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.
11 Stockfish is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>.
35 //// Local definitions
42 MovePicker::MovegenPhase PhaseTable[32];
43 int MainSearchPhaseIndex;
44 int EvasionsPhaseIndex;
45 int QsearchWithChecksPhaseIndex;
46 int QsearchWithoutChecksPhaseIndex;
57 /// Constructor for the MovePicker class. Apart from the position for which
58 /// it is asked to pick legal moves, MovePicker also wants some information
59 /// to help it to return the presumably good moves first, to decide which
60 /// moves to return (in the quiescence search, for instance, we only want to
61 /// search captures, promotions and some checks) and about how important good
62 /// move ordering is at the current node.
64 MovePicker::MovePicker(const Position& p, bool pvnode, Move ttm, Move mk,
65 Move k1, Move k2, Depth d) : pos(p) {
68 mateKiller = (mk == ttm)? MOVE_NONE : mk;
75 dc = p.discovered_check_candidates(p.side_to_move());
78 phaseIndex = EvasionsPhaseIndex;
79 else if (depth > Depth(0))
80 phaseIndex = MainSearchPhaseIndex;
81 else if (depth == Depth(0))
82 phaseIndex = QsearchWithChecksPhaseIndex;
84 phaseIndex = QsearchWithoutChecksPhaseIndex;
86 pinned = p.pinned_pieces(p.side_to_move());
92 /// MovePicker::get_next_move() is the most important method of the MovePicker
93 /// class. It returns a new legal move every time it is called, until there
94 /// are no more moves left of the types we are interested in.
96 Move MovePicker::get_next_move() {
102 // If we already have a list of generated moves, pick the best move from
103 // the list, and return it.
104 move = pick_move_from_list();
105 if (move != MOVE_NONE)
107 assert(move_is_ok(move));
113 switch (PhaseTable[phaseIndex]) {
116 if (ttMove != MOVE_NONE)
118 assert(move_is_ok(ttMove));
119 if (generate_move_if_legal(pos, ttMove, pinned) != MOVE_NONE)
125 if (mateKiller != MOVE_NONE)
127 assert(move_is_ok(mateKiller));
128 if (generate_move_if_legal(pos, mateKiller, pinned) != MOVE_NONE)
133 case PH_GOOD_CAPTURES:
134 numOfMoves = generate_captures(pos, moves);
139 case PH_BAD_CAPTURES:
140 badCapturesPicked = 0;
144 numOfMoves = generate_noncaptures(pos, moves);
150 assert(pos.is_check());
151 numOfMoves = generate_evasions(pos, moves);
157 numOfMoves = generate_captures(pos, moves);
163 numOfMoves = generate_checks(pos, moves, dc);
181 /// A variant of get_next_move() which takes a lock as a parameter, used to
182 /// prevent multiple threads from picking the same move at a split point.
184 Move MovePicker::get_next_move(Lock &lock) {
192 Move m = get_next_move();
200 /// MovePicker::score_captures(), MovePicker::score_noncaptures(),
201 /// MovePicker::score_evasions() and MovePicker::score_qcaptures() assign a
202 /// numerical move ordering score to each move in a move list. The moves
203 /// with highest scores will be picked first by pick_move_from_list().
205 void MovePicker::score_captures() {
206 // Winning and equal captures in the main search are ordered by MVV.
207 // Suprisingly, this appears to perform slightly better than SEE based
208 // move ordering. The reason is probably that in a position with a winning
209 // capture, capturing a more valuable (but sufficiently defended) piece
210 // first usually doesn't hurt. The opponent will have to recapture, and
211 // the hanging piece will still be hanging (except in the unusual cases
212 // where it is possible to recapture with the hanging piece). Exchanging
213 // big pieces before capturing a hanging piece probably helps to reduce
215 for (int i = 0; i < numOfMoves; i++)
217 Move m = moves[i].move;
218 moves[i].score = pos.see(m);
219 if (moves[i].score >= 0)
220 moves[i].score = move_promotion(m) ? QueenValueMidgame
221 : pos.midgame_value_of_piece_on(move_to(m));
225 void MovePicker::score_noncaptures() {
227 for (int i = 0; i < numOfMoves; i++)
229 Move m = moves[i].move;
231 moves[i].score = HistoryMax + 2;
232 else if (m == killer2)
233 moves[i].score = HistoryMax + 1;
235 moves[i].score = H.move_ordering_score(pos.piece_on(move_from(m)), m);
237 // Ensure moves in history are always sorted as first
238 if (moves[i].score > 0)
239 moves[i].score += 1000;
241 moves[i].score += pos.mg_pst_delta(moves[i].move);
245 void MovePicker::score_evasions() {
247 for (int i = 0; i < numOfMoves; i++)
249 Move m = moves[i].move;
251 moves[i].score = 2*HistoryMax;
252 else if (!pos.square_is_empty(move_to(m)))
254 moves[i].score = pos.see(m);
255 if (moves[i].score >= 0)
256 moves[i].score += HistoryMax;
259 moves[i].score = H.move_ordering_score(pos.piece_on(move_from(m)), m);
261 // FIXME try psqt also here
264 void MovePicker::score_qcaptures() {
267 for (int i = 0; i < numOfMoves; i++)
269 Move m = moves[i].move;
270 moves[i].score = move_promotion(m) ? QueenValueMidgame
271 : pos.midgame_value_of_piece_on(move_to(m));
276 /// find_best_index() loops across the moves and returns index of
277 /// the highest scored one.
279 int MovePicker::find_best_index() {
281 int bestScore = -10000000, bestIndex = -1;
283 for (int i = movesPicked; i < numOfMoves; i++)
284 if (moves[i].score > bestScore)
287 bestScore = moves[i].score;
293 /// MovePicker::pick_move_from_list() picks the move with the biggest score
294 /// from a list of generated moves (moves[] or badCaptures[], depending on
295 /// the current move generation phase). It takes care not to return the
296 /// transposition table move if that has already been serched previously.
297 /// While picking captures in the PH_GOOD_CAPTURES phase (i.e. while picking
298 /// non-losing captures in the main search), it moves all captures with
299 /// negative SEE values to the badCaptures[] array.
301 Move MovePicker::pick_move_from_list() {
306 switch (PhaseTable[phaseIndex]) {
308 case PH_GOOD_CAPTURES:
309 assert(!pos.is_check());
310 assert(movesPicked >= 0);
311 while (movesPicked < numOfMoves)
313 int bestScore = -10000000;
315 for (int i = movesPicked; i < numOfMoves; i++)
317 if (moves[i].score < 0)
319 // Losing capture, move it to the badCaptures[] array
320 assert(numOfBadCaptures < 63);
321 badCaptures[numOfBadCaptures++] = moves[i];
322 moves[i--] = moves[--numOfMoves];
324 else if (moves[i].score > bestScore)
327 bestScore = moves[i].score;
330 if (bestIndex != -1) // Found a good capture
332 move = moves[bestIndex].move;
\r
333 moves[bestIndex] = moves[movesPicked++];
335 && move != mateKiller
336 && pos.move_is_legal(move, pinned))
343 assert(!pos.is_check());
344 assert(movesPicked >= 0);
345 while (movesPicked < numOfMoves)
347 // If this is a PV node or we have only picked a few moves, scan
348 // the entire move list for the best move. If many moves have already
349 // been searched and it is not a PV node, we are probably failing low
350 // anyway, so we just pick the first move from the list.
351 bestIndex = (movesPicked < 12 || pvNode ? find_best_index() : movesPicked);
355 move = moves[bestIndex].move;
\r
356 moves[bestIndex] = moves[movesPicked++];
358 && move != mateKiller
359 && pos.move_is_legal(move, pinned))
366 assert(pos.is_check());
367 assert(movesPicked >= 0);
368 while (movesPicked < numOfMoves)
370 bestIndex = find_best_index();
374 move = moves[bestIndex].move;
\r
375 moves[bestIndex] = moves[movesPicked++];
381 case PH_BAD_CAPTURES:
382 assert(!pos.is_check());
383 assert(badCapturesPicked >= 0);
384 // It's probably a good idea to use SEE move ordering here, instead
385 // of just picking the first move. FIXME
386 while (badCapturesPicked < numOfBadCaptures)
388 move = badCaptures[badCapturesPicked++].move;
390 && move != mateKiller
391 && pos.move_is_legal(move, pinned))
397 assert(!pos.is_check());
398 assert(movesPicked >= 0);
399 while (movesPicked < numOfMoves)
401 // FIXME makes sens to score qcaps?
402 bestIndex = (movesPicked < 4 ? find_best_index() : movesPicked);
406 move = moves[bestIndex].move;
\r
407 moves[bestIndex] = moves[movesPicked++];
408 if (move != ttMove && pos.move_is_legal(move, pinned))
415 assert(!pos.is_check());
416 assert(movesPicked >= 0);
417 // Perhaps we should do something better than just picking the first
419 while (movesPicked < numOfMoves)
421 move = moves[movesPicked++].move;
422 if (move != ttMove && pos.move_is_legal(move, pinned))
434 /// MovePicker::current_move_type() returns the type of the just
435 /// picked next move. It can be used in search to further differentiate
436 /// according to the current move type: capture, non capture, escape, etc.
437 MovePicker::MovegenPhase MovePicker::current_move_type() const {
439 return PhaseTable[phaseIndex];
443 /// MovePicker::init_phase_table() initializes the PhaseTable[],
444 /// MainSearchPhaseIndex, EvasionPhaseIndex, QsearchWithChecksPhaseIndex
445 /// and QsearchWithoutChecksPhaseIndex variables. It is only called once
446 /// during program startup, and never again while the program is running.
448 void MovePicker::init_phase_table() {
453 MainSearchPhaseIndex = i - 1;
454 PhaseTable[i++] = PH_TT_MOVE;
455 PhaseTable[i++] = PH_MATE_KILLER;
456 PhaseTable[i++] = PH_GOOD_CAPTURES;
457 // PH_KILLER_1 and PH_KILLER_2 are not yet used.
458 // PhaseTable[i++] = PH_KILLER_1;
459 // PhaseTable[i++] = PH_KILLER_2;
460 PhaseTable[i++] = PH_NONCAPTURES;
461 PhaseTable[i++] = PH_BAD_CAPTURES;
462 PhaseTable[i++] = PH_STOP;
465 EvasionsPhaseIndex = i - 1;
466 PhaseTable[i++] = PH_EVASIONS;
467 PhaseTable[i++] = PH_STOP;
469 // Quiescence search with checks
470 QsearchWithChecksPhaseIndex = i - 1;
471 PhaseTable[i++] = PH_QCAPTURES;
472 PhaseTable[i++] = PH_QCHECKS;
473 PhaseTable[i++] = PH_STOP;
475 // Quiescence search without checks
476 QsearchWithoutChecksPhaseIndex = i - 1;
477 PhaseTable[i++] = PH_QCAPTURES;
478 PhaseTable[i++] = PH_STOP;