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(MoveStack* begin, MoveStack* end)
41 MoveStack tmp, *p, *q;
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 MoveStack& 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 MoveStack* pick_best(MoveStack* begin, MoveStack* 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 History& h,
74 Search::Stack* s, Move refutationMove, Value beta) : pos(p), Hist(h), depth(d) {
76 assert(d > DEPTH_ZERO);
80 endBadCaptures = moves + MAX_MOVES - 1;
90 killers[0].move = ss->killers[0];
91 killers[1].move = ss->killers[1];
92 killers[2].move = refutationMove;
94 // Consider sligtly negative captures as good if at low depth and far from beta
95 if (ss && ss->staticEval < beta - PawnValueMg && d < 3 * ONE_PLY)
96 captureThreshold = -PawnValueMg;
98 // Consider negative captures as good if still enough to reach beta
99 else if (ss && ss->staticEval > beta)
100 captureThreshold = beta - ss->staticEval;
103 ttMove = (ttm && pos.is_pseudo_legal(ttm) ? ttm : MOVE_NONE);
104 end += (ttMove != MOVE_NONE);
107 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h,
108 Square sq) : pos(p), Hist(h), cur(moves), end(moves) {
110 assert(d <= DEPTH_ZERO);
115 else if (d > DEPTH_QS_NO_CHECKS)
118 else if (d > DEPTH_QS_RECAPTURES)
122 // Skip TT move if is not a capture or a promotion, this avoids qsearch
123 // tree explosion due to a possible perpetual check or similar rare cases
124 // when TT table is full.
125 if (ttm && !pos.is_capture_or_promotion(ttm))
131 recaptureSquare = sq;
135 ttMove = (ttm && pos.is_pseudo_legal(ttm) ? ttm : MOVE_NONE);
136 end += (ttMove != MOVE_NONE);
139 MovePicker::MovePicker(const Position& p, Move ttm, const History& h, PieceType pt)
140 : pos(p), Hist(h), cur(moves), end(moves) {
142 assert(!pos.checkers());
146 // In ProbCut we generate only captures better than parent's captured piece
147 captureThreshold = PieceValue[MG][pt];
148 ttMove = (ttm && pos.is_pseudo_legal(ttm) ? ttm : MOVE_NONE);
150 if (ttMove && (!pos.is_capture(ttMove) || pos.see(ttMove) <= captureThreshold))
153 end += (ttMove != MOVE_NONE);
157 /// score() assign a numerical move ordering score to each move in a move list.
158 /// The moves with highest scores will be picked first.
160 void MovePicker::score<CAPTURES>() {
161 // Winning and equal captures in the main search are ordered by MVV/LVA.
162 // Suprisingly, this appears to perform slightly better than SEE based
163 // move ordering. The reason is probably that in a position with a winning
164 // capture, capturing a more valuable (but sufficiently defended) piece
165 // first usually doesn't hurt. The opponent will have to recapture, and
166 // the hanging piece will still be hanging (except in the unusual cases
167 // where it is possible to recapture with the hanging piece). Exchanging
168 // big pieces before capturing a hanging piece probably helps to reduce
170 // In main search we want to push captures with negative SEE values to
171 // badCaptures[] array, but instead of doing it now we delay till when
172 // the move has been picked up in pick_move_from_list(), this way we save
173 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
176 for (MoveStack* it = moves; it != end; ++it)
179 it->score = PieceValue[MG][pos.piece_on(to_sq(m))]
180 - type_of(pos.piece_moved(m));
182 if (type_of(m) == PROMOTION)
183 it->score += PieceValue[MG][promotion_type(m)] - PieceValue[MG][PAWN];
185 else if (type_of(m) == ENPASSANT)
186 it->score += PieceValue[MG][PAWN];
191 void MovePicker::score<QUIETS>() {
195 for (MoveStack* it = moves; it != end; ++it)
198 it->score = Hist[pos.piece_moved(m)][to_sq(m)];
203 void MovePicker::score<EVASIONS>() {
204 // Try good captures ordered by MVV/LVA, then non-captures if destination square
205 // is not under attack, ordered by history value, then bad-captures and quiet
206 // moves with a negative SEE. This last group is ordered by the SEE score.
210 for (MoveStack* it = moves; it != end; ++it)
213 if ((seeScore = pos.see_sign(m)) < 0)
214 it->score = seeScore - History::Max; // At the bottom
216 else if (pos.is_capture(m))
217 it->score = PieceValue[MG][pos.piece_on(to_sq(m))]
218 - type_of(pos.piece_moved(m)) + History::Max;
220 it->score = Hist[pos.piece_moved(m)][to_sq(m)];
225 /// generate_next() generates, scores and sorts the next bunch of moves, when
226 /// there are no more moves to try for the current phase.
228 void MovePicker::generate_next() {
234 case CAPTURES_S1: case CAPTURES_S3: case CAPTURES_S4: case CAPTURES_S5: case CAPTURES_S6:
235 end = generate<CAPTURES>(pos, moves);
245 endQuiets = end = generate<QUIETS>(pos, moves);
247 end = std::partition(cur, end, has_positive_score);
248 insertion_sort(cur, end);
254 if (depth >= 3 * ONE_PLY)
255 insertion_sort(cur, end);
258 case BAD_CAPTURES_S1:
259 // Just pick them in reverse order to get MVV/LVA ordering
260 cur = moves + MAX_MOVES - 1;
261 end = endBadCaptures;
265 end = generate<EVASIONS>(pos, moves);
270 case QUIET_CHECKS_S3:
271 end = generate<QUIET_CHECKS>(pos, moves);
274 case EVASION: case QSEARCH_0: case QSEARCH_1: case PROBCUT: case RECAPTURE:
277 end = cur + 1; // Avoid another next_phase() call
286 /// next_move() is the most important method of the MovePicker class. It returns
287 /// a new pseudo legal move every time is called, until there are no more moves
288 /// left. It picks the move with the biggest score from a list of generated moves
289 /// taking care not returning the ttMove if has already been searched previously.
291 Move MovePicker::next_move<false>() {
302 case MAIN_SEARCH: case EVASION: case QSEARCH_0: case QSEARCH_1: case PROBCUT:
307 move = pick_best(cur++, end)->move;
310 assert(captureThreshold <= 0); // Otherwise we cannot use see_sign()
312 if (pos.see_sign(move) >= captureThreshold)
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)
338 case BAD_CAPTURES_S1:
339 return (cur--)->move;
341 case EVASIONS_S2: case CAPTURES_S3: case CAPTURES_S4:
342 move = pick_best(cur++, end)->move;
348 move = pick_best(cur++, end)->move;
349 if (move != ttMove && pos.see(move) > captureThreshold)
354 move = pick_best(cur++, end)->move;
355 if (to_sq(move) == recaptureSquare)
359 case QUIET_CHECKS_S3:
360 move = (cur++)->move;
375 /// Version of next_move() to use at split point nodes where the move is grabbed
376 /// from the split point's shared MovePicker object. This function is not thread
377 /// safe so must be lock protected by the caller.
379 Move MovePicker::next_move<true>() { return ss->splitPoint->movePicker->next_move<false>(); }