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 MainSearchNoNullPhaseTable[] = { PH_STOP, PH_TT_MOVES, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP};
48 const MovegenPhaseT EvasionsPhaseTable[] = { PH_STOP, PH_EVASIONS, PH_STOP};
49 const MovegenPhaseT QsearchWithChecksPhaseTable[] = { PH_STOP, PH_TT_MOVES, PH_QCAPTURES, PH_QCHECKS, PH_STOP};
50 const MovegenPhaseT QsearchWithoutChecksPhaseTable[] = { PH_STOP, PH_TT_MOVES, PH_QCAPTURES, PH_STOP};
59 /// Constructor for the MovePicker class. Apart from the position for which
60 /// it is asked to pick legal moves, MovePicker also wants some information
61 /// to help it to return the presumably good moves first, to decide which
62 /// moves to return (in the quiescence search, for instance, we only want to
63 /// search captures, promotions and some checks) and about how important good
64 /// move ordering is at the current node.
66 MovePicker::MovePicker(const Position& p, Move ttm, Depth d,
67 const History& h, SearchStack* ss, bool useNullMove) : pos(p), H(h) {
71 ttMoves[1] = (ss->mateKiller == ttm)? MOVE_NONE : ss->mateKiller;
72 killers[0] = ss->killers[0];
73 killers[1] = ss->killers[1];
75 ttMoves[1] = killers[0] = killers[1] = MOVE_NONE;
77 movesPicked = numOfMoves = numOfBadCaptures = 0;
81 phasePtr = EvasionsPhaseTable;
82 else if (d > Depth(0))
83 phasePtr = useNullMove ? MainSearchPhaseTable : MainSearchNoNullPhaseTable;
84 else if (d == Depth(0))
85 phasePtr = QsearchWithChecksPhaseTable;
87 phasePtr = QsearchWithoutChecksPhaseTable;
89 Color us = pos.side_to_move();
91 dc = p.discovered_check_candidates(us);
92 pinned = p.pinned_pieces(us);
96 /// MovePicker::get_next_move() is the most important method of the MovePicker
97 /// class. It returns a new legal move every time it is called, until there
98 /// are no more moves left of the types we are interested in.
100 Move MovePicker::get_next_move() {
106 // If we already have a list of generated moves, pick the best move from
107 // the list, and return it.
108 move = pick_move_from_list();
109 if (move != MOVE_NONE)
111 assert(move_is_ok(move));
123 movesPicked = 0; // This is used as index to ttMoves[]
126 case PH_GOOD_CAPTURES:
127 numOfMoves = generate_captures(pos, moves);
129 std::sort(moves, moves + numOfMoves);
134 movesPicked = 0; // This is used as index to killers[]
138 numOfMoves = generate_noncaptures(pos, moves);
140 std::sort(moves, moves + numOfMoves);
144 case PH_BAD_CAPTURES:
145 // Bad captures SEE value is already calculated so just sort them
146 // to get SEE move ordering.
147 std::sort(badCaptures, badCaptures + numOfBadCaptures);
152 assert(pos.is_check());
153 numOfMoves = generate_evasions(pos, moves, pinned);
155 std::sort(moves, moves + numOfMoves);
160 numOfMoves = generate_captures(pos, moves);
162 std::sort(moves, moves + numOfMoves);
167 // Perhaps we should order moves move here? FIXME
168 numOfMoves = generate_non_capture_checks(pos, moves, dc);
183 /// A variant of get_next_move() which takes a lock as a parameter, used to
184 /// prevent multiple threads from picking the same move at a split point.
186 Move MovePicker::get_next_move(Lock &lock) {
194 Move m = get_next_move();
203 /// MovePicker::score_captures(), MovePicker::score_noncaptures(),
204 /// MovePicker::score_evasions() and MovePicker::score_qcaptures() assign a
205 /// numerical move ordering score to each move in a move list. The moves
206 /// with highest scores will be picked first by pick_move_from_list().
208 void MovePicker::score_captures() {
209 // Winning and equal captures in the main search are ordered by MVV/LVA.
210 // Suprisingly, this appears to perform slightly better than SEE based
211 // move ordering. The reason is probably that in a position with a winning
212 // capture, capturing a more valuable (but sufficiently defended) piece
213 // first usually doesn't hurt. The opponent will have to recapture, and
214 // the hanging piece will still be hanging (except in the unusual cases
215 // where it is possible to recapture with the hanging piece). Exchanging
216 // big pieces before capturing a hanging piece probably helps to reduce
218 // In main search we want to push captures with negative SEE values to
219 // badCaptures[] array, but instead of doing it now we delay till when
220 // the move has been picked up in pick_move_from_list(), this way we save
221 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
224 // Use MVV/LVA ordering
225 for (int i = 0; i < numOfMoves; i++)
228 if (move_is_promotion(m))
229 moves[i].score = QueenValueMidgame;
231 moves[i].score = int(pos.midgame_value_of_piece_on(move_to(m)))
232 -int(pos.type_of_piece_on(move_from(m)));
236 void MovePicker::score_noncaptures() {
237 // First score by history, when no history is available then use
238 // piece/square tables values. This seems to be better then a
239 // random choice when we don't have an history for any move.
244 for (int i = 0; i < numOfMoves; i++)
246 from = move_from(moves[i].move);
247 to = move_to(moves[i].move);
248 piece = pos.piece_on(from);
249 hs = H.move_ordering_score(piece, to);
251 // Ensure history is always preferred to pst
256 moves[i].score = hs + pos.pst_delta<Position::MidGame>(piece, from, to);
260 void MovePicker::score_evasions() {
262 for (int i = 0; i < numOfMoves; i++)
264 Move m = moves[i].move;
266 moves[i].score = 2*HistoryMax;
267 else if (!pos.square_is_empty(move_to(m)))
269 int seeScore = pos.see(m);
270 moves[i].score = (seeScore >= 0)? seeScore + HistoryMax : seeScore;
272 moves[i].score = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
277 /// MovePicker::pick_move_from_list() picks the move with the biggest score
278 /// from a list of generated moves (moves[] or badCaptures[], depending on
279 /// the current move generation phase). It takes care not to return the
280 /// transposition table move if that has already been serched previously.
282 Move MovePicker::pick_move_from_list() {
284 assert(movesPicked >= 0);
285 assert(!pos.is_check() || *phasePtr == PH_EVASIONS || *phasePtr == PH_STOP);
286 assert( pos.is_check() || *phasePtr != PH_EVASIONS);
291 while (movesPicked < 2) {
292 Move move = ttMoves[movesPicked++];
293 if ( move != MOVE_NONE
294 && move_is_legal(pos, move, pinned))
299 case PH_GOOD_CAPTURES:
300 while (movesPicked < numOfMoves)
302 Move move = moves[movesPicked++].move;
303 if ( move != ttMoves[0]
304 && move != ttMoves[1]
305 && pos.pl_move_is_legal(move, pinned))
307 // Check for a non negative SEE now
308 int seeValue = pos.see_sign(move);
312 // Losing capture, move it to the badCaptures[] array, note
313 // that move has now been already checked for legality.
314 assert(numOfBadCaptures < 63);
315 badCaptures[numOfBadCaptures].move = move;
316 badCaptures[numOfBadCaptures++].score = seeValue;
322 while (movesPicked < 2) {
323 Move move = killers[movesPicked++];
324 if ( move != MOVE_NONE
325 && move != ttMoves[0]
326 && move != ttMoves[1]
327 && move_is_legal(pos, move, pinned)
328 && !pos.move_is_capture(move))
334 while (movesPicked < numOfMoves)
336 Move move = moves[movesPicked++].move;
337 if ( move != ttMoves[0]
338 && move != ttMoves[1]
339 && move != killers[0]
340 && move != killers[1]
341 && pos.pl_move_is_legal(move, pinned))
347 if (movesPicked < numOfMoves)
348 return moves[movesPicked++].move;
351 case PH_BAD_CAPTURES:
352 if (movesPicked < numOfBadCaptures)
353 return badCaptures[movesPicked++].move;
358 while (movesPicked < numOfMoves)
360 Move move = moves[movesPicked++].move;
361 // Maybe postpone the legality check until after futility pruning?
362 if ( move != ttMoves[0]
363 && pos.pl_move_is_legal(move, pinned))