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 int MainSearchPhaseIndex;
47 int EvasionsPhaseIndex;
48 int QsearchWithChecksPhaseIndex;
49 int QsearchWithoutChecksPhaseIndex;
50 uint8_t PhaseTable[32];
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) : pos(p), H(h) {
71 mateKiller = (ss->mateKiller == ttm)? MOVE_NONE : ss->mateKiller;
72 killer1 = ss->killers[0];
73 killer2 = ss->killers[1];
75 mateKiller = killer1 = killer2 = MOVE_NONE;
77 movesPicked = numOfMoves = numOfBadCaptures = 0;
78 checkKillers = checkLegal = finished = false;
81 phaseIndex = EvasionsPhaseIndex;
82 else if (d > Depth(0))
83 phaseIndex = MainSearchPhaseIndex;
84 else if (d == Depth(0))
85 phaseIndex = QsearchWithChecksPhaseIndex;
87 phaseIndex = QsearchWithoutChecksPhaseIndex;
89 Color us = pos.side_to_move();
91 dc = p.discovered_check_candidates(us);
92 pinned = p.pinned_pieces(us);
98 /// MovePicker::get_next_move() is the most important method of the MovePicker
99 /// class. It returns a new legal move every time it is called, until there
100 /// are no more moves left of the types we are interested in.
102 Move MovePicker::get_next_move() {
108 // If we already have a list of generated moves, pick the best move from
109 // the list, and return it.
110 move = pick_move_from_list();
111 if (move != MOVE_NONE)
113 assert(move_is_ok(move));
119 switch (PhaseTable[phaseIndex]) {
122 if (ttMove != MOVE_NONE)
124 assert(move_is_ok(ttMove));
125 if (move_is_legal(pos, ttMove, pinned))
131 if (mateKiller != MOVE_NONE)
133 assert(move_is_ok(mateKiller));
134 if (move_is_legal(pos, mateKiller, pinned))
139 case PH_GOOD_CAPTURES:
140 numOfMoves = generate_captures(pos, moves);
142 std::sort(moves, moves + numOfMoves);
147 movesPicked = numOfMoves = 0;
148 if (killer1 != MOVE_NONE && move_is_legal(pos, killer1, pinned) && !pos.move_is_capture(killer1))
149 moves[numOfMoves++].move = killer1;
150 if (killer2 != MOVE_NONE && move_is_legal(pos, killer2, pinned) && !pos.move_is_capture(killer2))
151 moves[numOfMoves++].move = killer2;
155 checkKillers = (numOfMoves != 0); // previous phase is PH_KILLERS
157 numOfMoves = generate_noncaptures(pos, moves);
159 std::sort(moves, moves + numOfMoves);
163 case PH_BAD_CAPTURES:
164 // Bad captures SEE value is already calculated so just sort them
165 // to get SEE move ordering.
166 std::sort(badCaptures, badCaptures + numOfBadCaptures);
171 assert(pos.is_check());
172 numOfMoves = generate_evasions(pos, moves, pinned);
174 std::sort(moves, moves + numOfMoves);
179 numOfMoves = generate_captures(pos, moves);
181 std::sort(moves, moves + numOfMoves);
186 // Perhaps we should order moves move here? FIXME
187 numOfMoves = generate_non_capture_checks(pos, moves, dc);
202 /// A variant of get_next_move() which takes a lock as a parameter, used to
203 /// prevent multiple threads from picking the same move at a split point.
205 Move MovePicker::get_next_move(Lock &lock) {
213 Move m = get_next_move();
222 /// MovePicker::score_captures(), MovePicker::score_noncaptures(),
223 /// MovePicker::score_evasions() and MovePicker::score_qcaptures() assign a
224 /// numerical move ordering score to each move in a move list. The moves
225 /// with highest scores will be picked first by pick_move_from_list().
227 void MovePicker::score_captures() {
228 // Winning and equal captures in the main search are ordered by MVV/LVA.
229 // Suprisingly, this appears to perform slightly better than SEE based
230 // move ordering. The reason is probably that in a position with a winning
231 // capture, capturing a more valuable (but sufficiently defended) piece
232 // first usually doesn't hurt. The opponent will have to recapture, and
233 // the hanging piece will still be hanging (except in the unusual cases
234 // where it is possible to recapture with the hanging piece). Exchanging
235 // big pieces before capturing a hanging piece probably helps to reduce
237 // In main search we want to push captures with negative SEE values to
238 // badCaptures[] array, but instead of doing it now we delay till when
239 // the move has been picked up in pick_move_from_list(), this way we save
240 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
243 // Use MVV/LVA ordering
244 for (int i = 0; i < numOfMoves; i++)
247 if (move_is_promotion(m))
248 moves[i].score = QueenValueMidgame;
250 moves[i].score = int(pos.midgame_value_of_piece_on(move_to(m)))
251 -int(pos.type_of_piece_on(move_from(m)));
255 void MovePicker::score_noncaptures() {
256 // First score by history, when no history is available then use
257 // piece/square tables values. This seems to be better then a
258 // random choice when we don't have an history for any move.
263 for (int i = 0; i < numOfMoves; i++)
265 from = move_from(moves[i].move);
266 to = move_to(moves[i].move);
267 piece = pos.piece_on(from);
268 hs = H.move_ordering_score(piece, to);
270 // Ensure history is always preferred to pst
275 moves[i].score = hs + pos.pst_delta<Position::MidGame>(piece, from, to);
279 void MovePicker::score_evasions() {
281 for (int i = 0; i < numOfMoves; i++)
283 Move m = moves[i].move;
285 moves[i].score = 2*HistoryMax;
286 else if (!pos.square_is_empty(move_to(m)))
288 int seeScore = pos.see(m);
289 moves[i].score = (seeScore >= 0)? seeScore + HistoryMax : seeScore;
291 moves[i].score = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
296 /// MovePicker::pick_move_from_list() picks the move with the biggest score
297 /// from a list of generated moves (moves[] or badCaptures[], depending on
298 /// the current move generation phase). It takes care not to return the
299 /// transposition table move if that has already been serched previously.
301 Move MovePicker::pick_move_from_list() {
303 assert(movesPicked >= 0);
304 assert(!pos.is_check() || PhaseTable[phaseIndex] == PH_EVASIONS || PhaseTable[phaseIndex] == PH_STOP);
305 assert( pos.is_check() || PhaseTable[phaseIndex] != PH_EVASIONS);
307 switch (PhaseTable[phaseIndex]) {
309 case PH_GOOD_CAPTURES:
310 while (movesPicked < numOfMoves)
312 Move move = moves[movesPicked++].move;
314 && move != mateKiller
315 && pos.pl_move_is_legal(move, pinned))
317 // Check for a non negative SEE now
318 int seeValue = pos.see_sign(move);
322 // Losing capture, move it to the badCaptures[] array, note
323 // that move has now been already checked for legality.
324 assert(numOfBadCaptures < 63);
325 badCaptures[numOfBadCaptures].move = move;
326 badCaptures[numOfBadCaptures++].score = seeValue;
332 while (movesPicked < numOfMoves)
334 Move move = moves[movesPicked++].move;
336 && move != mateKiller
337 && (!checkKillers || (move != killer1 && move != killer2))
338 && (!checkLegal || pos.pl_move_is_legal(move, pinned)))
344 if (movesPicked < numOfMoves)
345 return moves[movesPicked++].move;
348 case PH_BAD_CAPTURES:
349 if (movesPicked < numOfBadCaptures)
350 return badCaptures[movesPicked++].move;
355 while (movesPicked < numOfMoves)
357 Move move = moves[movesPicked++].move;
358 // Maybe postpone the legality check until after futility pruning?
360 && pos.pl_move_is_legal(move, pinned))
372 /// MovePicker::init_phase_table() initializes the PhaseTable[],
373 /// MainSearchPhaseIndex, EvasionPhaseIndex, QsearchWithChecksPhaseIndex
374 /// and QsearchWithoutChecksPhaseIndex. It is only called once during
375 /// program startup, and never again while the program is running.
377 void MovePicker::init_phase_table() {
382 MainSearchPhaseIndex = i - 1;
383 PhaseTable[i++] = PH_TT_MOVE;
384 PhaseTable[i++] = PH_MATE_KILLER;
385 PhaseTable[i++] = PH_GOOD_CAPTURES;
386 PhaseTable[i++] = PH_KILLERS;
387 PhaseTable[i++] = PH_NONCAPTURES;
388 PhaseTable[i++] = PH_BAD_CAPTURES;
389 PhaseTable[i++] = PH_STOP;
392 EvasionsPhaseIndex = i - 1;
393 PhaseTable[i++] = PH_EVASIONS;
394 PhaseTable[i++] = PH_STOP;
396 // Quiescence search with checks
397 QsearchWithChecksPhaseIndex = i - 1;
398 PhaseTable[i++] = PH_TT_MOVE;
399 PhaseTable[i++] = PH_QCAPTURES;
400 PhaseTable[i++] = PH_QCHECKS;
401 PhaseTable[i++] = PH_STOP;
403 // Quiescence search without checks
404 QsearchWithoutChecksPhaseIndex = i - 1;
405 PhaseTable[i++] = PH_TT_MOVE;
406 PhaseTable[i++] = PH_QCAPTURES;
407 PhaseTable[i++] = PH_STOP;