2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2012 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/>.
31 MAIN_SEARCH, CAPTURES_S1, KILLERS_S1, QUIETS_1_S1, QUIETS_2_S1, BAD_CAPTURES_S1,
33 QSEARCH_0, CAPTURES_S3, QUIET_CHECKS_S3,
34 QSEARCH_1, CAPTURES_S4,
36 RECAPTURE, CAPTURES_S6,
40 // Unary predicate used by std::partition to split positive scores from remaining
41 // ones so to sort separately the two sets, and with the second sort delayed.
42 inline bool has_positive_score(const MoveStack& ms) { return ms.score > 0; }
44 // Picks and moves to the front the best move in the range [begin, end),
45 // it is faster than sorting all the moves in advance when moves are few, as
46 // normally are the possible captures.
47 inline MoveStack* pick_best(MoveStack* begin, MoveStack* end)
49 std::swap(*begin, *std::max_element(begin, end));
55 /// Constructors of the MovePicker class. As arguments we pass information
56 /// to help it to return the presumably good moves first, to decide which
57 /// moves to return (in the quiescence search, for instance, we only want to
58 /// search captures, promotions and some checks) and about how important good
59 /// move ordering is at the current node.
61 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h,
62 Search::Stack* s, Value beta) : pos(p), H(h), depth(d) {
64 assert(d > DEPTH_ZERO);
68 endBadCaptures = moves + MAX_MOVES - 1;
78 killers[0].move = ss->killers[0];
79 killers[1].move = ss->killers[1];
81 // Consider sligtly negative captures as good if at low depth and far from beta
82 if (ss && ss->staticEval < beta - PawnValueMg && d < 3 * ONE_PLY)
83 captureThreshold = -PawnValueMg;
85 // Consider negative captures as good if still enough to reach beta
86 else if (ss && ss->staticEval > beta)
87 captureThreshold = beta - ss->staticEval;
90 ttMove = (ttm && pos.is_pseudo_legal(ttm) ? ttm : MOVE_NONE);
91 end += (ttMove != MOVE_NONE);
94 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h,
95 Square sq) : pos(p), H(h), cur(moves), end(moves) {
97 assert(d <= DEPTH_ZERO);
102 else if (d > DEPTH_QS_NO_CHECKS)
105 else if (d > DEPTH_QS_RECAPTURES)
109 // Skip TT move if is not a capture or a promotion, this avoids qsearch
110 // tree explosion due to a possible perpetual check or similar rare cases
111 // when TT table is full.
112 if (ttm && !pos.is_capture_or_promotion(ttm))
118 recaptureSquare = sq;
122 ttMove = (ttm && pos.is_pseudo_legal(ttm) ? ttm : MOVE_NONE);
123 end += (ttMove != MOVE_NONE);
126 MovePicker::MovePicker(const Position& p, Move ttm, const History& h, PieceType pt)
127 : pos(p), H(h), cur(moves), end(moves) {
129 assert(!pos.checkers());
133 // In ProbCut we generate only captures better than parent's captured piece
134 captureThreshold = PieceValue[MG][pt];
135 ttMove = (ttm && pos.is_pseudo_legal(ttm) ? ttm : MOVE_NONE);
137 if (ttMove && (!pos.is_capture(ttMove) || pos.see(ttMove) <= captureThreshold))
140 end += (ttMove != MOVE_NONE);
144 /// MovePicker::score_captures(), MovePicker::score_noncaptures() and
145 /// MovePicker::score_evasions() assign a numerical move ordering score
146 /// to each move in a move list. The moves with highest scores will be
147 /// picked first by next_move().
149 void MovePicker::score_captures() {
150 // Winning and equal captures in the main search are ordered by MVV/LVA.
151 // Suprisingly, this appears to perform slightly better than SEE based
152 // move ordering. The reason is probably that in a position with a winning
153 // capture, capturing a more valuable (but sufficiently defended) piece
154 // first usually doesn't hurt. The opponent will have to recapture, and
155 // the hanging piece will still be hanging (except in the unusual cases
156 // where it is possible to recapture with the hanging piece). Exchanging
157 // big pieces before capturing a hanging piece probably helps to reduce
159 // In main search we want to push captures with negative SEE values to
160 // badCaptures[] array, but instead of doing it now we delay till when
161 // the move has been picked up in pick_move_from_list(), this way we save
162 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
165 for (MoveStack* it = moves; it != end; ++it)
168 it->score = PieceValue[MG][pos.piece_on(to_sq(m))]
169 - type_of(pos.piece_moved(m));
171 if (type_of(m) == PROMOTION)
172 it->score += PieceValue[MG][promotion_type(m)];
176 void MovePicker::score_noncaptures() {
180 for (MoveStack* it = moves; it != end; ++it)
183 it->score = H.value(pos.piece_moved(m), to_sq(m));
187 void MovePicker::score_evasions() {
188 // Try good captures ordered by MVV/LVA, then non-captures if destination square
189 // is not under attack, ordered by history value, then bad-captures and quiet
190 // moves with a negative SEE. This last group is ordered by the SEE score.
197 for (MoveStack* it = moves; it != end; ++it)
200 if ((seeScore = pos.see_sign(m)) < 0)
201 it->score = seeScore - History::MaxValue; // Be sure we are at the bottom
202 else if (pos.is_capture(m))
203 it->score = PieceValue[MG][pos.piece_on(to_sq(m))]
204 - type_of(pos.piece_moved(m)) + History::MaxValue;
206 it->score = H.value(pos.piece_moved(m), to_sq(m));
211 /// MovePicker::generate_next() generates, scores and sorts the next bunch of moves,
212 /// when there are no more moves to try for the current phase.
214 void MovePicker::generate_next() {
220 case CAPTURES_S1: case CAPTURES_S3: case CAPTURES_S4: case CAPTURES_S5: case CAPTURES_S6:
221 end = generate<CAPTURES>(pos, moves);
231 endQuiets = end = generate<QUIETS>(pos, moves);
233 end = std::partition(cur, end, has_positive_score);
234 sort<MoveStack>(cur, end);
240 if (depth >= 3 * ONE_PLY)
241 sort<MoveStack>(cur, end);
244 case BAD_CAPTURES_S1:
245 // Just pick them in reverse order to get MVV/LVA ordering
246 cur = moves + MAX_MOVES - 1;
247 end = endBadCaptures;
251 end = generate<EVASIONS>(pos, moves);
255 case QUIET_CHECKS_S3:
256 end = generate<QUIET_CHECKS>(pos, moves);
259 case EVASION: case QSEARCH_0: case QSEARCH_1: case PROBCUT: case RECAPTURE:
262 end = cur + 1; // Avoid another next_phase() call
271 /// MovePicker::next_move() is the most important method of the MovePicker class.
272 /// It returns a new pseudo legal move every time it is called, until there
273 /// are no more moves left. It picks the move with the biggest score from a list
274 /// of generated moves taking care not to return the tt move if has already been
275 /// searched previously.
277 Move MovePicker::next_move<false>() {
288 case MAIN_SEARCH: case EVASION: case QSEARCH_0: case QSEARCH_1: case PROBCUT:
293 move = pick_best(cur++, end)->move;
296 assert(captureThreshold <= 0); // Otherwise we cannot use see_sign()
298 if (pos.see_sign(move) >= captureThreshold)
301 // Losing capture, move it to the tail of the array
302 (endBadCaptures--)->move = move;
307 move = (cur++)->move;
308 if ( move != MOVE_NONE
309 && pos.is_pseudo_legal(move)
311 && !pos.is_capture(move))
315 case QUIETS_1_S1: case QUIETS_2_S1:
316 move = (cur++)->move;
318 && move != killers[0].move
319 && move != killers[1].move)
323 case BAD_CAPTURES_S1:
324 return (cur--)->move;
326 case EVASIONS_S2: case CAPTURES_S3: case CAPTURES_S4:
327 move = pick_best(cur++, end)->move;
333 move = pick_best(cur++, end)->move;
334 if (move != ttMove && pos.see(move) > captureThreshold)
339 move = pick_best(cur++, end)->move;
340 if (to_sq(move) == recaptureSquare)
344 case QUIET_CHECKS_S3:
345 move = (cur++)->move;
360 /// Version of next_move() to use at split point nodes where the move is grabbed
361 /// from the split point's shared MovePicker object. This function is not thread
362 /// safe so should be lock protected by the caller.
364 Move MovePicker::next_move<true>() { return ss->sp->mp->next_move<false>(); }