->staticEval = eval = evaluate(pos);
761
762 // Can ttValue be used as a better position evaluation?
763 if ( ttValue != VALUE_NONE
764 && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
765 eval = ttValue;
766 }
767 else
768 {
769 if ((ss-1)->currentMove != MOVE_NULL)
770 {
771 int bonus = -(ss-1)->statScore / 512;
772
773 ss->staticEval = eval = evaluate(pos) + bonus;
774 }
775 else
776 ss->staticEval = eval = -(ss-1)->staticEval + 2 * Eval::Tempo;
777
778 tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
779 }
780
781 // Step 7. Razoring (~2 Elo)
782 if ( !rootNode // The required rootNode PV handling is not available in qsearch
783 && depth < 2 * ONE_PLY
784 && eval <= alpha - RazorMargin)
785 return qsearch<NT>(pos, ss, alpha, beta);
786
787 improving = ss->staticEval >= (ss-2)->staticEval
788 || (ss-2)->staticEval == VALUE_NONE;
789
790 // Step 8. Futility pruning: child node (~30 Elo)
791 if ( !PvNode
792 && depth < 7 * ONE_PLY
793 && eval - futility_margin(depth, improving) >= beta
794 && eval < VALUE_KNOWN_WIN) // Do not return unproven wins
795 return eval;
796
797 // Step 9. Null move search with verification search (~40 Elo)
798 if ( !PvNode
799 && (ss-1)->currentMove != MOVE_NULL
800 && (ss-1)->statScore < 22661
801 && eval >= beta
802 && ss->staticEval >= beta - 33 * depth / ONE_PLY + 299
803 && !excludedMove
804 && pos.non_pawn_material(us)
805 && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
806 {
807 assert(eval - beta >= 0);
808
809 // Null move dynamic reduction based on depth and value
810 Depth R = ((835 + 70 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 185, 3)) * ONE_PLY;
811
812 ss->currentMove = MOVE_NULL;
813 ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
814
815 pos.do_null_move(st);
816
817 Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
818
819 pos.undo_null_move();
820
821 if (nullValue >= beta)
822 {
823 // Do not return unproven mate scores
824 if (nullValue >= VALUE_MATE_IN_MAX_PLY)
825 nullValue = beta;
826
827 if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13 * ONE_PLY))
828 return nullValue;
829
830 assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
831
832 // Do verification search at high depths, with null move pruning disabled
833 // for us, until ply exceeds nmpMinPly.
834 thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / (4 * ONE_PLY);
835 thisThread->nmpColor = us;
836
837 Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
838
839 thisThread->nmpMinPly = 0;
840
841 if (v >= beta)
842 return nullValue;
843 }
844 }
845
846 // Step 10. ProbCut (~10 Elo)
847 // If we have a good enough capture and a reduced search returns a value
848 // much above beta, we can (almost) safely prune the previous move.
849 if ( !PvNode
850 && depth >= 5 * ONE_PLY
851 && abs(beta) < VALUE_MATE_IN_MAX_PLY)
852 {
853 Value raisedBeta = std::min(beta + 191 - 46 * improving, VALUE_INFINITE);
854 MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
855 int probCutCount = 0;
856
857 while ( (move = mp.next_move()) != MOVE_NONE
858 && probCutCount < 2 + 2 * cutNode)
859 if (move != excludedMove && pos.legal(move))
860 {
861 probCutCount++;
862
863 ss->currentMove = move;
864 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
865
866 assert(depth >= 5 * ONE_PLY);
867
868 pos.do_move(move, st);
869
870 // Perform a preliminary qsearch to verify that the move holds
871 value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
872
873 // If the qsearch held, perform the regular search
874 if (value >= raisedBeta)
875 value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
876
877 pos.undo_move(move);
878
879 if (value >= raisedBeta)
880 return value;
881 }
882 }
883
884 // Step 11. Internal iterative deepening (~2 Elo)
885 if (depth >= 7 * ONE_PLY && !ttMove)
886 {
887 search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
888
889 tte = TT.probe(posKey, ttHit);
890 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
891 ttMove = ttHit ? tte->move() : MOVE_NONE;
892 }
893
894moves_loop: // When in check, search starts from here
895
896 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
897 nullptr, (ss-4)->continuationHistory,
898 nullptr, (ss-6)->continuationHistory };
899
900 Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
901
902 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
903 &thisThread->captureHistory,
904 contHist,
905 countermove,
906 ss->killers);
907
908 value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
909 moveCountPruning = false;
910 ttCapture = ttMove && pos.capture_or_promotion(ttMove);
911
912 // Mark this node as being searched
913 ThreadHolding th(thisThread, posKey, ss->ply);
914
915 // Step 12. Loop through all pseudo-legal moves until no moves remain
916 // or a beta cutoff occurs.
917 while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
918 {
919 assert(is_ok(move));
920
921 if (move == excludedMove)
922 continue;
923
924 // At root obey the "searchmoves" option and skip moves not listed in Root
925 // Move List. As a consequence any illegal move is also skipped. In MultiPV
926 // mode we also skip PV moves which have been already searched and those
927 // of lower "TB rank" if we are in a TB root position.
928 if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
929 thisThread->rootMoves.begin() + thisThread->pvLast, move))
930 continue;
931
932 ss->moveCount = ++moveCount;
933
934 if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
935 sync_cout << "info depth " << depth / ONE_PLY
936 << " currmove " << UCI::move(move, pos.is_chess960())
937 << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
938 if (PvNode)
939 (ss+1)->pv = nullptr;
940
941 extension = DEPTH_ZERO;
942 captureOrPromotion = pos.capture_or_promotion(move);
943 movedPiece = pos.moved_piece(move);
944 givesCheck = pos.gives_check(move);
945
946 // Step 13. Extensions (~70 Elo)
947
948 // Singular extension search (~60 Elo). If all moves but one fail low on a
949 // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
950 // then that move is singular and should be extended. To verify this we do
951 // a reduced search on all the other moves but the ttMove and if the
952 // result is lower than ttValue minus a margin then we will extend the ttMove.
953 if ( depth >= 6 * ONE_PLY
954 && move == ttMove
955 && !rootNode
956 && !excludedMove // Avoid recursive singular search
957 /* && ttValue != VALUE_NONE Already implicit in the next condition */
958 && abs(ttValue) < VALUE_KNOWN_WIN
959 && (tte->bound() & BOUND_LOWER)
960 && tte->depth() >= depth - 3 * ONE_PLY
961 && pos.legal(move))
962 {
963 Value singularBeta = ttValue - 2 * depth / ONE_PLY;
964 Depth halfDepth = depth / (2 * ONE_PLY) * ONE_PLY; // ONE_PLY invariant
965 ss->excludedMove = move;
966 value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, halfDepth, cutNode);
967 ss->excludedMove = MOVE_NONE;
968
969 if (value < singularBeta)
970 {
971 extension = ONE_PLY;
972 singularLMR++;
973
974 if (value < singularBeta - std::min(4 * depth / ONE_PLY, 36))
975 singularLMR++;
976 }
977
978 // Multi-cut pruning
979 // Our ttMove is assumed to fail high, and now we failed high also on a reduced
980 // search without the ttMove. So we assume this expected Cut-node is not singular,
981 // that multiple moves fail high, and we can prune the whole subtree by returning
982 // a soft bound.
983 else if ( eval >= beta
984 && singularBeta >= beta)
985 return singularBeta;
986 }
987
988 // Check extension (~2 Elo)
989 else if ( givesCheck
990 && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
991 extension = ONE_PLY;
992
993 // Castling extension
994 else if (type_of(move) == CASTLING)
995 extension = ONE_PLY;
996
997 // Shuffle extension
998 else if ( PvNode
999 && pos.rule50_count() > 18
1000 && depth < 3 * ONE_PLY
1001 && ++thisThread->shuffleExts < thisThread->nodes.load(std::memory_order_relaxed) / 4) // To avoid too many extensions
1002 extension = ONE_PLY;
1003
1004 // Passed pawn extension
1005 else if ( move == ss->killers[0]
1006 && pos.advanced_pawn_push(move)
1007 && pos.pawn_passed(us, to_sq(move)))
1008 extension = ONE_PLY;
1009
1010 // Calculate new depth for this move
1011 newDepth = depth - ONE_PLY + extension;
1012
1013 // Step 14. Pruning at shallow depth (~170 Elo)
1014 if ( !rootNode
1015 && pos.non_pawn_material(us)
1016 && bestValue > VALUE_MATED_IN_MAX_PLY)
1017 {
1018 // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
1019 moveCountPruning = moveCount >= futility_move_count(improving, depth / ONE_PLY);
1020
1021 if ( !captureOrPromotion
1022 && !givesCheck
1023 && (!pos.advanced_pawn_push(move) || pos.non_pawn_material(~us) > BishopValueMg))
1024 {
1025 // Move count based pruning
1026 if (moveCountPruning)
1027 continue;
1028
1029 // Reduced depth of the next LMR search
1030 int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), DEPTH_ZERO);
1031 lmrDepth /= ONE_PLY;
1032
1033 // Countermoves based pruning (~20 Elo)
1034 if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
1035 && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
1036 && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
1037 continue;
1038
1039 // Futility pruning: parent node (~2 Elo)
1040 if ( lmrDepth < 6
1041 && !inCheck
1042 && ss->staticEval + 250 + 211 * lmrDepth <= alpha)
1043 continue;
1044
1045 // Prune moves with negative SEE (~10 Elo)
1046 if (!pos.see_ge(move, Value(-(31 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
1047 continue;
1048 }
1049 else if ( (!givesCheck || !extension)
1050 && !pos.see_ge(move, Value(-199) * (depth / ONE_PLY))) // (~20 Elo)
1051 continue;
1052 }
1053
1054 // Speculative prefetch as early as possible
1055 prefetch(TT.first_entry(pos.key_after(move)));
1056
1057 // Check for legality just before making the move
1058 if (!rootNode && !pos.legal(move))
1059 {
1060 ss->moveCount = --moveCount;
1061 continue;
1062 }
1063
1064 // Update the current move (this must be done after singular extension search)
1065 ss->currentMove = move;
1066 ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
1067
1068 // Step 15. Make the move
1069 pos.do_move(move, st, givesCheck);
1070
1071 // Step 16. Reduced depth search (LMR). If the move fails high it will be
1072 // re-searched at full depth.
1073 if ( depth >= 3 * ONE_PLY
1074 && moveCount > 1 + 3 * rootNode
1075 && ( !captureOrPromotion
1076 || moveCountPruning
1077 || ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha))
1078 {
1079 Depth r = reduction(improving, depth, moveCount);
1080
1081 // Reduction if other threads are searching this position.
1082 if (th.marked())
1083 r += ONE_PLY;
1084
1085 // Decrease reduction if position is or has been on the PV
1086 if (ttPv)
1087 r -= 2 * ONE_PLY;
1088
1089 // Decrease reduction if opponent's move count is high (~10 Elo)
1090 if ((ss-1)->moveCount > 15)
1091 r -= ONE_PLY;
1092
1093 // Decrease reduction if move has been singularly extended
1094 r -= singularLMR * ONE_PLY;
1095
1096 if (!captureOrPromotion)
1097 {
1098 // Increase reduction if ttMove is a capture (~0 Elo)
1099 if (ttCapture)
1100 r += ONE_PLY;
1101
1102 // Increase reduction for cut nodes (~5 Elo)
1103 if (cutNode)
1104 r += 2 * ONE_PLY;
1105
1106 // Decrease reduction for moves that escape a capture. Filter out
1107 // castling moves, because they are coded as "king captures rook" and
1108 // hence break make_move(). (~5 Elo)
1109 else if ( type_of(move) == NORMAL
1110 && !pos.see_ge(make_move(to_sq(move), from_sq(move))))
1111 r -= 2 * ONE_PLY;
1112
1113 ss->statScore = thisThread->mainHistory[us][from_to(move)]
1114 + (*contHist[0])[movedPiece][to_sq(move)]
1115 + (*contHist[1])[movedPiece][to_sq(move)]
1116 + (*contHist[3])[movedPiece][to_sq(move)]
1117 - 4729;
1118
1119 // Reset statScore to zero if negative and most stats shows >= 0
1120 if ( ss->statScore < 0
1121 && (*contHist[0])[movedPiece][to_sq(move)] >= 0
1122 && (*contHist[1])[movedPiece][to_sq(move)] >= 0
1123 && thisThread->mainHistory[us][from_to(move)] >= 0)
1124 ss->statScore = 0;
1125
1126 // Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
1127 if (ss->statScore >= -99 && (ss-1)->statScore < -116)
1128 r -= ONE_PLY;
1129
1130 else if ((ss-1)->statScore >= -117 && ss->statScore < -144)
1131 r += ONE_PLY;
1132
1133 // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
1134 r -= ss->statScore / 16384 * ONE_PLY;
1135 }
1136
1137 Depth d = clamp(newDepth - r, ONE_PLY, newDepth);
1138
1139 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
1140
1141 doFullDepthSearch = (value > alpha && d != newDepth), doLMR = true;
1142 }
1143 else
1144 doFullDepthSearch = !PvNode || moveCount > 1, doLMR = false;
1145
1146 // Step 17. Full depth search when LMR is skipped or fails high
1147 if (doFullDepthSearch)
1148 {
1149 value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
1150
1151 if (doLMR && !captureOrPromotion)
1152 {
1153 int bonus = value > alpha ? stat_bonus(newDepth)
1154 : -stat_bonus(newDepth);
1155
1156 if (move == ss->killers[0])
1157 bonus += bonus / 4;
1158
1159 update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
1160 }
1161 }
1162
1163 // For PV nodes only, do a full PV search on the first move or after a fail
1164 // high (in the latter case search only if value < beta), otherwise let the
1165 // parent node fail low with value <= alpha and try another move.
1166 if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
1167 {
1168 (ss+1)->pv = pv;
1169 (ss+1)->pv[0] = MOVE_NONE;
1170
1171 value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
1172 }
1173
1174 // Step 18. Undo move
1175 pos.undo_move(move);
1176
1177 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1178
1179 // Step 19. Check for a new best move
1180 // Finished searching the move. If a stop occurred, the return value of
1181 // the search cannot be trusted, and we return immediately without
1182 // updating best move, PV and TT.
1183 if (Threads.stop.load(std::memory_order_relaxed))
1184 return VALUE_ZERO;
1185
1186 if (rootNode)
1187 {
1188 RootMove& rm = *std::find(thisThread->rootMoves.begin(),
1189 thisThread->rootMoves.end(), move);
1190
1191 // PV move or new best move?
1192 if (moveCount == 1 || value > alpha)
1193 {
1194 rm.score = value;
1195 rm.selDepth = thisThread->selDepth;
1196 rm.pv.resize(1);
1197
1198 assert((ss+1)->pv);
1199
1200 for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
1201 rm.pv.push_back(*m);
1202
1203 // We record how often the best move has been changed in each
1204 // iteration. This information is used for time management: When
1205 // the best move changes frequently, we allocate some more time.
1206 if (moveCount > 1)
1207 ++thisThread->bestMoveChanges;
1208 }
1209 else
1210 // All other moves but the PV are set to the lowest value: this
1211 // is not a problem when sorting because the sort is stable and the
1212 // move position in the list is preserved - just the PV is pushed up.
1213 rm.score = -VALUE_INFINITE;
1214 }
1215
1216 if (value > bestValue)
1217 {
1218 bestValue = value;
1219
1220 if (value > alpha)
1221 {
1222 bestMove = move;
1223
1224 if (PvNode && !rootNode) // Update pv even in fail-high case
1225 update_pv(ss->pv, move, (ss+1)->pv);
1226
1227 if (PvNode && value < beta) // Update alpha! Always alpha < beta
1228 alpha = value;
1229 else
1230 {
1231 assert(value >= beta); // Fail high
1232 ss->statScore = 0;
1233 break;
1234 }
1235 }
1236 }
1237
1238 if (move != bestMove)
1239 {
1240 if (captureOrPromotion && captureCount < 32)
1241 capturesSearched[captureCount++] = move;
1242
1243 else if (!captureOrPromotion && quietCount < 64)
1244 quietsSearched[quietCount++] = move;
1245 }
1246 }
1247
1248 // The following condition would detect a stop only after move loop has been
1249 // completed. But in this case bestValue is valid because we have fully
1250 // searched our subtree, and we can anyhow save the result in TT.
1251 /*
1252 if (Threads.stop)
1253 return VALUE_DRAW;
1254 */
1255
1256 // Step 20. Check for mate and stalemate
1257 // All legal moves have been searched and if there are no legal moves, it
1258 // must be a mate or a stalemate. If we are in a singular extension search then
1259 // return a fail low score.
1260
1261 assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
1262
1263 if (!moveCount)
1264 bestValue = excludedMove ? alpha
1265 : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
1266 else if (bestMove)
1267 {
1268 // Quiet best move: update move sorting heuristics
1269 if (!pos.capture_or_promotion(bestMove))
1270 update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
1271 stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
1272
1273 update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
1274
1275 // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
1276 if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
1277 && !pos.captured_piece())
1278 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
1279
1280 }
1281 // Bonus for prior countermove that caused the fail low
1282 else if ( (depth >= 3 * ONE_PLY || PvNode)
1283 && !pos.captured_piece())
1284 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
1285
1286 if (PvNode)
1287 bestValue = std::min(bestValue, maxValue);
1288
1289 if (!excludedMove)
1290 tte->save(posKey, value_to_tt(bestValue, ss->ply), ttPv,
1291 bestValue >= beta ? BOUND_LOWER :
1292 PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
1293 depth, bestMove, ss->staticEval);
1294
1295 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1296
1297 return bestValue;
1298 }
1299
1300
1301 // qsearch() is the quiescence search function, which is called by the main search
1302 // function with zero depth, or recursively with further decreasing depth per call.
1303 template <NodeType NT>
1304 Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
1305
1306 constexpr bool PvNode = NT == PV;
1307
1308 assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
1309 assert(PvNode || (alpha == beta - 1));
1310 assert(depth <= DEPTH_ZERO);
1311 assert(depth / ONE_PLY * ONE_PLY == depth);
1312
1313 Move pv[MAX_PLY+1];
1314 StateInfo st;
1315 TTEntry* tte;
1316 Key posKey;
1317 Move ttMove, move, bestMove;
1318 Depth ttDepth;
1319 Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
1320 bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
1321 int moveCount;
1322
1323 if (PvNode)
1324 {
1325 oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
1326 (ss+1)->pv = pv;
1327 ss->pv[0] = MOVE_NONE;
1328 }
1329
1330 Thread* thisThread = pos.this_thread();
1331 (ss+1)->ply = ss->ply + 1;
1332 bestMove = MOVE_NONE;
1333 inCheck = pos.checkers();
1334 moveCount = 0;
1335
1336 // Check for an immediate draw or maximum ply reached
1337 if ( pos.is_draw(ss->ply)
1338 || ss->ply >= MAX_PLY)
1339 return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
1340
1341 assert(0 <= ss->ply && ss->ply < MAX_PLY);
1342
1343 // Decide whether or not to include checks: this fixes also the type of
1344 // TT entry depth that we are going to use. Note that in qsearch we use
1345 // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
1346 ttDepth = inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
1347 : DEPTH_QS_NO_CHECKS;
1348 // Transposition table lookup
1349 posKey = pos.key();
1350 tte = TT.probe(posKey, ttHit);
1351 ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
1352 ttMove = ttHit ? tte->move() : MOVE_NONE;
1353 pvHit = ttHit && tte->is_pv();
1354
1355 if ( !PvNode
1356 && ttHit
1357 && tte->depth() >= ttDepth
1358 && ttValue != VALUE_NONE // Only in case of TT access race
1359 && (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
1360 : (tte->bound() & BOUND_UPPER)))
1361 return ttValue;
1362
1363 // Evaluate the position statically
1364 if (inCheck)
1365 {
1366 ss->staticEval = VALUE_NONE;
1367 bestValue = futilityBase = -VALUE_INFINITE;
1368 }
1369 else
1370 {
1371 if (ttHit)
1372 {
1373 // Never assume anything about values stored in TT
1374 if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
1375 ss->staticEval = bestValue = evaluate(pos);
1376
1377 // Can ttValue be used as a better position evaluation?
1378 if ( ttValue != VALUE_NONE
1379 && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER)))
1380 bestValue = ttValue;
1381 }
1382 else
1383 ss->staticEval = bestValue =
1384 (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
1385 : -(ss-1)->staticEval + 2 * Eval::Tempo;
1386
1387 // Stand pat. Return immediately if static value is at least beta
1388 if (bestValue >= beta)
1389 {
1390 if (!ttHit)
1391 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit, BOUND_LOWER,
1392 DEPTH_NONE, MOVE_NONE, ss->staticEval);
1393
1394 return bestValue;
1395 }
1396
1397 if (PvNode && bestValue > alpha)
1398 alpha = bestValue;
1399
1400 futilityBase = bestValue + 153;
1401 }
1402
1403 const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
1404 nullptr, (ss-4)->continuationHistory,
1405 nullptr, (ss-6)->continuationHistory };
1406
1407 // Initialize a MovePicker object for the current position, and prepare
1408 // to search the moves. Because the depth is <= 0 here, only captures,
1409 // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
1410 // be generated.
1411 MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
1412 &thisThread->captureHistory,
1413 contHist,
1414 to_sq((ss-1)->currentMove));
1415
1416 // Loop through the moves until no moves remain or a beta cutoff occurs
1417 while ((move = mp.next_move()) != MOVE_NONE)
1418 {
1419 assert(is_ok(move));
1420
1421 givesCheck = pos.gives_check(move);
1422
1423 moveCount++;
1424
1425 // Futility pruning
1426 if ( !inCheck
1427 && !givesCheck
1428 && futilityBase > -VALUE_KNOWN_WIN
1429 && !pos.advanced_pawn_push(move))
1430 {
1431 assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
1432
1433 futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
1434
1435 if (futilityValue <= alpha)
1436 {
1437 bestValue = std::max(bestValue, futilityValue);
1438 continue;
1439 }
1440
1441 if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
1442 {
1443 bestValue = std::max(bestValue, futilityBase);
1444 continue;
1445 }
1446 }
1447
1448 // Detect non-capture evasions that are candidates to be pruned
1449 evasionPrunable = inCheck
1450 && (depth != DEPTH_ZERO || moveCount > 2)
1451 && bestValue > VALUE_MATED_IN_MAX_PLY
1452 && !pos.capture(move);
1453
1454 // Don't search moves with negative SEE values
1455 if ( (!inCheck || evasionPrunable)
1456 && (!givesCheck || !(pos.blockers_for_king(~pos.side_to_move()) & from_sq(move)))
1457 && !pos.see_ge(move))
1458 continue;
1459
1460 // Speculative prefetch as early as possible
1461 prefetch(TT.first_entry(pos.key_after(move)));
1462
1463 // Check for legality just before making the move
1464 if (!pos.legal(move))
1465 {
1466 moveCount--;
1467 continue;
1468 }
1469
1470 ss->currentMove = move;
1471 ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
1472
1473 // Make and search the move
1474 pos.do_move(move, st, givesCheck);
1475 value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
1476 pos.undo_move(move);
1477
1478 assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
1479
1480 // Check for a new best move
1481 if (value > bestValue)
1482 {
1483 bestValue = value;
1484
1485 if (value > alpha)
1486 {
1487 bestMove = move;
1488
1489 if (PvNode) // Update pv even in fail-high case
1490 update_pv(ss->pv, move, (ss+1)->pv);
1491
1492 if (PvNode && value < beta) // Update alpha here!
1493 alpha = value;
1494 else
1495 break; // Fail high
1496 }
1497 }
1498 }
1499
1500 // All legal moves have been searched. A special case: If we're in check
1501 // and no legal moves were found, it is checkmate.
1502 if (inCheck && bestValue == -VALUE_INFINITE)
1503 return mated_in(ss->ply); // Plies to mate from the root
1504
1505 tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
1506 bestValue >= beta ? BOUND_LOWER :
1507 PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
1508 ttDepth, bestMove, ss->staticEval);
1509
1510 assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
1511
1512 return bestValue;
1513 }
1514
1515
1516 // value_to_tt() adjusts a mate score from "plies to mate from the root" to
1517 // "plies to mate from the current position". Non-mate scores are unchanged.
1518 // The function is called before storing a value in the transposition table.
1519
1520 Value value_to_tt(Value v, int ply) {
1521
1522 assert(v != VALUE_NONE);
1523
1524 return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
1525 : v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
1526 }
1527
1528
1529 // value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
1530 // from the transposition table (which refers to the plies to mate/be mated
1531 // from current position) to "plies to mate/be mated from the root".
1532
1533 Value value_from_tt(Value v, int ply) {
1534
1535 return v == VALUE_NONE ? VALUE_NONE
1536 : v >= VALUE_MATE_IN_MAX_PLY ? v - ply
1537 : v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
1538 }
1539
1540
1541 // update_pv() adds current move and appends child pv[]
1542
1543 void update_pv(Move* pv, Move move, Move* childPv) {
1544
1545 for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
1546 *pv++ = *childPv++;
1547 *pv = MOVE_NONE;
1548 }
1549
1550
1551 // update_continuation_histories() updates histories of the move pairs formed
1552 // by moves at ply -1, -2, and -4 with current move.
1553
1554 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
1555
1556 for (int i : {1, 2, 4, 6})
1557 if (is_ok((ss-i)->currentMove))
1558 (*(ss-i)->continuationHistory)[pc][to] << bonus;
1559 }
1560
1561
1562 // update_capture_stats() updates move sorting heuristics when a new capture best move is found
1563
1564 void update_capture_stats(const Position& pos, Move move,
1565 Move* captures, int captureCount, int bonus) {
1566
1567 CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
1568 Piece moved_piece = pos.moved_piece(move);
1569 PieceType captured = type_of(pos.piece_on(to_sq(move)));
1570
1571 if (pos.capture_or_promotion(move))
1572 captureHistory[moved_piece][to_sq(move)][captured] << bonus;
1573
1574 // Decrease all the other played capture moves
1575 for (int i = 0; i < captureCount; ++i)
1576 {
1577 moved_piece = pos.moved_piece(captures[i]);
1578 captured = type_of(pos.piece_on(to_sq(captures[i])));
1579 captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
1580 }
1581 }
1582
1583
1584 // update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
1585
1586 void update_quiet_stats(const Position& pos, Stack* ss, Move move,
1587 Move* quiets, int quietCount, int bonus) {
1588
1589 if (ss->killers[0] != move)
1590 {
1591 ss->killers[1] = ss->killers[0];
1592 ss->killers[0] = move;
1593 }
1594
1595 Color us = pos.side_to_move();
1596 Thread* thisThread = pos.this_thread();
1597 thisThread->mainHistory[us][from_to(move)] << bonus;
1598 update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
1599
1600 if (is_ok((ss-1)->currentMove))
1601 {
1602 Square prevSq = to_sq((ss-1)->currentMove);
1603 thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
1604 }
1605
1606 // Decrease all the other played quiet moves
1607 for (int i = 0; i < quietCount; ++i)
1608 {
1609 thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
1610 update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
1611 }
1612 }
1613
1614 // When playing with strength handicap, choose best move among a set of RootMoves
1615 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
1616
1617 Move Skill::pick_best(size_t multiPV) {
1618
1619 const RootMoves& rootMoves = Threads.main()->rootMoves;
1620 static PRNG rng(now()); // PRNG sequence should be non-deterministic
1621
1622 // RootMoves are already sorted by score in descending order
1623 Value topScore = rootMoves[0].score;
1624 int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
1625 int weakness = 120 - 2 * level;
1626 int maxScore = -VALUE_INFINITE;
1627
1628 // Choose best move. For each move score we add two terms, both dependent on
1629 // weakness. One is deterministic and bigger for weaker levels, and one is
1630 // random. Then we choose the move with the resulting highest score.
1631 for (size_t i = 0; i < multiPV; ++i)
1632 {
1633 // This is our magic formula
1634 int push = ( weakness * int(topScore - rootMoves[i].score)
1635 + delta * (rng.rand<unsigned>() % weakness)) / 128;
1636
1637 if (rootMoves[i].score + push >= maxScore)
1638 {
1639 maxScore = rootMoves[i].score + push;
1640 best = rootMoves[i].pv[0];
1641 }
1642 }
1643
1644 return best;
1645 }
1646
1647} // namespace
1648
1649/// MainThread::check_time() is used to print debug info and, more importantly,
1650/// to detect when we are out of available time and thus stop the search.
1651
1652void MainThread::check_time() {
1653
1654 if (--callsCnt > 0)
1655 return;
1656
1657 // When using nodes, ensure checking rate is not lower than 0.1% of nodes
1658 callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
1659
1660 static TimePoint lastInfoTime = now();
1661
1662 TimePoint elapsed = Time.elapsed();
1663 TimePoint tick = Limits.startTime + elapsed;
1664
1665 if (tick - lastInfoTime >= 1000)
1666 {
1667 lastInfoTime = tick;
1668 dbg_print();
1669 }
1670
1671 // We should not stop pondering until told so by the GUI
1672 if (ponder)
1673 return;
1674
1675 if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
1676 || (Limits.movetime && elapsed >= Limits.movetime)
1677 || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
1678 Threads.stop = true;
1679}
1680
1681
1682/// UCI::pv() formats PV information according to the UCI protocol. UCI requires
1683/// that all (if any) unsearched PV lines are sent using a previous search score.
1684
1685string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
1686
1687 std::stringstream ss;
1688 TimePoint elapsed = Time.elapsed() + 1;
1689 const RootMoves& rootMoves = pos.this_thread()->rootMoves;
1690 size_t pvIdx = pos.this_thread()->pvIdx;
1691 size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
1692 uint64_t nodesSearched = Threads.nodes_searched();
1693 uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
1694
1695 for (size_t i = 0; i < multiPV; ++i)
1696 {
1697 bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
1698
1699 if (depth == ONE_PLY && !updated)
1700 continue;
1701
1702 Depth d = updated ? depth : depth - ONE_PLY;
1703 Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore;
1704
1705 bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
1706 v = tb ? rootMoves[i].tbScore : v;
1707
1708 if (ss.rdbuf()->in_avail()) // Not at first line
1709 ss << "\n";
1710
1711 ss << "info"
1712 << " depth " << d / ONE_PLY
1713 << " seldepth " << rootMoves[i].selDepth
1714 << " multipv " << i + 1
1715 << " score " << UCI::value(v);
1716
1717 if (!tb && i == pvIdx)
1718 ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
1719
1720 ss << " nodes " << nodesSearched
1721 << " nps " << nodesSearched * 1000 / elapsed;
1722
1723 if (elapsed > 1000) // Earlier makes little sense
1724 ss << " hashfull " << TT.hashfull();
1725
1726 ss << " tbhits " << tbHits
1727 << " time " << elapsed
1728 << " pv";
1729
1730 for (Move m : rootMoves[i].pv)
1731 ss << " " << UCI::move(m, pos.is_chess960());
1732 }
1733
1734 return ss.str();
1735}
1736
1737
1738/// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
1739/// before exiting the search, for instance, in case we stop the search during a
1740/// fail high at root. We try hard to have a ponder move to return to the GUI,
1741/// otherwise in case of 'ponder on' we have nothing to think on.
1742
1743bool RootMove::extract_ponder_from_tt(Position& pos) {
1744
1745 StateInfo st;
1746 bool ttHit;
1747
1748 assert(pv.size() == 1);
1749
1750 if (pv[0] == MOVE_NONE)
1751 return false;
1752
1753 pos.do_move(pv[0], st);
1754 TTEntry* tte = TT.probe(pos.key(), ttHit);
1755
1756 if (ttHit)
1757 {
1758 Move m = tte->move(); // Local copy to be SMP safe
1759 if (MoveList<LEGAL>(pos).contains(m))
1760 pv.push_back(m);
1761 }
1762
1763 pos.undo_move(pv[0]);
1764 return pv.size() > 1;
1765}
1766
1767void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
1768
1769 RootInTB = false;
1770 UseRule50 = bool(Options["Syzygy50MoveRule"]);
1771 ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
1772 Cardinality = int(Options["SyzygyProbeLimit"]);
1773 bool dtz_available = true;
1774
1775 // Tables with fewer pieces than SyzygyProbeLimit are searched with
1776 // ProbeDepth == DEPTH_ZERO
1777 if (Cardinality > MaxCardinality)
1778 {
1779 Cardinality = MaxCardinality;
1780 ProbeDepth = DEPTH_ZERO;
1781 }
1782
1783 if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
1784 {
1785 // Rank moves using DTZ tables
1786 RootInTB = root_probe(pos, rootMoves);
1787
1788 if (!RootInTB)
1789 {
1790 // DTZ tables are missing; try to rank moves using WDL tables
1791 dtz_available = false;
1792 RootInTB = root_probe_wdl(pos, rootMoves);
1793 }
1794 }
1795
1796 if (RootInTB)
1797 {
1798 // Sort moves according to TB rank
1799 std::sort(rootMoves.begin(), rootMoves.end(),
1800 [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
1801
1802 // Probe during search only if DTZ is not available and we are winning
1803 if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
1804 Cardinality = 0;
1805 }
1806 else
1807 {
1808 // Clean up if root_probe() and root_probe_wdl() have failed
1809 for (auto& m : rootMoves)
1810 m.tbRank = 0;
1811 }
1812}
1813