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[1];
80 // With EvalInfo we are able to know how many captures are possible before
81 // generating them. So avoid generating in case we know are zero.
82 Color us = pos.side_to_move();
83 Color them = opposite_color(us);
85 && (ei->attackedBy[us][0] & pos.pieces_of_color(them)) == 0
86 && !ei->mi->specialized_eval_exists()
87 && (pos.ep_square() == SQ_NONE)
88 && !pos.has_pawn_on_7th(us);
91 phaseIndex = EvasionsPhaseIndex;
92 else if (depth > Depth(0))
93 phaseIndex = MainSearchPhaseIndex;
94 else if (depth == Depth(0))
95 phaseIndex = (noCaptures ? QsearchNoCapturesPhaseIndex : QsearchWithChecksPhaseIndex);
97 phaseIndex = (noCaptures ? NoMovesPhaseIndex : QsearchWithoutChecksPhaseIndex);
99 dc = p.discovered_check_candidates(us);
100 pinned = p.pinned_pieces(p.side_to_move());
106 /// MovePicker::get_next_move() is the most important method of the MovePicker
107 /// class. It returns a new legal move every time it is called, until there
108 /// are no more moves left of the types we are interested in.
110 Move MovePicker::get_next_move() {
116 // If we already have a list of generated moves, pick the best move from
117 // the list, and return it.
118 move = pick_move_from_list();
119 if (move != MOVE_NONE)
121 assert(move_is_ok(move));
127 switch (PhaseTable[phaseIndex]) {
130 if (ttMove != MOVE_NONE)
132 assert(move_is_ok(ttMove));
133 if (move_is_legal(pos, ttMove, pinned))
139 if (mateKiller != MOVE_NONE)
141 assert(move_is_ok(mateKiller));
142 if (move_is_legal(pos, mateKiller, pinned))
147 case PH_GOOD_CAPTURES:
148 numOfMoves = generate_captures(pos, moves);
153 case PH_BAD_CAPTURES:
154 badCapturesPicked = 0;
158 numOfMoves = generate_noncaptures(pos, moves);
164 assert(pos.is_check());
165 numOfMoves = generate_evasions(pos, moves);
171 numOfMoves = generate_captures(pos, moves);
177 numOfMoves = generate_checks(pos, moves, dc);
192 /// A variant of get_next_move() which takes a lock as a parameter, used to
193 /// prevent multiple threads from picking the same move at a split point.
195 Move MovePicker::get_next_move(Lock &lock) {
203 Move m = get_next_move();
212 /// MovePicker::score_captures(), MovePicker::score_noncaptures(),
213 /// MovePicker::score_evasions() and MovePicker::score_qcaptures() assign a
214 /// numerical move ordering score to each move in a move list. The moves
215 /// with highest scores will be picked first by pick_move_from_list().
217 void MovePicker::score_captures() {
218 // Winning and equal captures in the main search are ordered by MVV/LVA.
219 // Suprisingly, this appears to perform slightly better than SEE based
220 // move ordering. The reason is probably that in a position with a winning
221 // capture, capturing a more valuable (but sufficiently defended) piece
222 // first usually doesn't hurt. The opponent will have to recapture, and
223 // the hanging piece will still be hanging (except in the unusual cases
224 // where it is possible to recapture with the hanging piece). Exchanging
225 // big pieces before capturing a hanging piece probably helps to reduce
227 // While scoring captures it moves all captures with negative SEE values
228 // to the badCaptures[] array.
233 for (int i = 0; i < numOfMoves; i++)
239 bool hxl = ( int(pos.midgame_value_of_piece_on(from))
240 -int(pos.midgame_value_of_piece_on(to)) > 0)
241 || pos.type_of_piece_on(from) == KING;
243 // Avoid calling see() for LxH and equal captures because
244 // SEE is always >= 0 and we order for MVV/LVA anyway.
245 seeValue = (hxl ? pos.see(m) : 0);
249 if (move_promotion(m))
250 moves[i].score = QueenValueMidgame;
252 moves[i].score = int(pos.midgame_value_of_piece_on(to))
253 -int(pos.type_of_piece_on(from));
257 // Losing capture, move it to the badCaptures[] array
258 assert(numOfBadCaptures < 63);
259 moves[i].score = seeValue;
260 badCaptures[numOfBadCaptures++] = moves[i];
261 moves[i--] = moves[--numOfMoves];
266 void MovePicker::score_noncaptures() {
267 // First score by history, when no history is available then use
268 // piece/square tables values. This seems to be better then a
269 // random choice when we don't have an history for any move.
273 for (int i = 0; i < numOfMoves; i++)
279 else if (m == killer2)
282 hs = H.move_ordering_score(pos.piece_on(move_from(m)), 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)), 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 /// find_best_index() loops across the moves and returns index of
\r
325 /// the highest scored one. There is also a second version that
\r
326 /// lowers the priority of moves that attack the same square,
\r
327 /// so that if the best move that attack a square fails the next
\r
328 /// move picked attacks a different square if any, not the same one.
330 int MovePicker::find_best_index() {
332 int bestScore = -10000000, bestIndex = -1;
334 for (int i = movesPicked; i < numOfMoves; i++)
335 if (moves[i].score > bestScore)
338 bestScore = moves[i].score;
343 int MovePicker::find_best_index(Bitboard* squares, int values[]) {
\r
348 int bestScore = -10000000, bestIndex = -1;
\r
350 for (int i = movesPicked; i < numOfMoves; i++)
\r
355 if (!bit_is_set(*squares, to))
\r
357 // Init at first use
\r
358 set_bit(squares, to);
\r
362 hs = moves[i].score - values[to];
\r
363 if (hs > bestScore)
\r
370 if (bestIndex != -1)
\r
372 // Raise value of the picked square, so next attack
\r
373 // to the same square will get low priority.
\r
374 to = move_to(moves[bestIndex].move);
\r
375 values[to] += 0xB00;
\r
381 /// MovePicker::pick_move_from_list() picks the move with the biggest score
382 /// from a list of generated moves (moves[] or badCaptures[], depending on
383 /// the current move generation phase). It takes care not to return the
384 /// transposition table move if that has already been serched previously.
386 Move MovePicker::pick_move_from_list() {
391 switch (PhaseTable[phaseIndex]) {
392 case PH_GOOD_CAPTURES:
393 assert(!pos.is_check());
394 assert(movesPicked >= 0);
396 while (movesPicked < numOfMoves)
398 bestIndex = find_best_index();
400 if (bestIndex != -1) // Found a good capture
402 move = moves[bestIndex].move;
403 moves[bestIndex] = moves[movesPicked++];
405 && move != mateKiller
406 && pos.pl_move_is_legal(move, pinned))
413 assert(!pos.is_check());
414 assert(movesPicked >= 0);
416 while (movesPicked < numOfMoves)
418 // If this is a PV node or we have only picked a few moves, scan
419 // the entire move list for the best move. If many moves have already
420 // been searched and it is not a PV node, we are probably failing low
421 // anyway, so we just pick the first move from the list.
422 bestIndex = (pvNode || movesPicked < 12) ? find_best_index() : movesPicked;
426 move = moves[bestIndex].move;
427 moves[bestIndex] = moves[movesPicked++];
429 && move != mateKiller
430 && pos.pl_move_is_legal(move, pinned))
437 assert(pos.is_check());
438 assert(movesPicked >= 0);
440 while (movesPicked < numOfMoves)
442 bestIndex = find_best_index();
446 move = moves[bestIndex].move;
447 moves[bestIndex] = moves[movesPicked++];
453 case PH_BAD_CAPTURES:
454 assert(!pos.is_check());
455 assert(badCapturesPicked >= 0);
456 // It's probably a good idea to use SEE move ordering here, instead
457 // of just picking the first move. FIXME
458 while (badCapturesPicked < numOfBadCaptures)
460 move = badCaptures[badCapturesPicked++].move;
462 && move != mateKiller
463 && pos.pl_move_is_legal(move, pinned))
469 assert(!pos.is_check());
470 assert(movesPicked >= 0);
471 while (movesPicked < numOfMoves)
473 bestIndex = (movesPicked < 4 ? find_best_index() : movesPicked);
477 move = moves[bestIndex].move;
478 moves[bestIndex] = moves[movesPicked++];
479 // Remember to change the line below if we decide to hash the qsearch!
480 // Maybe also postpone the legality check until after futility pruning?
481 if (/* move != ttMove && */ pos.pl_move_is_legal(move, pinned))
488 assert(!pos.is_check());
489 assert(movesPicked >= 0);
490 // Perhaps we should do something better than just picking the first
492 while (movesPicked < numOfMoves)
494 move = moves[movesPicked++].move;
495 // Remember to change the line below if we decide to hash the qsearch!
496 if (/* move != ttMove && */ pos.pl_move_is_legal(move, pinned))
508 /// MovePicker::current_move_type() returns the type of the just
509 /// picked next move. It can be used in search to further differentiate
510 /// according to the current move type: capture, non capture, escape, etc.
511 MovePicker::MovegenPhase MovePicker::current_move_type() const {
513 return PhaseTable[phaseIndex];
517 /// MovePicker::init_phase_table() initializes the PhaseTable[],
518 /// MainSearchPhaseIndex, EvasionPhaseIndex, QsearchWithChecksPhaseIndex
519 /// QsearchNoCapturesPhaseIndex, QsearchWithoutChecksPhaseIndex and
520 /// NoMovesPhaseIndex variables. It is only called once during program
521 /// startup, and never again while the program is running.
523 void MovePicker::init_phase_table() {
528 MainSearchPhaseIndex = i - 1;
529 PhaseTable[i++] = PH_TT_MOVE;
530 PhaseTable[i++] = PH_MATE_KILLER;
531 PhaseTable[i++] = PH_GOOD_CAPTURES;
532 // PH_KILLER_1 and PH_KILLER_2 are not yet used.
533 // PhaseTable[i++] = PH_KILLER_1;
534 // PhaseTable[i++] = PH_KILLER_2;
535 PhaseTable[i++] = PH_NONCAPTURES;
536 PhaseTable[i++] = PH_BAD_CAPTURES;
537 PhaseTable[i++] = PH_STOP;
540 EvasionsPhaseIndex = i - 1;
541 PhaseTable[i++] = PH_EVASIONS;
542 PhaseTable[i++] = PH_STOP;
544 // Quiescence search with checks
545 QsearchWithChecksPhaseIndex = i - 1;
546 PhaseTable[i++] = PH_QCAPTURES;
547 PhaseTable[i++] = PH_QCHECKS;
548 PhaseTable[i++] = PH_STOP;
550 // Quiescence search with checks only and no captures
551 QsearchNoCapturesPhaseIndex = i - 1;
552 PhaseTable[i++] = PH_QCHECKS;
553 PhaseTable[i++] = PH_STOP;
555 // Quiescence search without checks
556 QsearchWithoutChecksPhaseIndex = i - 1;
557 PhaseTable[i++] = PH_QCAPTURES;
558 PhaseTable[i++] = PH_STOP;
560 // Do not generate any move
561 NoMovesPhaseIndex = i - 1;
562 PhaseTable[i++] = PH_STOP;