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 LowSearchPhaseTable[] = { PH_STOP, PH_TT_MOVES, PH_GOOD_CAPTURES, PH_NULL_MOVE, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP};
49 const MovegenPhaseT EvasionsPhaseTable[] = { PH_STOP, PH_EVASIONS, PH_STOP};
50 const MovegenPhaseT QsearchWithChecksPhaseTable[] = { PH_STOP, PH_TT_MOVES, PH_QCAPTURES, PH_QCHECKS, PH_STOP};
51 const MovegenPhaseT QsearchWithoutChecksPhaseTable[] = { PH_STOP, PH_TT_MOVES, PH_QCAPTURES, PH_STOP};
60 /// Constructor for the MovePicker class. Apart from the position for which
61 /// it is asked to pick legal moves, MovePicker also wants some information
62 /// to help it to return the presumably good moves first, to decide which
63 /// moves to return (in the quiescence search, for instance, we only want to
64 /// search captures, promotions and some checks) and about how important good
65 /// move ordering is at the current node.
67 MovePicker::MovePicker(const Position& p, Move ttm, Depth d,
68 const History& h, SearchStack* ss, bool useNullMove) : pos(p), H(h) {
72 ttMoves[1] = (ss->mateKiller == ttm)? MOVE_NONE : ss->mateKiller;
73 killers[0] = ss->killers[0];
74 killers[1] = ss->killers[1];
76 ttMoves[1] = killers[0] = killers[1] = MOVE_NONE;
78 movesPicked = numOfMoves = numOfBadCaptures = 0;
82 phasePtr = EvasionsPhaseTable;
83 else if (d >= Depth(3 * OnePly))
84 phasePtr = useNullMove ? MainSearchPhaseTable : MainSearchNoNullPhaseTable;
85 else if (d > Depth(0))
86 phasePtr = useNullMove ? LowSearchPhaseTable : MainSearchNoNullPhaseTable;
87 else if (d == Depth(0))
88 phasePtr = QsearchWithChecksPhaseTable;
90 phasePtr = QsearchWithoutChecksPhaseTable;
92 Color us = pos.side_to_move();
94 dc = p.discovered_check_candidates(us);
95 pinned = p.pinned_pieces(us);
99 /// MovePicker::get_next_move() is the most important method of the MovePicker
100 /// class. It returns a new legal move every time it is called, until there
101 /// are no more moves left of the types we are interested in.
103 Move MovePicker::get_next_move() {
109 // If we already have a list of generated moves, pick the best move from
110 // the list, and return it.
111 move = pick_move_from_list();
112 if (move != MOVE_NONE)
114 assert(move_is_ok(move));
126 movesPicked = 0; // This is used as index to ttMoves[]
129 case PH_GOOD_CAPTURES:
130 numOfMoves = generate_captures(pos, moves);
132 std::sort(moves, moves + numOfMoves);
137 movesPicked = 0; // This is used as index to killers[]
141 numOfMoves = generate_noncaptures(pos, moves);
143 std::sort(moves, moves + numOfMoves);
147 case PH_BAD_CAPTURES:
148 // Bad captures SEE value is already calculated so just sort them
149 // to get SEE move ordering.
150 std::sort(badCaptures, badCaptures + numOfBadCaptures);
155 assert(pos.is_check());
156 numOfMoves = generate_evasions(pos, moves, pinned);
158 std::sort(moves, moves + numOfMoves);
163 numOfMoves = generate_captures(pos, moves);
165 std::sort(moves, moves + numOfMoves);
170 // Perhaps we should order moves move here? FIXME
171 numOfMoves = generate_non_capture_checks(pos, moves, dc);
186 /// A variant of get_next_move() which takes a lock as a parameter, used to
187 /// prevent multiple threads from picking the same move at a split point.
189 Move MovePicker::get_next_move(Lock &lock) {
197 Move m = get_next_move();
206 /// MovePicker::score_captures(), MovePicker::score_noncaptures(),
207 /// MovePicker::score_evasions() and MovePicker::score_qcaptures() assign a
208 /// numerical move ordering score to each move in a move list. The moves
209 /// with highest scores will be picked first by pick_move_from_list().
211 void MovePicker::score_captures() {
212 // Winning and equal captures in the main search are ordered by MVV/LVA.
213 // Suprisingly, this appears to perform slightly better than SEE based
214 // move ordering. The reason is probably that in a position with a winning
215 // capture, capturing a more valuable (but sufficiently defended) piece
216 // first usually doesn't hurt. The opponent will have to recapture, and
217 // the hanging piece will still be hanging (except in the unusual cases
218 // where it is possible to recapture with the hanging piece). Exchanging
219 // big pieces before capturing a hanging piece probably helps to reduce
221 // In main search we want to push captures with negative SEE values to
222 // badCaptures[] array, but instead of doing it now we delay till when
223 // the move has been picked up in pick_move_from_list(), this way we save
224 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
227 // Use MVV/LVA ordering
228 for (int i = 0; i < numOfMoves; i++)
231 if (move_is_promotion(m))
232 moves[i].score = QueenValueMidgame;
234 moves[i].score = int(pos.midgame_value_of_piece_on(move_to(m)))
235 -int(pos.type_of_piece_on(move_from(m)));
239 void MovePicker::score_noncaptures() {
240 // First score by history, when no history is available then use
241 // piece/square tables values. This seems to be better then a
242 // random choice when we don't have an history for any move.
247 for (int i = 0; i < numOfMoves; i++)
249 from = move_from(moves[i].move);
250 to = move_to(moves[i].move);
251 piece = pos.piece_on(from);
252 hs = H.move_ordering_score(piece, to);
254 // Ensure history is always preferred to pst
259 moves[i].score = hs + pos.pst_delta<Position::MidGame>(piece, from, to);
263 void MovePicker::score_evasions() {
265 for (int i = 0; i < numOfMoves; i++)
267 Move m = moves[i].move;
269 moves[i].score = 2*HistoryMax;
270 else if (!pos.square_is_empty(move_to(m)))
272 int seeScore = pos.see(m);
273 moves[i].score = (seeScore >= 0)? seeScore + HistoryMax : seeScore;
275 moves[i].score = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
280 /// MovePicker::pick_move_from_list() picks the move with the biggest score
281 /// from a list of generated moves (moves[] or badCaptures[], depending on
282 /// the current move generation phase). It takes care not to return the
283 /// transposition table move if that has already been serched previously.
285 Move MovePicker::pick_move_from_list() {
287 assert(movesPicked >= 0);
288 assert(!pos.is_check() || *phasePtr == PH_EVASIONS || *phasePtr == PH_STOP);
289 assert( pos.is_check() || *phasePtr != PH_EVASIONS);
294 while (movesPicked < 2) {
295 Move move = ttMoves[movesPicked++];
296 if ( move != MOVE_NONE
297 && move_is_legal(pos, move, pinned))
302 case PH_GOOD_CAPTURES:
303 while (movesPicked < numOfMoves)
305 Move move = moves[movesPicked++].move;
306 if ( move != ttMoves[0]
307 && move != ttMoves[1]
308 && pos.pl_move_is_legal(move, pinned))
310 // Check for a non negative SEE now
311 int seeValue = pos.see_sign(move);
315 // Losing capture, move it to the badCaptures[] array, note
316 // that move has now been already checked for legality.
317 assert(numOfBadCaptures < 63);
318 badCaptures[numOfBadCaptures].move = move;
319 badCaptures[numOfBadCaptures++].score = seeValue;
325 while (movesPicked < 2) {
326 Move move = killers[movesPicked++];
327 if ( move != MOVE_NONE
328 && move != ttMoves[0]
329 && move != ttMoves[1]
330 && move_is_legal(pos, move, pinned)
331 && !pos.move_is_capture(move))
337 while (movesPicked < numOfMoves)
339 Move move = moves[movesPicked++].move;
340 if ( move != ttMoves[0]
341 && move != ttMoves[1]
342 && move != killers[0]
343 && move != killers[1]
344 && pos.pl_move_is_legal(move, pinned))
350 if (movesPicked < numOfMoves)
351 return moves[movesPicked++].move;
354 case PH_BAD_CAPTURES:
355 if (movesPicked < numOfBadCaptures)
356 return badCaptures[movesPicked++].move;
361 while (movesPicked < numOfMoves)
363 Move move = moves[movesPicked++].move;
364 // Maybe postpone the legality check until after futility pruning?
365 if ( move != ttMoves[0]
366 && pos.pl_move_is_legal(move, pinned))