2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2013 Marco Costalba, Joona Kiiski, Tord Romstad
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/>.
29 MAIN_SEARCH, CAPTURES_S1, KILLERS_S1, QUIETS_1_S1, QUIETS_2_S1, BAD_CAPTURES_S1,
31 QSEARCH_0, CAPTURES_S3, QUIET_CHECKS_S3,
32 QSEARCH_1, CAPTURES_S4,
34 RECAPTURE, CAPTURES_S6,
38 // Our insertion sort, guaranteed to be stable, as is needed
39 void insertion_sort(ExtMove* begin, ExtMove* end)
43 for (p = begin + 1; p < end; ++p)
46 for (q = p; q != begin && *(q-1) < tmp; --q)
52 // Unary predicate used by std::partition to split positive scores from remaining
53 // ones so to sort separately the two sets, and with the second sort delayed.
54 inline bool has_positive_score(const ExtMove& ms) { return ms.score > 0; }
56 // Picks and moves to the front the best move in the range [begin, end),
57 // it is faster than sorting all the moves in advance when moves are few, as
58 // normally are the possible captures.
59 inline ExtMove* pick_best(ExtMove* begin, ExtMove* end)
61 std::swap(*begin, *std::max_element(begin, end));
67 /// Constructors of the MovePicker class. As arguments we pass information
68 /// to help it to return the presumably good moves first, to decide which
69 /// moves to return (in the quiescence search, for instance, we only want to
70 /// search captures, promotions and some checks) and about how important good
71 /// move ordering is at the current node.
73 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const HistoryStats& h,
74 Move* cm, Search::Stack* s) : pos(p), history(h), depth(d) {
76 assert(d > DEPTH_ZERO);
78 cur = end = moves = pos.this_thread()->get_moves_array();
79 endBadCaptures = moves + MAX_MOVES - 1;
89 ttMove = (ttm && pos.is_pseudo_legal(ttm) ? ttm : MOVE_NONE);
90 end += (ttMove != MOVE_NONE);
93 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const HistoryStats& h,
94 Square sq) : pos(p), history(h) {
96 assert(d <= DEPTH_ZERO);
98 cur = end = moves = pos.this_thread()->get_moves_array();
102 else if (d > DEPTH_QS_NO_CHECKS)
105 else if (d > DEPTH_QS_RECAPTURES)
109 // Skip TT move if is not a capture or a promotion, this avoids qsearch
110 // tree explosion due to a possible perpetual check or similar rare cases
111 // when TT table is full.
112 if (ttm && !pos.is_capture_or_promotion(ttm))
118 recaptureSquare = sq;
122 ttMove = (ttm && pos.is_pseudo_legal(ttm) ? ttm : MOVE_NONE);
123 end += (ttMove != MOVE_NONE);
126 MovePicker::MovePicker(const Position& p, Move ttm, const HistoryStats& h, PieceType pt)
127 : pos(p), history(h) {
129 assert(!pos.checkers());
131 cur = end = moves = pos.this_thread()->get_moves_array();
134 // In ProbCut we generate only captures better than parent's captured piece
135 captureThreshold = PieceValue[MG][pt];
136 ttMove = (ttm && pos.is_pseudo_legal(ttm) ? ttm : MOVE_NONE);
138 if (ttMove && (!pos.is_capture(ttMove) || pos.see(ttMove) <= captureThreshold))
141 end += (ttMove != MOVE_NONE);
144 MovePicker::~MovePicker() { pos.this_thread()->free_moves_array(); }
146 /// score() assign a numerical move ordering score to each move in a move list.
147 /// The moves with highest scores will be picked first.
149 void MovePicker::score<CAPTURES>() {
150 // Winning and equal captures in the main search are ordered by MVV/LVA.
151 // Suprisingly, this appears to perform slightly better than SEE based
152 // move ordering. The reason is probably that in a position with a winning
153 // capture, capturing a more valuable (but sufficiently defended) piece
154 // first usually doesn't hurt. The opponent will have to recapture, and
155 // the hanging piece will still be hanging (except in the unusual cases
156 // where it is possible to recapture with the hanging piece). Exchanging
157 // big pieces before capturing a hanging piece probably helps to reduce
159 // In main search we want to push captures with negative SEE values to
160 // badCaptures[] array, but instead of doing it now we delay till when
161 // the move has been picked up in pick_move_from_list(), this way we save
162 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
165 for (ExtMove* it = moves; it != end; ++it)
168 it->score = PieceValue[MG][pos.piece_on(to_sq(m))]
169 - type_of(pos.piece_moved(m));
171 if (type_of(m) == PROMOTION)
172 it->score += PieceValue[MG][promotion_type(m)] - PieceValue[MG][PAWN];
174 else if (type_of(m) == ENPASSANT)
175 it->score += PieceValue[MG][PAWN];
180 void MovePicker::score<QUIETS>() {
184 for (ExtMove* it = moves; it != end; ++it)
187 it->score = history[pos.piece_moved(m)][to_sq(m)];
192 void MovePicker::score<EVASIONS>() {
193 // Try good captures ordered by MVV/LVA, then non-captures if destination square
194 // is not under attack, ordered by history value, then bad-captures and quiet
195 // moves with a negative SEE. This last group is ordered by the SEE score.
199 for (ExtMove* it = moves; it != end; ++it)
202 if ((seeScore = pos.see_sign(m)) < 0)
203 it->score = seeScore - HistoryStats::Max; // At the bottom
205 else if (pos.is_capture(m))
206 it->score = PieceValue[MG][pos.piece_on(to_sq(m))]
207 - type_of(pos.piece_moved(m)) + HistoryStats::Max;
209 it->score = history[pos.piece_moved(m)][to_sq(m)];
214 /// generate_next() generates, scores and sorts the next bunch of moves, when
215 /// there are no more moves to try for the current phase.
217 void MovePicker::generate_next() {
223 case CAPTURES_S1: case CAPTURES_S3: case CAPTURES_S4: case CAPTURES_S5: case CAPTURES_S6:
224 end = generate<CAPTURES>(pos, moves);
232 killers[0].move = ss->killers[0];
233 killers[1].move = ss->killers[1];
234 killers[2].move = killers[3].move = MOVE_NONE;
236 // Be sure countermoves are different from killers
237 for (int i = 0; i < 2; ++i)
238 if (countermoves[i] != cur->move && countermoves[i] != (cur+1)->move)
239 (end++)->move = countermoves[i];
241 if (countermoves[1] && countermoves[1] == countermoves[0]) // Due to SMP races
242 killers[3].move = MOVE_NONE;
247 endQuiets = end = generate<QUIETS>(pos, moves);
249 end = std::partition(cur, end, has_positive_score);
250 insertion_sort(cur, end);
256 if (depth >= 3 * ONE_PLY)
257 insertion_sort(cur, end);
260 case BAD_CAPTURES_S1:
261 // Just pick them in reverse order to get MVV/LVA ordering
262 cur = moves + MAX_MOVES - 1;
263 end = endBadCaptures;
267 end = generate<EVASIONS>(pos, moves);
272 case QUIET_CHECKS_S3:
273 end = generate<QUIET_CHECKS>(pos, moves);
276 case EVASION: case QSEARCH_0: case QSEARCH_1: case PROBCUT: case RECAPTURE:
279 end = cur + 1; // Avoid another next_phase() call
288 /// next_move() is the most important method of the MovePicker class. It returns
289 /// a new pseudo legal move every time is called, until there are no more moves
290 /// left. It picks the move with the biggest score from a list of generated moves
291 /// taking care not returning the ttMove if has already been searched previously.
293 Move MovePicker::next_move<false>() {
304 case MAIN_SEARCH: case EVASION: case QSEARCH_0: case QSEARCH_1: case PROBCUT:
309 move = pick_best(cur++, end)->move;
312 if (pos.see_sign(move) >= 0)
315 // Losing capture, move it to the tail of the array
316 (endBadCaptures--)->move = move;
321 move = (cur++)->move;
322 if ( move != MOVE_NONE
323 && pos.is_pseudo_legal(move)
325 && !pos.is_capture(move))
329 case QUIETS_1_S1: case QUIETS_2_S1:
330 move = (cur++)->move;
332 && move != killers[0].move
333 && move != killers[1].move
334 && move != killers[2].move
335 && move != killers[3].move)
339 case BAD_CAPTURES_S1:
340 return (cur--)->move;
342 case EVASIONS_S2: case CAPTURES_S3: case CAPTURES_S4:
343 move = pick_best(cur++, end)->move;
349 move = pick_best(cur++, end)->move;
350 if (move != ttMove && pos.see(move) > captureThreshold)
355 move = pick_best(cur++, end)->move;
356 if (to_sq(move) == recaptureSquare)
360 case QUIET_CHECKS_S3:
361 move = (cur++)->move;
376 /// Version of next_move() to use at split point nodes where the move is grabbed
377 /// from the split point's shared MovePicker object. This function is not thread
378 /// safe so must be lock protected by the caller.
380 Move MovePicker::next_move<true>() { return ss->splitPoint->movePicker->next_move<false>(); }