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) 2015-2017 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
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.
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.
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/>.
29 MAIN_SEARCH, CAPTURES_INIT, GOOD_CAPTURES, KILLERS, COUNTERMOVE, QUIET_INIT, QUIET, BAD_CAPTURES,
30 EVASION, EVASIONS_INIT, ALL_EVASIONS,
31 PROBCUT, PROBCUT_INIT, PROBCUT_CAPTURES,
32 QSEARCH_WITH_CHECKS, QCAPTURES_1_INIT, QCAPTURES_1, QCHECKS,
33 QSEARCH_NO_CHECKS, QCAPTURES_2_INIT, QCAPTURES_2,
34 QSEARCH_RECAPTURES, QRECAPTURES
37 // partial_insertion_sort() sorts moves in descending order up to and including
38 // a given limit. The order of moves smaller than the limit is left unspecified.
39 // To keep the implementation simple, *begin is always included in the sorted moves.
40 void partial_insertion_sort(ExtMove* begin, ExtMove* end, int limit) {
42 for (ExtMove *sortedEnd = begin + 1, *p = begin + 1; p < end; ++p)
43 if (p->value >= limit)
47 for (q = sortedEnd; q != begin && *(q-1) < tmp; --q)
54 // pick_best() finds the best move in the range (begin, end) and moves it to
55 // the front. It's faster than sorting all the moves in advance when there
56 // are few moves, e.g., the possible captures.
57 Move pick_best(ExtMove* begin, ExtMove* end) {
59 std::swap(*begin, *std::max_element(begin, end));
66 /// Constructors of the MovePicker class. As arguments we pass information
67 /// to help it to return the (presumably) good moves first, to decide which
68 /// moves to return (in the quiescence search, for instance, we only want to
69 /// search captures, promotions, and some checks) and how important good move
70 /// ordering is at the current node.
72 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, Search::Stack* s)
73 : pos(p), ss(s), depth(d) {
75 assert(d > DEPTH_ZERO);
77 Square prevSq = to_sq((ss-1)->currentMove);
78 countermove = pos.this_thread()->counterMoves[pos.piece_on(prevSq)][prevSq];
79 killers[0] = ss->killers[0];
80 killers[1] = ss->killers[1];
82 stage = pos.checkers() ? EVASION : MAIN_SEARCH;
83 ttMove = ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE;
84 stage += (ttMove == MOVE_NONE);
87 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, Square s)
90 assert(d <= DEPTH_ZERO);
95 else if (d > DEPTH_QS_NO_CHECKS)
96 stage = QSEARCH_WITH_CHECKS;
98 else if (d > DEPTH_QS_RECAPTURES)
99 stage = QSEARCH_NO_CHECKS;
103 stage = QSEARCH_RECAPTURES;
108 ttMove = ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE;
109 stage += (ttMove == MOVE_NONE);
112 MovePicker::MovePicker(const Position& p, Move ttm, Value th)
113 : pos(p), threshold(th) {
115 assert(!pos.checkers());
119 // In ProbCut we generate captures with SEE higher than or equal to the given threshold
121 && pos.pseudo_legal(ttm)
123 && pos.see_ge(ttm, threshold) ? ttm : MOVE_NONE;
125 stage += (ttMove == MOVE_NONE);
129 /// score() assigns a numerical value to each move in a move list. The moves with
130 /// highest values will be picked first.
132 void MovePicker::score<CAPTURES>() {
133 // Winning and equal captures in the main search are ordered by MVV, preferring
134 // captures near our home rank. Surprisingly, this appears to perform slightly
135 // better than SEE-based move ordering: exchanging big pieces before capturing
136 // a hanging piece probably helps to reduce the subtree size.
137 // In the main search we want to push captures with negative SEE values to the
138 // badCaptures[] array, but instead of doing it now we delay until the move
139 // has been picked up, saving some SEE calls in case we get a cutoff.
140 for (auto& m : *this)
141 m.value = PieceValue[MG][pos.piece_on(to_sq(m))]
142 - Value(200 * relative_rank(pos.side_to_move(), to_sq(m)));
146 void MovePicker::score<QUIETS>() {
148 const HistoryStats& history = pos.this_thread()->history;
150 const CounterMoveStats& cmh = *(ss-1)->counterMoves;
151 const CounterMoveStats& fmh = *(ss-2)->counterMoves;
152 const CounterMoveStats& fm2 = *(ss-4)->counterMoves;
154 Color c = pos.side_to_move();
156 for (auto& m : *this)
157 m.value = cmh[pos.moved_piece(m)][to_sq(m)]
158 + fmh[pos.moved_piece(m)][to_sq(m)]
159 + fm2[pos.moved_piece(m)][to_sq(m)]
164 void MovePicker::score<EVASIONS>() {
165 // Try captures ordered by MVV/LVA, then non-captures ordered by stats heuristics
166 const HistoryStats& history = pos.this_thread()->history;
167 Color c = pos.side_to_move();
169 for (auto& m : *this)
171 m.value = PieceValue[MG][pos.piece_on(to_sq(m))]
172 - Value(type_of(pos.moved_piece(m))) + HistoryStats::Max;
174 m.value = history.get(c, m);
178 /// next_move() is the most important method of the MovePicker class. It returns
179 /// a new pseudo legal move every time it is called, until there are no more moves
180 /// left. It picks the move with the biggest value from a list of generated moves
181 /// taking care not to return the ttMove if it has already been searched.
183 Move MovePicker::next_move(bool skipQuiets) {
189 case MAIN_SEARCH: case EVASION: case QSEARCH_WITH_CHECKS:
190 case QSEARCH_NO_CHECKS: case PROBCUT:
195 endBadCaptures = cur = moves;
196 endMoves = generate<CAPTURES>(pos, cur);
201 while (cur < endMoves)
203 move = pick_best(cur++, endMoves);
206 if (pos.see_ge(move, VALUE_ZERO))
209 // Losing capture, move it to the beginning of the array
210 *endBadCaptures++ = move;
215 move = killers[0]; // First killer move
216 if ( move != MOVE_NONE
218 && pos.pseudo_legal(move)
219 && !pos.capture(move))
224 move = killers[1]; // Second killer move
225 if ( move != MOVE_NONE
227 && pos.pseudo_legal(move)
228 && !pos.capture(move))
234 if ( move != MOVE_NONE
236 && move != killers[0]
237 && move != killers[1]
238 && pos.pseudo_legal(move)
239 && !pos.capture(move))
243 cur = endBadCaptures;
244 endMoves = generate<QUIETS>(pos, cur);
246 partial_insertion_sort(cur, endMoves, -4000 * depth / ONE_PLY);
250 while ( cur < endMoves
251 && (!skipQuiets || cur->value >= VALUE_ZERO))
256 && move != killers[0]
257 && move != killers[1]
258 && move != countermove)
262 cur = moves; // Point to beginning of bad captures
265 if (cur < endBadCaptures)
271 endMoves = generate<EVASIONS>(pos, cur);
276 while (cur < endMoves)
278 move = pick_best(cur++, endMoves);
286 endMoves = generate<CAPTURES>(pos, cur);
290 case PROBCUT_CAPTURES:
291 while (cur < endMoves)
293 move = pick_best(cur++, endMoves);
295 && pos.see_ge(move, threshold))
300 case QCAPTURES_1_INIT: case QCAPTURES_2_INIT:
302 endMoves = generate<CAPTURES>(pos, cur);
306 case QCAPTURES_1: case QCAPTURES_2:
307 while (cur < endMoves)
309 move = pick_best(cur++, endMoves);
313 if (stage == QCAPTURES_2)
316 endMoves = generate<QUIET_CHECKS>(pos, cur);
320 while (cur < endMoves)
328 case QSEARCH_RECAPTURES:
330 endMoves = generate<CAPTURES>(pos, cur);
335 while (cur < endMoves)
337 move = pick_best(cur++, endMoves);
338 if (to_sq(move) == recaptureSquare)