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
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.
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_S1, KILLERS_S1, QUIETS_1_S1, QUIETS_2_S1, BAD_CAPTURES_S1,
31 QSEARCH_0, CAPTURES_S3, QUIET_CHECKS_S3,
32 QSEARCH_1, CAPTURES_S4,
34 RECAPTURE, CAPTURES_S6,
38 // Our insertion sort, which is guaranteed to be stable, as it should be
39 void insertion_sort(ExtMove* begin, ExtMove* end)
43 for (p = begin + 1; p < end; ++p)
46 for (q = p; q != begin && *(q-1) < tmp; --q)
52 // pick_best() finds the best move in the range (begin, end) and moves it to
53 // the front. It's faster than sorting all the moves in advance when there
54 // are few moves e.g. the possible captures.
55 inline Move pick_best(ExtMove* begin, ExtMove* end)
57 std::swap(*begin, *std::max_element(begin, end));
64 /// Constructors of the MovePicker class. As arguments we pass information
65 /// to help it to return the (presumably) good moves first, to decide which
66 /// moves to return (in the quiescence search, for instance, we only want to
67 /// search captures, promotions and some checks) and how important good move
68 /// ordering is at the current node.
70 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const HistoryStats& h,
71 Move* cm, Move* fm, Search::Stack* s) : pos(p), history(h), depth(d) {
73 assert(d > DEPTH_ZERO);
75 endBadCaptures = moves + MAX_MOVES - 1;
86 ttMove = (ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE);
87 endMoves += (ttMove != MOVE_NONE);
90 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const HistoryStats& h,
91 Square s) : pos(p), history(h) {
93 assert(d <= DEPTH_ZERO);
98 else if (d > DEPTH_QS_NO_CHECKS)
101 else if (d > DEPTH_QS_RECAPTURES)
111 ttMove = (ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE);
112 endMoves += (ttMove != MOVE_NONE);
115 MovePicker::MovePicker(const Position& p, Move ttm, const HistoryStats& h, PieceType pt)
116 : pos(p), history(h) {
118 assert(!pos.checkers());
122 // In ProbCut we generate only captures that are better than the parent's
124 captureThreshold = PieceValue[MG][pt];
125 ttMove = (ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE);
127 if (ttMove && (!pos.capture(ttMove) || pos.see(ttMove) <= captureThreshold))
130 endMoves += (ttMove != MOVE_NONE);
134 /// score() assign a numerical value to each move in a move list. The moves with
135 /// highest values will be picked first.
137 void MovePicker::score<CAPTURES>() {
138 // Winning and equal captures in the main search are ordered by MVV/LVA.
139 // Suprisingly, this appears to perform slightly better than SEE based
140 // move ordering. The reason is probably that in a position with a winning
141 // capture, capturing a more valuable (but sufficiently defended) piece
142 // first usually doesn't hurt. The opponent will have to recapture, and
143 // the hanging piece will still be hanging (except in the unusual cases
144 // where it is possible to recapture with the hanging piece). Exchanging
145 // big pieces before capturing a hanging piece probably helps to reduce
147 // In main search we want to push captures with negative SEE values to the
148 // badCaptures[] array, but instead of doing it now we delay until the move
149 // has been picked up in pick_move_from_list(). This way we save some SEE
150 // calls in case we get a cutoff.
151 for (auto& m : *this)
152 if (type_of(m) == ENPASSANT)
153 m.value = PieceValue[MG][PAWN] - Value(PAWN);
155 else if (type_of(m) == PROMOTION)
156 m.value = PieceValue[MG][pos.piece_on(to_sq(m))] - Value(PAWN)
157 + PieceValue[MG][promotion_type(m)] - PieceValue[MG][PAWN];
159 m.value = PieceValue[MG][pos.piece_on(to_sq(m))]
160 - Value(type_of(pos.moved_piece(m)));
164 void MovePicker::score<QUIETS>() {
165 for (auto& m : *this)
166 m.value = history[pos.moved_piece(m)][to_sq(m)];
170 void MovePicker::score<EVASIONS>() {
171 // Try good captures ordered by MVV/LVA, then non-captures if destination square
172 // is not under attack, ordered by history value, then bad-captures and quiet
173 // moves with a negative SEE. This last group is ordered by the SEE value.
176 for (auto& m : *this)
177 if ((see = pos.see_sign(m)) < VALUE_ZERO)
178 m.value = see - HistoryStats::Max; // At the bottom
180 else if (pos.capture(m))
181 m.value = PieceValue[MG][pos.piece_on(to_sq(m))]
182 - Value(type_of(pos.moved_piece(m))) + HistoryStats::Max;
184 m.value = history[pos.moved_piece(m)][to_sq(m)];
188 /// generate_next_stage() generates, scores and sorts the next bunch of moves,
189 /// when there are no more moves to try for the current stage.
191 void MovePicker::generate_next_stage() {
197 case CAPTURES_S1: case CAPTURES_S3: case CAPTURES_S4: case CAPTURES_S5: case CAPTURES_S6:
198 endMoves = generate<CAPTURES>(pos, moves);
206 killers[0] = ss->killers[0];
207 killers[1] = ss->killers[1];
208 killers[2].move = killers[3].move = MOVE_NONE;
209 killers[4].move = killers[5].move = MOVE_NONE;
211 // In SMP case countermoves[] and followupmoves[] could have duplicated entries
212 // in rare cases (less than 1 out of a million). This is harmless.
214 // Be sure countermoves and followupmoves are different from killers
215 for (int i = 0; i < 2; ++i)
216 if ( countermoves[i] != killers[0]
217 && countermoves[i] != killers[1])
218 *endMoves++ = countermoves[i];
220 for (int i = 0; i < 2; ++i)
221 if ( followupmoves[i] != killers[0]
222 && followupmoves[i] != killers[1]
223 && followupmoves[i] != killers[2]
224 && followupmoves[i] != killers[3])
225 *endMoves++ = followupmoves[i];
229 endQuiets = endMoves = generate<QUIETS>(pos, moves);
231 endMoves = std::partition(cur, endMoves, [](const ExtMove& m) { return m.value > VALUE_ZERO; });
232 insertion_sort(cur, endMoves);
237 endMoves = endQuiets;
238 if (depth >= 3 * ONE_PLY)
239 insertion_sort(cur, endMoves);
242 case BAD_CAPTURES_S1:
243 // Just pick them in reverse order to get MVV/LVA ordering
244 cur = moves + MAX_MOVES - 1;
245 endMoves = endBadCaptures;
249 endMoves = generate<EVASIONS>(pos, moves);
250 if (endMoves - moves > 1)
254 case QUIET_CHECKS_S3:
255 endMoves = generate<QUIET_CHECKS>(pos, moves);
258 case EVASION: case QSEARCH_0: case QSEARCH_1: case PROBCUT: case RECAPTURE:
263 endMoves = cur + 1; // Avoid another generate_next_stage() call
272 /// next_move() is the most important method of the MovePicker class. It returns
273 /// a new pseudo legal move every time it is called, until there are no more moves
274 /// left. It picks the move with the biggest value from a list of generated moves
275 /// taking care not to return the ttMove if it has already been searched.
277 Move MovePicker::next_move<false>() {
283 while (cur == endMoves)
284 generate_next_stage();
288 case MAIN_SEARCH: case EVASION: case QSEARCH_0: case QSEARCH_1: case PROBCUT:
293 move = pick_best(cur++, endMoves);
296 if (pos.see_sign(move) >= VALUE_ZERO)
299 // Losing capture, move it to the tail of the array
300 *endBadCaptures-- = move;
306 if ( move != MOVE_NONE
308 && pos.pseudo_legal(move)
309 && !pos.capture(move))
313 case QUIETS_1_S1: case QUIETS_2_S1:
316 && move != killers[0]
317 && move != killers[1]
318 && move != killers[2]
319 && move != killers[3]
320 && move != killers[4]
321 && move != killers[5])
325 case BAD_CAPTURES_S1:
328 case EVASIONS_S2: case CAPTURES_S3: case CAPTURES_S4:
329 move = pick_best(cur++, endMoves);
335 move = pick_best(cur++, endMoves);
336 if (move != ttMove && pos.see(move) > captureThreshold)
341 move = pick_best(cur++, endMoves);
342 if (to_sq(move) == recaptureSquare)
346 case QUIET_CHECKS_S3:
362 /// Version of next_move() to use at split point nodes where the move is grabbed
363 /// from the split point's shared MovePicker object. This function is not thread
364 /// safe so must be lock protected by the caller.
366 Move MovePicker::next_move<true>() { return ss->splitPoint->movePicker->next_move<false>(); }