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_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) : 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));
119 movesPicked = 0; // This is used as index to ttMoves[]
122 case PH_GOOD_CAPTURES:
123 numOfMoves = generate_captures(pos, moves);
125 std::sort(moves, moves + numOfMoves);
130 movesPicked = 0; // This is used as index to killers[]
134 numOfMoves = generate_noncaptures(pos, moves);
136 std::sort(moves, moves + numOfMoves);
140 case PH_BAD_CAPTURES:
141 // Bad captures SEE value is already calculated so just sort them
142 // to get SEE move ordering.
143 std::sort(badCaptures, badCaptures + numOfBadCaptures);
148 assert(pos.is_check());
149 numOfMoves = generate_evasions(pos, moves, pinned);
151 std::sort(moves, moves + numOfMoves);
156 numOfMoves = generate_captures(pos, moves);
158 std::sort(moves, moves + numOfMoves);
163 // Perhaps we should order moves move here? FIXME
164 numOfMoves = generate_non_capture_checks(pos, moves, dc);
179 /// A variant of get_next_move() which takes a lock as a parameter, used to
180 /// prevent multiple threads from picking the same move at a split point.
182 Move MovePicker::get_next_move(Lock &lock) {
190 Move m = get_next_move();
199 /// MovePicker::score_captures(), MovePicker::score_noncaptures(),
200 /// MovePicker::score_evasions() and MovePicker::score_qcaptures() assign a
201 /// numerical move ordering score to each move in a move list. The moves
202 /// with highest scores will be picked first by pick_move_from_list().
204 void MovePicker::score_captures() {
205 // Winning and equal captures in the main search are ordered by MVV/LVA.
206 // Suprisingly, this appears to perform slightly better than SEE based
207 // move ordering. The reason is probably that in a position with a winning
208 // capture, capturing a more valuable (but sufficiently defended) piece
209 // first usually doesn't hurt. The opponent will have to recapture, and
210 // the hanging piece will still be hanging (except in the unusual cases
211 // where it is possible to recapture with the hanging piece). Exchanging
212 // big pieces before capturing a hanging piece probably helps to reduce
214 // In main search we want to push captures with negative SEE values to
215 // badCaptures[] array, but instead of doing it now we delay till when
216 // the move has been picked up in pick_move_from_list(), this way we save
217 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
220 // Use MVV/LVA ordering
221 for (int i = 0; i < numOfMoves; i++)
224 if (move_is_promotion(m))
225 moves[i].score = QueenValueMidgame;
227 moves[i].score = int(pos.midgame_value_of_piece_on(move_to(m)))
228 -int(pos.type_of_piece_on(move_from(m)));
232 void MovePicker::score_noncaptures() {
233 // First score by history, when no history is available then use
234 // piece/square tables values. This seems to be better then a
235 // random choice when we don't have an history for any move.
240 for (int i = 0; i < numOfMoves; i++)
242 from = move_from(moves[i].move);
243 to = move_to(moves[i].move);
244 piece = pos.piece_on(from);
245 hs = H.move_ordering_score(piece, to);
247 // Ensure history is always preferred to pst
252 moves[i].score = hs + pos.pst_delta<Position::MidGame>(piece, from, to);
256 void MovePicker::score_evasions() {
258 for (int i = 0; i < numOfMoves; i++)
260 Move m = moves[i].move;
262 moves[i].score = 2*HistoryMax;
263 else if (!pos.square_is_empty(move_to(m)))
265 int seeScore = pos.see(m);
266 moves[i].score = (seeScore >= 0)? seeScore + HistoryMax : seeScore;
268 moves[i].score = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
273 /// MovePicker::pick_move_from_list() picks the move with the biggest score
274 /// from a list of generated moves (moves[] or badCaptures[], depending on
275 /// the current move generation phase). It takes care not to return the
276 /// transposition table move if that has already been serched previously.
278 Move MovePicker::pick_move_from_list() {
280 assert(movesPicked >= 0);
281 assert(!pos.is_check() || *phasePtr == PH_EVASIONS || *phasePtr == PH_STOP);
282 assert( pos.is_check() || *phasePtr != PH_EVASIONS);
287 while (movesPicked < 2) {
288 Move move = ttMoves[movesPicked++];
289 if ( move != MOVE_NONE
290 && move_is_legal(pos, move, pinned))
295 case PH_GOOD_CAPTURES:
296 while (movesPicked < numOfMoves)
298 Move move = moves[movesPicked++].move;
299 if ( move != ttMoves[0]
300 && move != ttMoves[1]
301 && pos.pl_move_is_legal(move, pinned))
303 // Check for a non negative SEE now
304 int seeValue = pos.see_sign(move);
308 // Losing capture, move it to the badCaptures[] array, note
309 // that move has now been already checked for legality.
310 assert(numOfBadCaptures < 63);
311 badCaptures[numOfBadCaptures].move = move;
312 badCaptures[numOfBadCaptures++].score = seeValue;
318 while (movesPicked < 2) {
319 Move move = killers[movesPicked++];
320 if ( move != MOVE_NONE
321 && move != ttMoves[0]
322 && move != ttMoves[1]
323 && move_is_legal(pos, move, pinned)
324 && !pos.move_is_capture(move))
330 while (movesPicked < numOfMoves)
332 Move move = moves[movesPicked++].move;
333 if ( move != ttMoves[0]
334 && move != ttMoves[1]
335 && move != killers[0]
336 && move != killers[1]
337 && pos.pl_move_is_legal(move, pinned))
343 if (movesPicked < numOfMoves)
344 return moves[movesPicked++].move;
347 case PH_BAD_CAPTURES:
348 if (movesPicked < numOfBadCaptures)
349 return badCaptures[movesPicked++].move;
354 while (movesPicked < numOfMoves)
356 Move move = moves[movesPicked++].move;
357 // Maybe postpone the legality check until after futility pruning?
358 if ( move != ttMoves[0]
359 && pos.pl_move_is_legal(move, pinned))