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/>.
30 MAIN_SEARCH, TT_MOVE_S1, CAPTURES_S1, KILLERS_S1, QUIETS_1_S1,
31 QUIETS_2_S1, BAD_CAPTURES_S1, STOP_S1,
32 EVASIONS, TT_MOVE_S2, EVASIONS_S2, STOP_S2,
33 CAPTURES_AND_CHECKS, TT_MOVE_S3, CAPTURES_S3, QUIET_CHECKS_S3, STOP_S3,
34 CAPTURES, TT_MOVE_S4, CAPTURES_S4, STOP_S4,
35 PROBCUT, TT_MOVE_S5, CAPTURES_S5, STOP_S5,
36 RECAPTURES, CAPTURES_S6, STOP_S6
39 // Unary predicate used by std::partition to split positive scores from remaining
40 // ones so to sort separately the two sets, and with the second sort delayed.
41 inline bool has_positive_score(const MoveStack& move) { return move.score > 0; }
43 // Picks and moves to the front the best move in the range [firstMove, lastMove),
44 // it is faster than sorting all the moves in advance when moves are few, as
45 // normally are the possible captures.
46 inline MoveStack* pick_best(MoveStack* firstMove, MoveStack* lastMove)
48 std::swap(*firstMove, *std::max_element(firstMove, lastMove));
54 /// Constructors of the MovePicker class. As arguments we pass information
55 /// to help it to return the presumably good moves first, to decide which
56 /// moves to return (in the quiescence search, for instance, we only want to
57 /// search captures, promotions and some checks) and about how important good
58 /// move ordering is at the current node.
60 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h,
61 Search::Stack* ss, Value beta) : pos(p), H(h), depth(d) {
63 assert(d > DEPTH_ZERO);
66 curMove = lastMove = 0;
67 badCaptures = moves + MAX_MOVES;
74 killers[0].move = ss->killers[0];
75 killers[1].move = ss->killers[1];
77 // Consider sligtly negative captures as good if at low depth and far from beta
78 if (ss && ss->eval < beta - PawnValueMidgame && d < 3 * ONE_PLY)
79 captureThreshold = -PawnValueMidgame;
81 // Consider negative captures as good if still enough to reach beta
82 else if (ss && ss->eval > beta)
83 captureThreshold = beta - ss->eval;
88 ttMove = (ttm && pos.is_pseudo_legal(ttm) ? ttm : MOVE_NONE);
89 phase += (ttMove == MOVE_NONE);
92 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h,
93 Square sq) : pos(p), H(h), curMove(0), lastMove(0) {
95 assert(d <= DEPTH_ZERO);
100 else if (d >= DEPTH_QS_CHECKS)
101 phase = CAPTURES_AND_CHECKS;
103 else if (d >= DEPTH_QS_RECAPTURES)
107 // Skip TT move if is not a capture or a promotion, this avoids qsearch
108 // tree explosion due to a possible perpetual check or similar rare cases
109 // when TT table is full.
110 if (ttm && !pos.is_capture_or_promotion(ttm))
115 phase = RECAPTURES - 1;
116 recaptureSquare = sq;
120 ttMove = (ttm && pos.is_pseudo_legal(ttm) ? ttm : MOVE_NONE);
121 phase += (ttMove == MOVE_NONE);
124 MovePicker::MovePicker(const Position& p, Move ttm, const History& h,
125 PieceType parentCapture) : pos(p), H(h) {
127 assert (!pos.in_check());
129 // In ProbCut we consider only captures better than parent's move
130 captureThreshold = PieceValueMidgame[Piece(parentCapture)];
131 curMove = lastMove = 0;
134 if ( ttm != MOVE_NONE
135 && (!pos.is_capture(ttm) || pos.see(ttm) <= captureThreshold))
138 ttMove = (ttm && pos.is_pseudo_legal(ttm) ? ttm : MOVE_NONE);
139 phase += (ttMove == MOVE_NONE);
143 /// MovePicker::score_captures(), MovePicker::score_noncaptures() and
144 /// MovePicker::score_evasions() assign a numerical move ordering score
145 /// to each move in a move list. The moves with highest scores will be
146 /// picked first by next_move().
148 void MovePicker::score_captures() {
149 // Winning and equal captures in the main search are ordered by MVV/LVA.
150 // Suprisingly, this appears to perform slightly better than SEE based
151 // move ordering. The reason is probably that in a position with a winning
152 // capture, capturing a more valuable (but sufficiently defended) piece
153 // first usually doesn't hurt. The opponent will have to recapture, and
154 // the hanging piece will still be hanging (except in the unusual cases
155 // where it is possible to recapture with the hanging piece). Exchanging
156 // big pieces before capturing a hanging piece probably helps to reduce
158 // In main search we want to push captures with negative SEE values to
159 // badCaptures[] array, but instead of doing it now we delay till when
160 // the move has been picked up in pick_move_from_list(), this way we save
161 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
164 for (MoveStack* cur = moves; cur != lastMove; cur++)
167 cur->score = PieceValueMidgame[pos.piece_on(to_sq(m))]
168 - type_of(pos.piece_moved(m));
171 cur->score += PieceValueMidgame[Piece(promotion_piece_type(m))];
175 void MovePicker::score_noncaptures() {
180 for (MoveStack* cur = moves; cur != lastMove; cur++)
184 cur->score = H.value(pos.piece_on(from), to_sq(m));
188 void MovePicker::score_evasions() {
189 // Try good captures ordered by MVV/LVA, then non-captures if destination square
190 // is not under attack, ordered by history value, then bad-captures and quiet
191 // moves with a negative SEE. This last group is ordered by the SEE score.
195 if (lastMove < moves + 2)
198 for (MoveStack* cur = moves; cur != lastMove; cur++)
201 if ((seeScore = pos.see_sign(m)) < 0)
202 cur->score = seeScore - History::MaxValue; // Be sure we are at the bottom
203 else if (pos.is_capture(m))
204 cur->score = PieceValueMidgame[pos.piece_on(to_sq(m))]
205 - type_of(pos.piece_moved(m)) + History::MaxValue;
207 cur->score = H.value(pos.piece_moved(m), to_sq(m));
212 /// MovePicker::next_phase() generates, scores and sorts the next bunch of moves,
213 /// when there are no more moves to try for the current phase.
215 void MovePicker::next_phase() {
221 case TT_MOVE_S1: case TT_MOVE_S2: case TT_MOVE_S3: case TT_MOVE_S4: case TT_MOVE_S5:
222 lastMove = curMove + 1;
225 case CAPTURES_S1: case CAPTURES_S3: case CAPTURES_S4:
226 case CAPTURES_S5: case CAPTURES_S6:
227 lastMove = generate<MV_CAPTURE>(pos, moves);
233 lastMove = curMove + 2;
237 lastQuiet = lastMove = generate<MV_QUIET>(pos, moves);
239 lastMove = std::partition(curMove, lastMove, has_positive_score);
240 sort<MoveStack>(curMove, lastMove);
245 lastMove = lastQuiet;
246 if (depth >= 3 * ONE_PLY)
247 sort<MoveStack>(curMove, lastMove);
250 case BAD_CAPTURES_S1:
251 // Bad captures SEE value is already calculated so just pick them in order
252 // to get SEE move ordering.
253 curMove = badCaptures;
254 lastMove = moves + MAX_MOVES;
258 assert(pos.in_check());
259 lastMove = generate<MV_EVASION>(pos, moves);
263 case QUIET_CHECKS_S3:
264 lastMove = generate<MV_QUIET_CHECK>(pos, moves);
267 case STOP_S1: case STOP_S2: case STOP_S3: case STOP_S4: case STOP_S5: case STOP_S6:
268 lastMove = curMove + 1; // Avoid another next_phase() call
277 /// MovePicker::next_move() is the most important method of the MovePicker class.
278 /// It returns a new pseudo legal move every time it is called, until there
279 /// are no more moves left. It picks the move with the biggest score from a list
280 /// of generated moves taking care not to return the tt move if has already been
281 /// searched previously. Note that this function is not thread safe so should be
282 /// lock protected by caller when accessed through a shared MovePicker object.
284 Move MovePicker::next_move() {
290 while (curMove == lastMove)
295 case TT_MOVE_S1: case TT_MOVE_S2: case TT_MOVE_S3: case TT_MOVE_S4: case TT_MOVE_S5:
301 move = pick_best(curMove++, lastMove)->move;
304 assert(captureThreshold <= 0); // Otherwise we cannot use see_sign()
306 int seeScore = pos.see_sign(move);
307 if (seeScore >= captureThreshold)
310 // Losing capture, move it to the tail of the array
311 (--badCaptures)->move = move;
312 badCaptures->score = seeScore;
317 move = (curMove++)->move;
318 if ( move != MOVE_NONE
319 && pos.is_pseudo_legal(move)
321 && !pos.is_capture(move))
327 move = (curMove++)->move;
329 && move != killers[0].move
330 && move != killers[1].move)
334 case BAD_CAPTURES_S1:
335 move = pick_best(curMove++, lastMove)->move;
339 case CAPTURES_S3: case CAPTURES_S4:
340 move = pick_best(curMove++, lastMove)->move;
346 move = pick_best(curMove++, lastMove)->move;
348 && pos.see(move) > captureThreshold)
353 move = pick_best(curMove++, lastMove)->move;
354 if (to_sq(move) == recaptureSquare)
358 case QUIET_CHECKS_S3:
359 move = (curMove++)->move;
364 case STOP_S1: case STOP_S2: case STOP_S3: case STOP_S4: case STOP_S5: case STOP_S6: