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 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/>.
37 //// Local definitions
44 MovePicker::MovegenPhase PhaseTable[32];
45 int MainSearchPhaseIndex;
46 int EvasionsPhaseIndex;
47 int QsearchWithChecksPhaseIndex;
48 int QsearchNoCapturesPhaseIndex;
49 int QsearchWithoutChecksPhaseIndex;
50 int NoMovesPhaseIndex;
61 /// Constructor for the MovePicker class. Apart from the position for which
62 /// it is asked to pick legal moves, MovePicker also wants some information
63 /// to help it to return the presumably good moves first, to decide which
64 /// moves to return (in the quiescence search, for instance, we only want to
65 /// search captures, promotions and some checks) and about how important good
66 /// move ordering is at the current node.
68 MovePicker::MovePicker(const Position& p, bool pv, Move ttm,
69 const SearchStack& ss, Depth d, EvalInfo* ei) : pos(p) {
72 mateKiller = (ss.mateKiller == ttm)? MOVE_NONE : ss.mateKiller;
73 killer1 = ss.killers[0];
74 killer2 = ss.killers[0];
80 // With EvalInfo we are able to know how many captures are possible before
81 // generating them. So avoid generating them in case we know are zero.
82 Color us = pos.side_to_move();
83 Color them = opposite_color(us);
84 bool noAttacks = ei && (ei->attackedBy[us][0] & pos.pieces_of_color(them)) == 0;
85 bool noCaptures = noAttacks && (pos.ep_square() == SQ_NONE) && !pos.has_pawn_on_7th(us);
88 phaseIndex = EvasionsPhaseIndex;
89 else if (depth > Depth(0))
90 phaseIndex = MainSearchPhaseIndex;
91 else if (depth == Depth(0))
92 phaseIndex = (noCaptures ? QsearchNoCapturesPhaseIndex : QsearchWithChecksPhaseIndex);
94 phaseIndex = (noCaptures ? NoMovesPhaseIndex : QsearchWithoutChecksPhaseIndex);
97 dc = p.discovered_check_candidates(us);
98 pinned = p.pinned_pieces(p.side_to_move());
104 /// MovePicker::get_next_move() is the most important method of the MovePicker
105 /// class. It returns a new legal move every time it is called, until there
106 /// are no more moves left of the types we are interested in.
108 Move MovePicker::get_next_move() {
114 // If we already have a list of generated moves, pick the best move from
115 // the list, and return it.
116 move = pick_move_from_list();
117 if (move != MOVE_NONE)
119 assert(move_is_ok(move));
125 switch (PhaseTable[phaseIndex]) {
128 if (ttMove != MOVE_NONE)
130 assert(move_is_ok(ttMove));
131 if (move_is_legal(pos, ttMove, pinned))
137 if (mateKiller != MOVE_NONE)
139 assert(move_is_ok(mateKiller));
140 if (move_is_legal(pos, mateKiller, pinned))
145 case PH_GOOD_CAPTURES:
146 numOfMoves = generate_captures(pos, moves);
151 case PH_BAD_CAPTURES:
152 badCapturesPicked = 0;
156 numOfMoves = generate_noncaptures(pos, moves);
162 assert(pos.is_check());
163 numOfMoves = generate_evasions(pos, moves);
169 numOfMoves = generate_captures(pos, moves);
175 numOfMoves = generate_checks(pos, moves, dc);
190 /// A variant of get_next_move() which takes a lock as a parameter, used to
191 /// prevent multiple threads from picking the same move at a split point.
193 Move MovePicker::get_next_move(Lock &lock) {
201 Move m = get_next_move();
210 /// MovePicker::score_captures(), MovePicker::score_noncaptures(),
211 /// MovePicker::score_evasions() and MovePicker::score_qcaptures() assign a
212 /// numerical move ordering score to each move in a move list. The moves
213 /// with highest scores will be picked first by pick_move_from_list().
215 void MovePicker::score_captures() {
216 // Winning and equal captures in the main search are ordered by MVV/LVA.
217 // Suprisingly, this appears to perform slightly better than SEE based
218 // move ordering. The reason is probably that in a position with a winning
219 // capture, capturing a more valuable (but sufficiently defended) piece
220 // first usually doesn't hurt. The opponent will have to recapture, and
221 // the hanging piece will still be hanging (except in the unusual cases
222 // where it is possible to recapture with the hanging piece). Exchanging
223 // big pieces before capturing a hanging piece probably helps to reduce
225 // While scoring captures it moves all captures with negative SEE values
226 // to the badCaptures[] array.
230 for (int i = 0; i < numOfMoves; i++)
233 seeValue = pos.see(m);
236 if (move_promotion(m))
237 moves[i].score = QueenValueMidgame;
239 moves[i].score = int(pos.midgame_value_of_piece_on(move_to(m)))
240 -int(pos.type_of_piece_on(move_from(m)));
244 // Losing capture, move it to the badCaptures[] array
245 assert(numOfBadCaptures < 63);
246 moves[i].score = seeValue;
247 badCaptures[numOfBadCaptures++] = moves[i];
248 moves[i--] = moves[--numOfMoves];
253 void MovePicker::score_noncaptures() {
254 // First score by history, when no history is available then use
255 // piece/square tables values. This seems to be better then a
256 // random choice when we don't have an history for any move.
260 for (int i = 0; i < numOfMoves; i++)
266 else if (m == killer2)
269 hs = H.move_ordering_score(pos.piece_on(move_from(m)), m);
271 // Ensure moves in history are always sorted as first
275 moves[i].score = hs + pos.mg_pst_delta(m);
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)), m);
293 // FIXME try psqt also here
296 void MovePicker::score_qcaptures() {
298 // Use MVV/LVA ordering
299 for (int i = 0; i < numOfMoves; i++)
301 Move m = moves[i].move;
302 if (move_promotion(m))
303 moves[i].score = QueenValueMidgame;
305 moves[i].score = int(pos.midgame_value_of_piece_on(move_to(m)))
306 -int(pos.type_of_piece_on(move_from(m)));
311 /// find_best_index() loops across the moves and returns index of
\r
312 /// the highest scored one. There is also a second version that
\r
313 /// lowers the priority of moves that attack the same square,
\r
314 /// so that if the best move that attack a square fails the next
\r
315 /// move picked attacks a different square if any, not the same one.
317 int MovePicker::find_best_index() {
319 int bestScore = -10000000, bestIndex = -1;
321 for (int i = movesPicked; i < numOfMoves; i++)
322 if (moves[i].score > bestScore)
325 bestScore = moves[i].score;
330 int MovePicker::find_best_index(Bitboard* squares, int values[]) {
\r
335 int bestScore = -10000000, bestIndex = -1;
\r
337 for (int i = movesPicked; i < numOfMoves; i++)
\r
342 if (!bit_is_set(*squares, to))
\r
344 // Init at first use
\r
345 set_bit(squares, to);
\r
349 hs = moves[i].score - values[to];
\r
350 if (hs > bestScore)
\r
357 if (bestIndex != -1)
\r
359 // Raise value of the picked square, so next attack
\r
360 // to the same square will get low priority.
\r
361 to = move_to(moves[bestIndex].move);
\r
362 values[to] += 0xB00;
\r
368 /// MovePicker::pick_move_from_list() picks the move with the biggest score
369 /// from a list of generated moves (moves[] or badCaptures[], depending on
370 /// the current move generation phase). It takes care not to return the
371 /// transposition table move if that has already been serched previously.
373 Move MovePicker::pick_move_from_list() {
378 switch (PhaseTable[phaseIndex]) {
379 case PH_GOOD_CAPTURES:
380 assert(!pos.is_check());
381 assert(movesPicked >= 0);
383 while (movesPicked < numOfMoves)
385 bestIndex = find_best_index();
387 if (bestIndex != -1) // Found a good capture
389 move = moves[bestIndex].move;
390 moves[bestIndex] = moves[movesPicked++];
392 && move != mateKiller
393 && pos.pl_move_is_legal(move, pinned))
400 assert(!pos.is_check());
401 assert(movesPicked >= 0);
403 while (movesPicked < numOfMoves)
405 // If this is a PV node or we have only picked a few moves, scan
406 // the entire move list for the best move. If many moves have already
407 // been searched and it is not a PV node, we are probably failing low
408 // anyway, so we just pick the first move from the list.
409 bestIndex = (pvNode || movesPicked < 12) ? find_best_index() : movesPicked;
413 move = moves[bestIndex].move;
414 moves[bestIndex] = moves[movesPicked++];
416 && move != mateKiller
417 && pos.pl_move_is_legal(move, pinned))
424 assert(pos.is_check());
425 assert(movesPicked >= 0);
427 while (movesPicked < numOfMoves)
429 bestIndex = find_best_index();
433 move = moves[bestIndex].move;
434 moves[bestIndex] = moves[movesPicked++];
440 case PH_BAD_CAPTURES:
441 assert(!pos.is_check());
442 assert(badCapturesPicked >= 0);
443 // It's probably a good idea to use SEE move ordering here, instead
444 // of just picking the first move. FIXME
445 while (badCapturesPicked < numOfBadCaptures)
447 move = badCaptures[badCapturesPicked++].move;
449 && move != mateKiller
450 && pos.pl_move_is_legal(move, pinned))
456 assert(!pos.is_check());
457 assert(movesPicked >= 0);
458 while (movesPicked < numOfMoves)
460 bestIndex = (movesPicked < 4 ? find_best_index() : movesPicked);
464 move = moves[bestIndex].move;
465 moves[bestIndex] = moves[movesPicked++];
466 // Remember to change the line below if we decide to hash the qsearch!
467 // Maybe also postpone the legality check until after futility pruning?
468 if (/* move != ttMove && */ pos.pl_move_is_legal(move, pinned))
475 assert(!pos.is_check());
476 assert(movesPicked >= 0);
477 // Perhaps we should do something better than just picking the first
479 while (movesPicked < numOfMoves)
481 move = moves[movesPicked++].move;
482 // Remember to change the line below if we decide to hash the qsearch!
483 if (/* move != ttMove && */ pos.pl_move_is_legal(move, pinned))
495 /// MovePicker::current_move_type() returns the type of the just
496 /// picked next move. It can be used in search to further differentiate
497 /// according to the current move type: capture, non capture, escape, etc.
498 MovePicker::MovegenPhase MovePicker::current_move_type() const {
500 return PhaseTable[phaseIndex];
504 /// MovePicker::init_phase_table() initializes the PhaseTable[],
505 /// MainSearchPhaseIndex, EvasionPhaseIndex, QsearchWithChecksPhaseIndex
506 /// QsearchNoCapturesPhaseIndex, QsearchWithoutChecksPhaseIndex and
507 /// NoMovesPhaseIndex variables. It is only called once during program
508 /// startup, and never again while the program is running.
510 void MovePicker::init_phase_table() {
515 MainSearchPhaseIndex = i - 1;
516 PhaseTable[i++] = PH_TT_MOVE;
517 PhaseTable[i++] = PH_MATE_KILLER;
518 PhaseTable[i++] = PH_GOOD_CAPTURES;
519 // PH_KILLER_1 and PH_KILLER_2 are not yet used.
520 // PhaseTable[i++] = PH_KILLER_1;
521 // PhaseTable[i++] = PH_KILLER_2;
522 PhaseTable[i++] = PH_NONCAPTURES;
523 PhaseTable[i++] = PH_BAD_CAPTURES;
524 PhaseTable[i++] = PH_STOP;
527 EvasionsPhaseIndex = i - 1;
528 PhaseTable[i++] = PH_EVASIONS;
529 PhaseTable[i++] = PH_STOP;
531 // Quiescence search with checks
532 QsearchWithChecksPhaseIndex = i - 1;
533 PhaseTable[i++] = PH_QCAPTURES;
534 PhaseTable[i++] = PH_QCHECKS;
535 PhaseTable[i++] = PH_STOP;
537 // Quiescence search with checks only and no captures
538 QsearchNoCapturesPhaseIndex = i - 1;
539 PhaseTable[i++] = PH_QCHECKS;
540 PhaseTable[i++] = PH_STOP;
542 // Quiescence search without checks
543 QsearchWithoutChecksPhaseIndex = i - 1;
544 PhaseTable[i++] = PH_QCAPTURES;
545 PhaseTable[i++] = PH_STOP;
547 // Do not generate any move
548 NoMovesPhaseIndex = i - 1;
549 PhaseTable[i++] = PH_STOP;