2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2009 Marco Costalba
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/>.
36 //// Local definitions
42 PH_TT_MOVES, // Transposition table move and mate killer
43 PH_GOOD_CAPTURES, // Queen promotions and captures with SEE values >= 0
44 PH_KILLERS, // Killer moves from the current ply
45 PH_NONCAPTURES, // Non-captures and underpromotions
46 PH_BAD_CAPTURES, // Queen promotions and captures with SEE values < 0
47 PH_EVASIONS, // Check evasions
48 PH_QCAPTURES, // Captures in quiescence search
49 PH_QCHECKS, // Non-capture checks in quiescence search
54 const uint8_t MainSearchPhaseTable[] = { PH_TT_MOVES, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP};
55 const uint8_t EvasionsPhaseTable[] = { PH_TT_MOVES, PH_EVASIONS, PH_STOP};
56 const uint8_t QsearchWithChecksPhaseTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_QCHECKS, PH_STOP};
57 const uint8_t QsearchWithoutChecksPhaseTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_STOP};
66 /// Constructor for the MovePicker class. Apart from the position for which
67 /// it is asked to pick legal moves, MovePicker also wants some information
68 /// to help it to return the presumably good moves first, to decide which
69 /// moves to return (in the quiescence search, for instance, we only want to
70 /// search captures, promotions and some checks) and about how important good
71 /// move ordering is at the current node.
73 MovePicker::MovePicker(const Position& p, Move ttm, Depth d,
74 const History& h, SearchStack* ss) : pos(p), H(h) {
76 ttMoves[0].move = ttm;
77 lastBadCapture = badCaptures;
79 pinned = p.pinned_pieces(pos.side_to_move());
81 if (ss && !p.is_check())
83 ttMoves[1].move = (ss->mateKiller == ttm)? MOVE_NONE : ss->mateKiller;
84 searchTT |= ttMoves[1].move;
85 killers[0].move = ss->killers[0];
86 killers[1].move = ss->killers[1];
88 ttMoves[1].move = killers[0].move = killers[1].move = MOVE_NONE;
91 phasePtr = EvasionsPhaseTable;
92 else if (d > Depth(0))
93 phasePtr = MainSearchPhaseTable;
94 else if (d == Depth(0))
95 phasePtr = QsearchWithChecksPhaseTable;
97 phasePtr = QsearchWithoutChecksPhaseTable;
99 phasePtr += !searchTT - 1;
104 /// MovePicker::go_next_phase() generates, scores and sorts the next bunch
105 /// of moves when there are no more moves to try for the current phase.
107 void MovePicker::go_next_phase() {
110 phase = *(++phasePtr);
115 lastMove = curMove + 2;
118 case PH_GOOD_CAPTURES:
119 lastMove = generate_captures(pos, moves);
125 lastMove = curMove + 2;
129 lastMove = generate_noncaptures(pos, moves);
131 sort_moves(moves, lastMove);
134 case PH_BAD_CAPTURES:
135 // Bad captures SEE value is already calculated so just sort them
136 // to get SEE move ordering.
137 curMove = badCaptures;
138 lastMove = lastBadCapture;
142 assert(pos.is_check());
143 lastMove = generate_evasions(pos, moves);
144 score_evasions_or_checks();
148 lastMove = generate_captures(pos, moves);
153 lastMove = generate_non_capture_checks(pos, moves);
154 score_evasions_or_checks();
158 lastMove = curMove + 1; // Avoids another go_next_phase() call
168 /// MovePicker::score_captures(), MovePicker::score_noncaptures() and
169 /// MovePicker::score_evasions() assign a numerical move ordering score
170 /// to each move in a move list. The moves with highest scores will be
171 /// picked first by get_next_move().
173 void MovePicker::score_captures() {
174 // Winning and equal captures in the main search are ordered by MVV/LVA.
175 // Suprisingly, this appears to perform slightly better than SEE based
176 // move ordering. The reason is probably that in a position with a winning
177 // capture, capturing a more valuable (but sufficiently defended) piece
178 // first usually doesn't hurt. The opponent will have to recapture, and
179 // the hanging piece will still be hanging (except in the unusual cases
180 // where it is possible to recapture with the hanging piece). Exchanging
181 // big pieces before capturing a hanging piece probably helps to reduce
183 // In main search we want to push captures with negative SEE values to
184 // badCaptures[] array, but instead of doing it now we delay till when
185 // the move has been picked up in pick_move_from_list(), this way we save
186 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
189 // Use MVV/LVA ordering
190 for (MoveStack* cur = moves; cur != lastMove; cur++)
193 if (move_is_promotion(m))
194 cur->score = QueenValueMidgame;
196 cur->score = pos.midgame_value_of_piece_on(move_to(m))
197 - pos.type_of_piece_on(move_from(m));
201 void MovePicker::score_noncaptures() {
202 // First score by history, when no history is available then use
203 // piece/square tables values. This seems to be better then a
204 // random choice when we don't have an history for any move.
210 for (MoveStack* cur = moves; cur != lastMove; cur++)
215 piece = pos.piece_on(from);
216 hs = H.move_ordering_score(piece, to);
218 // Ensure history is always preferred to pst
223 cur->score = hs + mg_value(pos.pst_delta(piece, from, to));
227 void MovePicker::score_evasions_or_checks() {
228 // Try good captures ordered by MVV/LVA, then non-captures if
229 // destination square is not under attack, ordered by history
230 // value, and at the end bad-captures and non-captures with a
231 // negative SEE. This last group is ordered by the SEE score.
235 // Skip if we don't have at least two moves to order
236 if (lastMove < moves + 2)
239 for (MoveStack* cur = moves; cur != lastMove; cur++)
242 if ((seeScore = pos.see_sign(m)) < 0)
243 cur->score = seeScore;
244 else if (pos.move_is_capture(m))
245 cur->score = pos.midgame_value_of_piece_on(move_to(m))
246 - pos.type_of_piece_on(move_from(m)) + HistoryMax;
248 cur->score = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
252 /// MovePicker::get_next_move() is the most important method of the MovePicker
253 /// class. It returns a new legal move every time it is called, until there
254 /// are no more moves left.
255 /// It picks the move with the biggest score from a list of generated moves taking
256 /// care not to return the tt move if has already been searched previously.
258 Move MovePicker::get_next_move() {
264 while (curMove != lastMove)
269 move = (curMove++)->move;
270 if ( move != MOVE_NONE
271 && move_is_legal(pos, move, pinned))
275 case PH_GOOD_CAPTURES:
276 move = pick_best(curMove++, lastMove).move;
277 if ( move != ttMoves[0].move
278 && move != ttMoves[1].move
279 && pos.pl_move_is_legal(move, pinned))
281 // Check for a non negative SEE now
282 int seeValue = pos.see_sign(move);
286 // Losing capture, move it to the badCaptures[] array, note
287 // that move has now been already checked for legality.
288 assert(int(lastBadCapture - badCaptures) < 63);
289 lastBadCapture->move = move;
290 lastBadCapture->score = seeValue;
296 move = (curMove++)->move;
297 if ( move != MOVE_NONE
298 && move != ttMoves[0].move
299 && move != ttMoves[1].move
300 && move_is_legal(pos, move, pinned)
301 && !pos.move_is_capture(move))
306 move = (curMove++)->move;
307 if ( move != ttMoves[0].move
308 && move != ttMoves[1].move
309 && move != killers[0].move
310 && move != killers[1].move
311 && pos.pl_move_is_legal(move, pinned))
315 case PH_BAD_CAPTURES:
316 move = pick_best(curMove++, lastMove).move;
321 move = pick_best(curMove++, lastMove).move;
322 if ( move != ttMoves[0].move
323 && pos.pl_move_is_legal(move, pinned))
328 move = (curMove++)->move;
329 if ( move != ttMoves[0].move
330 && pos.pl_move_is_legal(move, pinned))
346 /// A variant of get_next_move() which takes a lock as a parameter, used to
347 /// prevent multiple threads from picking the same move at a split point.
349 Move MovePicker::get_next_move(Lock &lock) {
353 // Note that it is safe to call many times
354 // get_next_move() when phase == PH_STOP
355 Move m = get_next_move();