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, SearchStack* ss) : pos(p) {
72 mateKiller = (ss->mateKiller == ttm)? MOVE_NONE : ss->mateKiller;
73 killer1 = ss->killers[0];
74 killer2 = ss->killers[1];
76 mateKiller = killer1 = killer2 = MOVE_NONE;
82 checkKillers = checkLegal = false;
85 phaseIndex = EvasionsPhaseIndex;
86 else if (depth > Depth(0))
87 phaseIndex = MainSearchPhaseIndex;
88 else if (depth == Depth(0))
89 phaseIndex = QsearchWithChecksPhaseIndex;
91 phaseIndex = QsearchWithoutChecksPhaseIndex;
93 Color us = pos.side_to_move();
95 dc = p.discovered_check_candidates(us);
96 pinned = p.pinned_pieces(us);
102 /// MovePicker::get_next_move() is the most important method of the MovePicker
103 /// class. It returns a new legal move every time it is called, until there
104 /// are no more moves left of the types we are interested in.
106 Move MovePicker::get_next_move() {
112 // If we already have a list of generated moves, pick the best move from
113 // the list, and return it.
114 move = pick_move_from_list();
115 if (move != MOVE_NONE)
117 assert(move_is_ok(move));
123 switch (PhaseTable[phaseIndex]) {
126 if (ttMove != MOVE_NONE)
128 assert(move_is_ok(ttMove));
129 if (move_is_legal(pos, ttMove, pinned))
135 if (mateKiller != MOVE_NONE)
137 assert(move_is_ok(mateKiller));
138 if (move_is_legal(pos, mateKiller, pinned))
143 case PH_GOOD_CAPTURES:
144 numOfMoves = generate_captures(pos, moves);
146 std::sort(moves, moves + numOfMoves);
152 movesPicked = numOfMoves = 0;
154 if (killer1 != MOVE_NONE && move_is_legal(pos, killer1, pinned) && !pos.move_is_capture(killer1))
155 moves[numOfMoves++].move = killer1;
156 if (killer2 != MOVE_NONE && move_is_legal(pos, killer2, pinned) && !pos.move_is_capture(killer2) )
157 moves[numOfMoves++].move = killer2;
161 checkKillers = (numOfMoves != 0); // previous phase is PH_KILLERS
162 numOfMoves = generate_noncaptures(pos, moves);
164 std::sort(moves, moves + numOfMoves);
169 case PH_BAD_CAPTURES:
170 // Bad captures SEE value is already calculated by score_captures()
171 // so just sort them to get SEE move ordering.
172 std::sort(badCaptures, badCaptures + numOfBadCaptures);
177 assert(pos.is_check());
178 numOfMoves = generate_evasions(pos, moves, pinned);
180 std::sort(moves, moves + numOfMoves);
185 numOfMoves = generate_captures(pos, moves);
187 std::sort(moves, moves + numOfMoves);
192 // Perhaps we should order moves move here? FIXME
193 numOfMoves = generate_non_capture_checks(pos, moves, dc);
208 /// A variant of get_next_move() which takes a lock as a parameter, used to
209 /// prevent multiple threads from picking the same move at a split point.
211 Move MovePicker::get_next_move(Lock &lock) {
219 Move m = get_next_move();
228 /// MovePicker::score_captures(), MovePicker::score_noncaptures(),
229 /// MovePicker::score_evasions() and MovePicker::score_qcaptures() assign a
230 /// numerical move ordering score to each move in a move list. The moves
231 /// with highest scores will be picked first by pick_move_from_list().
233 void MovePicker::score_captures() {
234 // Winning and equal captures in the main search are ordered by MVV/LVA.
235 // Suprisingly, this appears to perform slightly better than SEE based
236 // move ordering. The reason is probably that in a position with a winning
237 // capture, capturing a more valuable (but sufficiently defended) piece
238 // first usually doesn't hurt. The opponent will have to recapture, and
239 // the hanging piece will still be hanging (except in the unusual cases
240 // where it is possible to recapture with the hanging piece). Exchanging
241 // big pieces before capturing a hanging piece probably helps to reduce
243 // While scoring captures it moves all captures with negative SEE values
244 // to the badCaptures[] array.
248 for (int i = 0; i < numOfMoves; i++)
251 seeValue = pos.see(m);
254 if (move_promotion(m))
255 moves[i].score = QueenValueMidgame;
257 moves[i].score = int(pos.midgame_value_of_piece_on(move_to(m)))
258 -int(pos.type_of_piece_on(move_from(m)));
262 // Losing capture, move it to the badCaptures[] array
263 assert(numOfBadCaptures < 63);
264 moves[i].score = seeValue;
265 badCaptures[numOfBadCaptures++] = moves[i];
266 moves[i--] = moves[--numOfMoves];
271 void MovePicker::score_noncaptures() {
272 // First score by history, when no history is available then use
273 // piece/square tables values. This seems to be better then a
274 // random choice when we don't have an history for any move.
278 for (int i = 0; i < numOfMoves; i++)
281 hs = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
283 // Ensure history is always preferred to pst
288 moves[i].score = hs + pos.mg_pst_delta(m);
292 void MovePicker::score_evasions() {
294 for (int i = 0; i < numOfMoves; i++)
296 Move m = moves[i].move;
298 moves[i].score = 2*HistoryMax;
299 else if (!pos.square_is_empty(move_to(m)))
301 int seeScore = pos.see(m);
302 moves[i].score = (seeScore >= 0)? seeScore + HistoryMax : seeScore;
304 moves[i].score = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
308 void MovePicker::score_qcaptures() {
310 // Use MVV/LVA ordering
311 for (int i = 0; i < numOfMoves; i++)
313 Move m = moves[i].move;
314 if (move_promotion(m))
315 moves[i].score = QueenValueMidgame;
317 moves[i].score = int(pos.midgame_value_of_piece_on(move_to(m)))
318 -int(pos.type_of_piece_on(move_from(m)));
323 /// MovePicker::pick_move_from_list() picks the move with the biggest score
324 /// from a list of generated moves (moves[] or badCaptures[], depending on
325 /// the current move generation phase). It takes care not to return the
326 /// transposition table move if that has already been serched previously.
328 Move MovePicker::pick_move_from_list() {
330 assert(movesPicked >= 0);
331 assert(!pos.is_check() || PhaseTable[phaseIndex] == PH_EVASIONS || PhaseTable[phaseIndex] == PH_STOP);
332 assert( pos.is_check() || PhaseTable[phaseIndex] != PH_EVASIONS);
334 switch (PhaseTable[phaseIndex]) {
336 case PH_GOOD_CAPTURES:
339 while (movesPicked < numOfMoves)
341 Move move = moves[movesPicked++].move;
343 && move != mateKiller
344 && (!checkKillers || (move != killer1 && move != killer2))
345 && (!checkLegal || pos.pl_move_is_legal(move, pinned)))
351 if (movesPicked < numOfMoves)
352 return moves[movesPicked++].move;
356 case PH_BAD_CAPTURES:
357 while (movesPicked < numOfBadCaptures)
359 Move move = badCaptures[movesPicked++].move;
361 && move != mateKiller
362 && pos.pl_move_is_legal(move, pinned))
369 while (movesPicked < numOfMoves)
371 Move move = moves[movesPicked++].move;
372 // Maybe postpone the legality check until after futility pruning?
374 && pos.pl_move_is_legal(move, pinned))
386 /// MovePicker::init_phase_table() initializes the PhaseTable[],
387 /// MainSearchPhaseIndex, EvasionPhaseIndex, QsearchWithChecksPhaseIndex
388 /// and QsearchWithoutChecksPhaseIndex. It is only called once during
389 /// program startup, and never again while the program is running.
391 void MovePicker::init_phase_table() {
396 MainSearchPhaseIndex = i - 1;
397 PhaseTable[i++] = PH_TT_MOVE;
398 PhaseTable[i++] = PH_MATE_KILLER;
399 PhaseTable[i++] = PH_GOOD_CAPTURES;
400 PhaseTable[i++] = PH_KILLERS;
401 PhaseTable[i++] = PH_NONCAPTURES;
402 PhaseTable[i++] = PH_BAD_CAPTURES;
403 PhaseTable[i++] = PH_STOP;
406 EvasionsPhaseIndex = i - 1;
407 PhaseTable[i++] = PH_EVASIONS;
408 PhaseTable[i++] = PH_STOP;
410 // Quiescence search with checks
411 QsearchWithChecksPhaseIndex = i - 1;
412 PhaseTable[i++] = PH_TT_MOVE;
413 PhaseTable[i++] = PH_QCAPTURES;
414 PhaseTable[i++] = PH_QCHECKS;
415 PhaseTable[i++] = PH_STOP;
417 // Quiescence search without checks
418 QsearchWithoutChecksPhaseIndex = i - 1;
419 PhaseTable[i++] = PH_TT_MOVE;
420 PhaseTable[i++] = PH_QCAPTURES;
421 PhaseTable[i++] = PH_STOP;