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, which is guaranteed (and also needed) to be stable
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 as to sort the two sets separately, with the second sort delayed.
54 inline bool has_positive_score(const ExtMove& ms) { return ms.score > 0; }
56 // Picks the best move in the range (begin, end) and moves it to the front.
57 // It's faster than sorting all the moves in advance when there are few
58 // moves e.g. 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 how important good move
71 /// 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);
79 endBadCaptures = moves + MAX_MOVES - 1;
89 ttMove = (ttm && pos.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), cur(moves), end(moves) {
96 assert(d <= DEPTH_ZERO);
101 else if (d > DEPTH_QS_NO_CHECKS)
104 else if (d > DEPTH_QS_RECAPTURES)
108 // Skip TT move if is not a capture or a promotion. This avoids qsearch
109 // tree explosion due to a possible perpetual check or similar rare cases
110 // when TT table is full.
111 if (ttm && !pos.capture_or_promotion(ttm))
117 recaptureSquare = sq;
121 ttMove = (ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE);
122 end += (ttMove != MOVE_NONE);
125 MovePicker::MovePicker(const Position& p, Move ttm, const HistoryStats& h, PieceType pt)
126 : pos(p), history(h), cur(moves), end(moves) {
128 assert(!pos.checkers());
132 // In ProbCut we generate only captures that are better than the parent's
134 captureThreshold = PieceValue[MG][pt];
135 ttMove = (ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE);
137 if (ttMove && (!pos.capture(ttMove) || pos.see(ttMove) <= captureThreshold))
140 end += (ttMove != MOVE_NONE);
144 /// score() assign a numerical move ordering score to each move in a move list.
145 /// The moves with highest scores will be picked first.
147 void MovePicker::score<CAPTURES>() {
148 // Winning and equal captures in the main search are ordered by MVV/LVA.
149 // Suprisingly, this appears to perform slightly better than SEE based
150 // move ordering. The reason is probably that in a position with a winning
151 // capture, capturing a more valuable (but sufficiently defended) piece
152 // first usually doesn't hurt. The opponent will have to recapture, and
153 // the hanging piece will still be hanging (except in the unusual cases
154 // where it is possible to recapture with the hanging piece). Exchanging
155 // big pieces before capturing a hanging piece probably helps to reduce
157 // In main search we want to push captures with negative SEE values to the
158 // badCaptures[] array, but instead of doing it now we delay until the move
159 // has been picked up in pick_move_from_list(). This way we save some SEE
160 // calls in case we get a cutoff.
163 for (ExtMove* it = moves; it != end; ++it)
166 it->score = PieceValue[MG][pos.piece_on(to_sq(m))]
167 - type_of(pos.moved_piece(m));
169 if (type_of(m) == PROMOTION)
170 it->score += PieceValue[MG][promotion_type(m)] - PieceValue[MG][PAWN];
172 else if (type_of(m) == ENPASSANT)
173 it->score += PieceValue[MG][PAWN];
178 void MovePicker::score<QUIETS>() {
182 for (ExtMove* it = moves; it != end; ++it)
185 it->score = history[pos.moved_piece(m)][to_sq(m)];
190 void MovePicker::score<EVASIONS>() {
191 // Try good captures ordered by MVV/LVA, then non-captures if destination square
192 // is not under attack, ordered by history value, then bad-captures and quiet
193 // moves with a negative SEE. This last group is ordered by the SEE score.
197 for (ExtMove* it = moves; it != end; ++it)
200 if ((seeScore = pos.see_sign(m)) < 0)
201 it->score = seeScore - HistoryStats::Max; // At the bottom
203 else if (pos.capture(m))
204 it->score = PieceValue[MG][pos.piece_on(to_sq(m))]
205 - type_of(pos.moved_piece(m)) + HistoryStats::Max;
207 it->score = history[pos.moved_piece(m)][to_sq(m)];
212 /// generate_next() generates, scores and sorts the next bunch of moves, when
213 /// there are no more moves to try for the current phase.
215 void MovePicker::generate_next() {
221 case CAPTURES_S1: case CAPTURES_S3: case CAPTURES_S4: case CAPTURES_S5: case CAPTURES_S6:
222 end = generate<CAPTURES>(pos, moves);
230 killers[0].move = ss->killers[0];
231 killers[1].move = ss->killers[1];
232 killers[2].move = killers[3].move = MOVE_NONE;
234 // Be sure countermoves are different from killers
235 for (int i = 0; i < 2; ++i)
236 if (countermoves[i] != cur->move && countermoves[i] != (cur+1)->move)
237 (end++)->move = countermoves[i];
239 if (countermoves[1] && countermoves[1] == countermoves[0]) // Due to SMP races
240 killers[3].move = MOVE_NONE;
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 it 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 it 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 if (pos.see_sign(move) >= 0)
313 // Losing capture, move it to the tail of the array
314 (endBadCaptures--)->move = move;
319 move = (cur++)->move;
320 if ( move != MOVE_NONE
321 && pos.pseudo_legal(move)
323 && !pos.capture(move))
327 case QUIETS_1_S1: case QUIETS_2_S1:
328 move = (cur++)->move;
330 && move != killers[0].move
331 && move != killers[1].move
332 && move != killers[2].move
333 && move != killers[3].move)
337 case BAD_CAPTURES_S1:
338 return (cur--)->move;
340 case EVASIONS_S2: case CAPTURES_S3: case CAPTURES_S4:
341 move = pick_best(cur++, end)->move;
347 move = pick_best(cur++, end)->move;
348 if (move != ttMove && pos.see(move) > captureThreshold)
353 move = pick_best(cur++, end)->move;
354 if (to_sq(move) == recaptureSquare)
358 case QUIET_CHECKS_S3:
359 move = (cur++)->move;
374 /// Version of next_move() to use at split point nodes where the move is grabbed
375 /// from the split point's shared MovePicker object. This function is not thread
376 /// safe so must be lock protected by the caller.
378 Move MovePicker::next_move<true>() { return ss->splitPoint->movePicker->next_move<false>(); }