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/>.
38 //// Local definitions
46 const MovegenPhaseT MainSearchPhaseTable[] = { PH_STOP, PH_NULL_MOVE, PH_TT_MOVES, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP};
47 const MovegenPhaseT EvasionsPhaseTable[] = { PH_STOP, PH_EVASIONS, PH_STOP};
48 const MovegenPhaseT QsearchWithChecksPhaseTable[] = { PH_STOP, PH_TT_MOVES, PH_QCAPTURES, PH_QCHECKS, PH_STOP};
49 const MovegenPhaseT QsearchWithoutChecksPhaseTable[] = { PH_STOP, PH_TT_MOVES, PH_QCAPTURES, PH_STOP};
58 /// Constructor for the MovePicker class. Apart from the position for which
59 /// it is asked to pick legal moves, MovePicker also wants some information
60 /// to help it to return the presumably good moves first, to decide which
61 /// moves to return (in the quiescence search, for instance, we only want to
62 /// search captures, promotions and some checks) and about how important good
63 /// move ordering is at the current node.
65 MovePicker::MovePicker(const Position& p, Move ttm, Depth d,
66 const History& h, SearchStack* ss, bool useNullMove) : pos(p), H(h) {
70 ttMoves[1] = (ss->mateKiller == ttm)? MOVE_NONE : ss->mateKiller;
71 killers[0] = ss->killers[0];
72 killers[1] = ss->killers[1];
74 ttMoves[1] = killers[0] = killers[1] = MOVE_NONE;
76 movesPicked = numOfMoves = numOfBadCaptures = 0;
80 phasePtr = EvasionsPhaseTable;
81 else if (d > Depth(0))
82 phasePtr = MainSearchPhaseTable;
83 else if (d == Depth(0))
84 phasePtr = QsearchWithChecksPhaseTable;
86 phasePtr = QsearchWithoutChecksPhaseTable;
88 Color us = pos.side_to_move();
90 dc = p.discovered_check_candidates(us);
91 pinned = p.pinned_pieces(us);
95 /// MovePicker::get_next_move() is the most important method of the MovePicker
96 /// class. It returns a new legal move every time it is called, until there
97 /// are no more moves left of the types we are interested in.
99 Move MovePicker::get_next_move() {
105 // If we already have a list of generated moves, pick the best move from
106 // the list, and return it.
107 move = pick_move_from_list();
108 if (move != MOVE_NONE)
110 assert(move_is_ok(move));
122 movesPicked = 0; // This is used as index to ttMoves[]
125 case PH_GOOD_CAPTURES:
126 numOfMoves = generate_captures(pos, moves);
128 std::sort(moves, moves + numOfMoves);
133 movesPicked = 0; // This is used as index to killers[]
137 numOfMoves = generate_noncaptures(pos, moves);
139 std::sort(moves, moves + numOfMoves);
143 case PH_BAD_CAPTURES:
144 // Bad captures SEE value is already calculated so just sort them
145 // to get SEE move ordering.
146 std::sort(badCaptures, badCaptures + numOfBadCaptures);
151 assert(pos.is_check());
152 numOfMoves = generate_evasions(pos, moves, pinned);
154 std::sort(moves, moves + numOfMoves);
159 numOfMoves = generate_captures(pos, moves);
161 std::sort(moves, moves + numOfMoves);
166 // Perhaps we should order moves move here? FIXME
167 numOfMoves = generate_non_capture_checks(pos, moves, dc);
182 /// A variant of get_next_move() which takes a lock as a parameter, used to
183 /// prevent multiple threads from picking the same move at a split point.
185 Move MovePicker::get_next_move(Lock &lock) {
193 Move m = get_next_move();
202 /// MovePicker::score_captures(), MovePicker::score_noncaptures(),
203 /// MovePicker::score_evasions() and MovePicker::score_qcaptures() assign a
204 /// numerical move ordering score to each move in a move list. The moves
205 /// with highest scores will be picked first by pick_move_from_list().
207 void MovePicker::score_captures() {
208 // Winning and equal captures in the main search are ordered by MVV/LVA.
209 // Suprisingly, this appears to perform slightly better than SEE based
210 // move ordering. The reason is probably that in a position with a winning
211 // capture, capturing a more valuable (but sufficiently defended) piece
212 // first usually doesn't hurt. The opponent will have to recapture, and
213 // the hanging piece will still be hanging (except in the unusual cases
214 // where it is possible to recapture with the hanging piece). Exchanging
215 // big pieces before capturing a hanging piece probably helps to reduce
217 // In main search we want to push captures with negative SEE values to
218 // badCaptures[] array, but instead of doing it now we delay till when
219 // the move has been picked up in pick_move_from_list(), this way we save
220 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
223 // Use MVV/LVA ordering
224 for (int i = 0; i < numOfMoves; i++)
227 if (move_is_promotion(m))
228 moves[i].score = QueenValueMidgame;
230 moves[i].score = int(pos.midgame_value_of_piece_on(move_to(m)))
231 -int(pos.type_of_piece_on(move_from(m)));
235 void MovePicker::score_noncaptures() {
236 // First score by history, when no history is available then use
237 // piece/square tables values. This seems to be better then a
238 // random choice when we don't have an history for any move.
243 for (int i = 0; i < numOfMoves; i++)
245 from = move_from(moves[i].move);
246 to = move_to(moves[i].move);
247 piece = pos.piece_on(from);
248 hs = H.move_ordering_score(piece, to);
250 // Ensure history is always preferred to pst
255 moves[i].score = hs + pos.pst_delta<Position::MidGame>(piece, from, to);
259 void MovePicker::score_evasions() {
261 for (int i = 0; i < numOfMoves; i++)
263 Move m = moves[i].move;
265 moves[i].score = 2*HistoryMax;
266 else if (!pos.square_is_empty(move_to(m)))
268 int seeScore = pos.see(m);
269 moves[i].score = (seeScore >= 0)? seeScore + HistoryMax : seeScore;
271 moves[i].score = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
276 /// MovePicker::pick_move_from_list() picks the move with the biggest score
277 /// from a list of generated moves (moves[] or badCaptures[], depending on
278 /// the current move generation phase). It takes care not to return the
279 /// transposition table move if that has already been serched previously.
281 Move MovePicker::pick_move_from_list() {
283 assert(movesPicked >= 0);
284 assert(!pos.is_check() || *phasePtr == PH_EVASIONS || *phasePtr == PH_STOP);
285 assert( pos.is_check() || *phasePtr != PH_EVASIONS);
290 while (movesPicked < 2) {
291 Move move = ttMoves[movesPicked++];
292 if ( move != MOVE_NONE
293 && move_is_legal(pos, move, pinned))
298 case PH_GOOD_CAPTURES:
299 while (movesPicked < numOfMoves)
301 Move move = moves[movesPicked++].move;
302 if ( move != ttMoves[0]
303 && move != ttMoves[1]
304 && pos.pl_move_is_legal(move, pinned))
306 // Check for a non negative SEE now
307 int seeValue = pos.see_sign(move);
311 // Losing capture, move it to the badCaptures[] array, note
312 // that move has now been already checked for legality.
313 assert(numOfBadCaptures < 63);
314 badCaptures[numOfBadCaptures].move = move;
315 badCaptures[numOfBadCaptures++].score = seeValue;
321 while (movesPicked < 2) {
322 Move move = killers[movesPicked++];
323 if ( move != MOVE_NONE
324 && move != ttMoves[0]
325 && move != ttMoves[1]
326 && move_is_legal(pos, move, pinned)
327 && !pos.move_is_capture(move))
333 while (movesPicked < numOfMoves)
335 Move move = moves[movesPicked++].move;
336 if ( move != ttMoves[0]
337 && move != ttMoves[1]
338 && move != killers[0]
339 && move != killers[1]
340 && pos.pl_move_is_legal(move, pinned))
346 if (movesPicked < numOfMoves)
347 return moves[movesPicked++].move;
350 case PH_BAD_CAPTURES:
351 if (movesPicked < numOfBadCaptures)
352 return badCaptures[movesPicked++].move;
357 while (movesPicked < numOfMoves)
359 Move move = moves[movesPicked++].move;
360 // Maybe postpone the legality check until after futility pruning?
361 if ( move != ttMoves[0]
362 && pos.pl_move_is_legal(move, pinned))