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
45 MovePicker::MovegenPhase PhaseTable[32];
46 int MainSearchPhaseIndex;
47 int EvasionsPhaseIndex;
48 int QsearchWithChecksPhaseIndex;
49 int QsearchWithoutChecksPhaseIndex;
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, bool pv, Move ttm, Depth d,
67 const History& h, SearchStack* ss) : pos(p), H(h) {
73 mateKiller = (ss->mateKiller == ttm)? MOVE_NONE : ss->mateKiller;
74 killer1 = ss->killers[0];
75 killer2 = ss->killers[1];
77 mateKiller = killer1 = killer2 = MOVE_NONE;
83 checkKillers = checkLegal = false;
86 phaseIndex = EvasionsPhaseIndex;
87 else if (depth > Depth(0))
88 phaseIndex = MainSearchPhaseIndex;
89 else if (depth == Depth(0))
90 phaseIndex = QsearchWithChecksPhaseIndex;
92 phaseIndex = QsearchWithoutChecksPhaseIndex;
94 Color us = pos.side_to_move();
96 dc = p.discovered_check_candidates(us);
97 pinned = p.pinned_pieces(us);
103 /// MovePicker::get_next_move() is the most important method of the MovePicker
104 /// class. It returns a new legal move every time it is called, until there
105 /// are no more moves left of the types we are interested in.
107 Move MovePicker::get_next_move() {
113 // If we already have a list of generated moves, pick the best move from
114 // the list, and return it.
115 move = pick_move_from_list();
116 if (move != MOVE_NONE)
118 assert(move_is_ok(move));
124 switch (PhaseTable[phaseIndex]) {
127 if (ttMove != MOVE_NONE)
129 assert(move_is_ok(ttMove));
130 if (move_is_legal(pos, ttMove, pinned))
136 if (mateKiller != MOVE_NONE)
138 assert(move_is_ok(mateKiller));
139 if (move_is_legal(pos, mateKiller, pinned))
144 case PH_GOOD_CAPTURES:
145 numOfMoves = generate_captures(pos, moves);
147 std::sort(moves, moves + numOfMoves);
153 movesPicked = numOfMoves = 0;
155 if (killer1 != MOVE_NONE && move_is_legal(pos, killer1, pinned) && !pos.move_is_capture(killer1))
156 moves[numOfMoves++].move = killer1;
157 if (killer2 != MOVE_NONE && move_is_legal(pos, killer2, pinned) && !pos.move_is_capture(killer2) )
158 moves[numOfMoves++].move = killer2;
162 checkKillers = (numOfMoves != 0); // previous phase is PH_KILLERS
163 numOfMoves = generate_noncaptures(pos, moves);
165 std::sort(moves, moves + numOfMoves);
170 case PH_BAD_CAPTURES:
171 // Bad captures SEE value is already calculated by score_captures()
172 // so just sort them to get SEE move ordering.
173 std::sort(badCaptures, badCaptures + numOfBadCaptures);
178 assert(pos.is_check());
179 numOfMoves = generate_evasions(pos, moves, pinned);
181 std::sort(moves, moves + numOfMoves);
186 numOfMoves = generate_captures(pos, moves);
188 std::sort(moves, moves + numOfMoves);
193 // Perhaps we should order moves move here? FIXME
194 numOfMoves = generate_non_capture_checks(pos, moves, dc);
209 /// A variant of get_next_move() which takes a lock as a parameter, used to
210 /// prevent multiple threads from picking the same move at a split point.
212 Move MovePicker::get_next_move(Lock &lock) {
220 Move m = get_next_move();
229 /// MovePicker::score_captures(), MovePicker::score_noncaptures(),
230 /// MovePicker::score_evasions() and MovePicker::score_qcaptures() assign a
231 /// numerical move ordering score to each move in a move list. The moves
232 /// with highest scores will be picked first by pick_move_from_list().
234 void MovePicker::score_captures() {
235 // Winning and equal captures in the main search are ordered by MVV/LVA.
236 // Suprisingly, this appears to perform slightly better than SEE based
237 // move ordering. The reason is probably that in a position with a winning
238 // capture, capturing a more valuable (but sufficiently defended) piece
239 // first usually doesn't hurt. The opponent will have to recapture, and
240 // the hanging piece will still be hanging (except in the unusual cases
241 // where it is possible to recapture with the hanging piece). Exchanging
242 // big pieces before capturing a hanging piece probably helps to reduce
244 // While scoring captures it moves all captures with negative SEE values
245 // to the badCaptures[] array.
249 for (int i = 0; i < numOfMoves; i++)
252 seeValue = pos.see(m);
255 if (move_promotion(m))
256 moves[i].score = QueenValueMidgame;
258 moves[i].score = int(pos.midgame_value_of_piece_on(move_to(m)))
259 -int(pos.type_of_piece_on(move_from(m)));
263 // Losing capture, move it to the badCaptures[] array
264 assert(numOfBadCaptures < 63);
265 moves[i].score = seeValue;
266 badCaptures[numOfBadCaptures++] = moves[i];
267 moves[i--] = moves[--numOfMoves];
272 void MovePicker::score_noncaptures() {
273 // First score by history, when no history is available then use
274 // piece/square tables values. This seems to be better then a
275 // random choice when we don't have an history for any move.
279 for (int i = 0; i < numOfMoves; i++)
282 hs = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
284 // Ensure history is always preferred to pst
289 moves[i].score = hs + pos.mg_pst_delta(m);
293 void MovePicker::score_evasions() {
295 for (int i = 0; i < numOfMoves; i++)
297 Move m = moves[i].move;
299 moves[i].score = 2*HistoryMax;
300 else if (!pos.square_is_empty(move_to(m)))
302 int seeScore = pos.see(m);
303 moves[i].score = (seeScore >= 0)? seeScore + HistoryMax : seeScore;
305 moves[i].score = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
309 void MovePicker::score_qcaptures() {
311 // Use MVV/LVA ordering
312 for (int i = 0; i < numOfMoves; i++)
314 Move m = moves[i].move;
315 if (move_promotion(m))
316 moves[i].score = QueenValueMidgame;
318 moves[i].score = int(pos.midgame_value_of_piece_on(move_to(m)))
319 -int(pos.type_of_piece_on(move_from(m)));
324 /// MovePicker::pick_move_from_list() picks the move with the biggest score
325 /// from a list of generated moves (moves[] or badCaptures[], depending on
326 /// the current move generation phase). It takes care not to return the
327 /// transposition table move if that has already been serched previously.
329 Move MovePicker::pick_move_from_list() {
331 assert(movesPicked >= 0);
332 assert(!pos.is_check() || PhaseTable[phaseIndex] == PH_EVASIONS || PhaseTable[phaseIndex] == PH_STOP);
333 assert( pos.is_check() || PhaseTable[phaseIndex] != PH_EVASIONS);
335 switch (PhaseTable[phaseIndex]) {
337 case PH_GOOD_CAPTURES:
340 while (movesPicked < numOfMoves)
342 Move move = moves[movesPicked++].move;
344 && move != mateKiller
345 && (!checkKillers || (move != killer1 && move != killer2))
346 && (!checkLegal || pos.pl_move_is_legal(move, pinned)))
352 if (movesPicked < numOfMoves)
353 return moves[movesPicked++].move;
357 case PH_BAD_CAPTURES:
358 while (movesPicked < numOfBadCaptures)
360 Move move = badCaptures[movesPicked++].move;
362 && move != mateKiller
363 && pos.pl_move_is_legal(move, pinned))
370 while (movesPicked < numOfMoves)
372 Move move = moves[movesPicked++].move;
373 // Maybe postpone the legality check until after futility pruning?
375 && pos.pl_move_is_legal(move, pinned))
387 /// MovePicker::init_phase_table() initializes the PhaseTable[],
388 /// MainSearchPhaseIndex, EvasionPhaseIndex, QsearchWithChecksPhaseIndex
389 /// and QsearchWithoutChecksPhaseIndex. It is only called once during
390 /// program startup, and never again while the program is running.
392 void MovePicker::init_phase_table() {
397 MainSearchPhaseIndex = i - 1;
398 PhaseTable[i++] = PH_TT_MOVE;
399 PhaseTable[i++] = PH_MATE_KILLER;
400 PhaseTable[i++] = PH_GOOD_CAPTURES;
401 PhaseTable[i++] = PH_KILLERS;
402 PhaseTable[i++] = PH_NONCAPTURES;
403 PhaseTable[i++] = PH_BAD_CAPTURES;
404 PhaseTable[i++] = PH_STOP;
407 EvasionsPhaseIndex = i - 1;
408 PhaseTable[i++] = PH_EVASIONS;
409 PhaseTable[i++] = PH_STOP;
411 // Quiescence search with checks
412 QsearchWithChecksPhaseIndex = i - 1;
413 PhaseTable[i++] = PH_TT_MOVE;
414 PhaseTable[i++] = PH_QCAPTURES;
415 PhaseTable[i++] = PH_QCHECKS;
416 PhaseTable[i++] = PH_STOP;
418 // Quiescence search without checks
419 QsearchWithoutChecksPhaseIndex = i - 1;
420 PhaseTable[i++] = PH_TT_MOVE;
421 PhaseTable[i++] = PH_QCAPTURES;
422 PhaseTable[i++] = PH_STOP;