1 /*
2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2021 The Stockfish developers (see AUTHORS file)
4
5 Stockfish is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation, either version 3 of the License, or
8 (at your option) any later version.
9
10 Stockfish is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
14
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>.
17 */
18
19 #include <cassert>
20
21 #include "movepick.h"
22
23 namespace Stockfish {
24
25 namespace {
26
27 enum Stages {
28 MAIN_TT, CAPTURE_INIT, GOOD_CAPTURE, REFUTATION, QUIET_INIT, QUIET, BAD_CAPTURE,
29 EVASION_TT, EVASION_INIT, EVASION,
30 PROBCUT_TT, PROBCUT_INIT, PROBCUT,
31 QSEARCH_TT, QCAPTURE_INIT, QCAPTURE, QCHECK_INIT, QCHECK
32 };
33
34 // partial_insertion_sort() sorts moves in descending order up to and including
35 // a given limit. The order of moves smaller than the limit is left unspecified.
partial_insertion_sort(ExtMove * begin,ExtMove * end,int limit)36 void partial_insertion_sort(ExtMove* begin, ExtMove* end, int limit) {
37
38 for (ExtMove *sortedEnd = begin, *p = begin + 1; p < end; ++p)
39 if (p->value >= limit)
40 {
41 ExtMove tmp = *p, *q;
42 *p = *++sortedEnd;
43 for (q = sortedEnd; q != begin && *(q - 1) < tmp; --q)
44 *q = *(q - 1);
45 *q = tmp;
46 }
47 }
48
49 } // namespace
50
51
52 /// Constructors of the MovePicker class. As arguments we pass information
53 /// to help it to return the (presumably) good moves first, to decide which
54 /// moves to return (in the quiescence search, for instance, we only want to
55 /// search captures, promotions, and some checks) and how important good move
56 /// ordering is at the current node.
57
58 /// MovePicker constructor for the main search
MovePicker(const Position & p,Move ttm,Depth d,const ButterflyHistory * mh,const LowPlyHistory * lp,const CapturePieceToHistory * cph,const PieceToHistory ** ch,Move cm,const Move * killers,int pl)59 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHistory* mh, const LowPlyHistory* lp,
60 const CapturePieceToHistory* cph, const PieceToHistory** ch, Move cm, const Move* killers, int pl)
61 : pos(p), mainHistory(mh), lowPlyHistory(lp), captureHistory(cph), continuationHistory(ch),
62 ttMove(ttm), refutations{{killers[0], 0}, {killers[1], 0}, {cm, 0}}, depth(d), ply(pl) {
63
64 assert(d > 0);
65
66 stage = (pos.checkers() ? EVASION_TT : MAIN_TT) +
67 !(ttm && pos.pseudo_legal(ttm));
68 }
69
70 /// MovePicker constructor for quiescence search
MovePicker(const Position & p,Move ttm,Depth d,const ButterflyHistory * mh,const CapturePieceToHistory * cph,const PieceToHistory ** ch,Square rs)71 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHistory* mh,
72 const CapturePieceToHistory* cph, const PieceToHistory** ch, Square rs)
73 : pos(p), mainHistory(mh), captureHistory(cph), continuationHistory(ch), ttMove(ttm), recaptureSquare(rs), depth(d) {
74
75 assert(d <= 0);
76
77 stage = (pos.checkers() ? EVASION_TT : QSEARCH_TT) +
78 !( ttm
79 && (pos.checkers() || depth > DEPTH_QS_RECAPTURES || to_sq(ttm) == recaptureSquare)
80 && pos.pseudo_legal(ttm));
81 }
82
83 /// MovePicker constructor for ProbCut: we generate captures with SEE greater
84 /// than or equal to the given threshold.
MovePicker(const Position & p,Move ttm,Value th,const CapturePieceToHistory * cph)85 MovePicker::MovePicker(const Position& p, Move ttm, Value th, const CapturePieceToHistory* cph)
86 : pos(p), captureHistory(cph), ttMove(ttm), threshold(th) {
87
88 assert(!pos.checkers());
89
90 stage = PROBCUT_TT + !(ttm && pos.capture(ttm)
91 && pos.pseudo_legal(ttm)
92 && pos.see_ge(ttm, threshold));
93 }
94
95 /// MovePicker::score() assigns a numerical value to each move in a list, used
96 /// for sorting. Captures are ordered by Most Valuable Victim (MVV), preferring
97 /// captures with a good history. Quiets moves are ordered using the histories.
98 template<GenType Type>
score()99 void MovePicker::score() {
100
101 static_assert(Type == CAPTURES || Type == QUIETS || Type == EVASIONS, "Wrong type");
102
103 for (auto& m : *this)
104 if constexpr (Type == CAPTURES)
105 m.value = int(PieceValue[MG][pos.piece_on(to_sq(m))]) * 6
106 + (*captureHistory)[pos.moved_piece(m)][to_sq(m)][type_of(pos.piece_on(to_sq(m)))];
107
108 else if constexpr (Type == QUIETS)
109 m.value = (*mainHistory)[pos.side_to_move()][from_to(m)]
110 + 2 * (*continuationHistory[0])[pos.moved_piece(m)][to_sq(m)]
111 + (*continuationHistory[1])[pos.moved_piece(m)][to_sq(m)]
112 + (*continuationHistory[3])[pos.moved_piece(m)][to_sq(m)]
113 + (*continuationHistory[5])[pos.moved_piece(m)][to_sq(m)]
114 + (ply < MAX_LPH ? std::min(4, depth / 3) * (*lowPlyHistory)[ply][from_to(m)] : 0);
115
116 else // Type == EVASIONS
117 {
118 if (pos.capture(m))
119 m.value = PieceValue[MG][pos.piece_on(to_sq(m))]
120 - Value(type_of(pos.moved_piece(m)));
121 else
122 m.value = (*mainHistory)[pos.side_to_move()][from_to(m)]
123 + 2 * (*continuationHistory[0])[pos.moved_piece(m)][to_sq(m)]
124 - (1 << 28);
125 }
126 }
127
128 /// MovePicker::select() returns the next move satisfying a predicate function.
129 /// It never returns the TT move.
130 template<MovePicker::PickType T, typename Pred>
select(Pred filter)131 Move MovePicker::select(Pred filter) {
132
133 while (cur < endMoves)
134 {
135 if (T == Best)
136 std::swap(*cur, *std::max_element(cur, endMoves));
137
138 if (*cur != ttMove && filter())
139 return *cur++;
140
141 cur++;
142 }
143 return MOVE_NONE;
144 }
145
146 /// MovePicker::next_move() is the most important method of the MovePicker class. It
147 /// returns a new pseudo-legal move every time it is called until there are no more
148 /// moves left, picking the move with the highest score from a list of generated moves.
next_move(bool skipQuiets)149 Move MovePicker::next_move(bool skipQuiets) {
150
151 top:
152 switch (stage) {
153
154 case MAIN_TT:
155 case EVASION_TT:
156 case QSEARCH_TT:
157 case PROBCUT_TT:
158 ++stage;
159 return ttMove;
160
161 case CAPTURE_INIT:
162 case PROBCUT_INIT:
163 case QCAPTURE_INIT:
164 cur = endBadCaptures = moves;
165 endMoves = generate<CAPTURES>(pos, cur);
166
167 score<CAPTURES>();
168 ++stage;
169 goto top;
170
171 case GOOD_CAPTURE:
172 if (select<Best>([&](){
173 return pos.see_ge(*cur, Value(-69 * cur->value / 1024)) ?
174 // Move losing capture to endBadCaptures to be tried later
175 true : (*endBadCaptures++ = *cur, false); }))
176 return *(cur - 1);
177
178 // Prepare the pointers to loop over the refutations array
179 cur = std::begin(refutations);
180 endMoves = std::end(refutations);
181
182 // If the countermove is the same as a killer, skip it
183 if ( refutations[0].move == refutations[2].move
184 || refutations[1].move == refutations[2].move)
185 --endMoves;
186
187 ++stage;
188 [[fallthrough]];
189
190 case REFUTATION:
191 if (select<Next>([&](){ return *cur != MOVE_NONE
192 && !pos.capture(*cur)
193 && pos.pseudo_legal(*cur); }))
194 return *(cur - 1);
195 ++stage;
196 [[fallthrough]];
197
198 case QUIET_INIT:
199 if (!skipQuiets)
200 {
201 cur = endBadCaptures;
202 endMoves = generate<QUIETS>(pos, cur);
203
204 score<QUIETS>();
205 partial_insertion_sort(cur, endMoves, -3000 * depth);
206 }
207
208 ++stage;
209 [[fallthrough]];
210
211 case QUIET:
212 if ( !skipQuiets
213 && select<Next>([&](){return *cur != refutations[0].move
214 && *cur != refutations[1].move
215 && *cur != refutations[2].move;}))
216 return *(cur - 1);
217
218 // Prepare the pointers to loop over the bad captures
219 cur = moves;
220 endMoves = endBadCaptures;
221
222 ++stage;
223 [[fallthrough]];
224
225 case BAD_CAPTURE:
226 return select<Next>([](){ return true; });
227
228 case EVASION_INIT:
229 cur = moves;
230 endMoves = generate<EVASIONS>(pos, cur);
231
232 score<EVASIONS>();
233 ++stage;
234 [[fallthrough]];
235
236 case EVASION:
237 return select<Best>([](){ return true; });
238
239 case PROBCUT:
240 return select<Best>([&](){ return pos.see_ge(*cur, threshold); });
241
242 case QCAPTURE:
243 if (select<Best>([&](){ return depth > DEPTH_QS_RECAPTURES
244 || to_sq(*cur) == recaptureSquare; }))
245 return *(cur - 1);
246
247 // If we did not find any move and we do not try checks, we have finished
248 if (depth != DEPTH_QS_CHECKS)
249 return MOVE_NONE;
250
251 ++stage;
252 [[fallthrough]];
253
254 case QCHECK_INIT:
255 cur = moves;
256 endMoves = generate<QUIET_CHECKS>(pos, cur);
257
258 ++stage;
259 [[fallthrough]];
260
261 case QCHECK:
262 return select<Next>([](){ return true; });
263 }
264
265 assert(false);
266 return MOVE_NONE; // Silence warning
267 }
268
269 } // namespace Stockfish
270