2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2014 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 values from remaining
53 // ones so as to sort the two sets separately, with the second sort delayed.
54 inline bool has_positive_value(const ExtMove& ms) { return ms.value > 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, Move* fm, Search::Stack* s) : pos(p), history(h), depth(d) {
76 assert(d > DEPTH_ZERO);
79 endBadCaptures = moves + MAX_MOVES - 1;
90 ttMove = (ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE);
91 end += (ttMove != MOVE_NONE);
94 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const HistoryStats& h,
95 Square sq) : pos(p), history(h), cur(moves), end(moves) {
97 assert(d <= DEPTH_ZERO);
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.capture_or_promotion(ttm))
118 recaptureSquare = sq;
122 ttMove = (ttm && pos.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), cur(moves), end(moves) {
129 assert(!pos.checkers());
133 // In ProbCut we generate only captures that are better than the parent's
135 captureThreshold = PieceValue[MG][pt];
136 ttMove = (ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE);
138 if (ttMove && (!pos.capture(ttMove) || pos.see(ttMove) <= captureThreshold))
141 end += (ttMove != MOVE_NONE);
145 /// score() assign a numerical value to each move in a move list. The moves with
146 /// highest values will be picked first.
148 void MovePicker::score<CAPTURES>() {
149 // Winning and equal captures in the main search are ordered by MVV/LVA.
150 // Suprisingly, this appears to perform slightly better than SEE based
151 // move ordering. The reason is probably that in a position with a winning
152 // capture, capturing a more valuable (but sufficiently defended) piece
153 // first usually doesn't hurt. The opponent will have to recapture, and
154 // the hanging piece will still be hanging (except in the unusual cases
155 // where it is possible to recapture with the hanging piece). Exchanging
156 // big pieces before capturing a hanging piece probably helps to reduce
158 // In main search we want to push captures with negative SEE values to the
159 // badCaptures[] array, but instead of doing it now we delay until the move
160 // has been picked up in pick_move_from_list(). This way we save some SEE
161 // calls in case we get a cutoff.
164 for (ExtMove* it = moves; it != end; ++it)
167 it->value = PieceValue[MG][pos.piece_on(to_sq(m))]
168 - type_of(pos.moved_piece(m));
170 if (type_of(m) == PROMOTION)
171 it->value += PieceValue[MG][promotion_type(m)] - PieceValue[MG][PAWN];
173 else if (type_of(m) == ENPASSANT)
174 it->value += PieceValue[MG][PAWN];
179 void MovePicker::score<QUIETS>() {
183 for (ExtMove* it = moves; it != end; ++it)
186 it->value = history[pos.moved_piece(m)][to_sq(m)];
191 void MovePicker::score<EVASIONS>() {
192 // Try good captures ordered by MVV/LVA, then non-captures if destination square
193 // is not under attack, ordered by history value, then bad-captures and quiet
194 // moves with a negative SEE. This last group is ordered by the SEE value.
198 for (ExtMove* it = moves; it != end; ++it)
201 if ((see = pos.see_sign(m)) < VALUE_ZERO)
202 it->value = see - HistoryStats::Max; // At the bottom
204 else if (pos.capture(m))
205 it->value = PieceValue[MG][pos.piece_on(to_sq(m))]
206 - type_of(pos.moved_piece(m)) + HistoryStats::Max;
208 it->value = history[pos.moved_piece(m)][to_sq(m)];
213 /// generate_next_stage() generates, scores and sorts the next bunch of moves,
214 /// when there are no more moves to try for the current stage.
216 void MovePicker::generate_next_stage() {
222 case CAPTURES_S1: case CAPTURES_S3: case CAPTURES_S4: case CAPTURES_S5: case CAPTURES_S6:
223 end = generate<CAPTURES>(pos, moves);
231 killers[0].move = ss->killers[0];
232 killers[1].move = ss->killers[1];
233 killers[2].move = killers[3].move = MOVE_NONE;
234 killers[4].move = killers[5].move = MOVE_NONE;
236 // Please note that following code is racy and could yield to rare (less
237 // than 1 out of a million) duplicated entries in SMP case. This is harmless.
239 // Be sure countermoves are different from killers
240 for (int i = 0; i < 2; ++i)
241 if ( countermoves[i] != (cur+0)->move
242 && countermoves[i] != (cur+1)->move)
243 (end++)->move = countermoves[i];
245 // Be sure followupmoves are different from killers and countermoves
246 for (int i = 0; i < 2; ++i)
247 if ( followupmoves[i] != (cur+0)->move
248 && followupmoves[i] != (cur+1)->move
249 && followupmoves[i] != (cur+2)->move
250 && followupmoves[i] != (cur+3)->move)
251 (end++)->move = followupmoves[i];
255 endQuiets = end = generate<QUIETS>(pos, moves);
257 end = std::partition(cur, end, has_positive_value);
258 insertion_sort(cur, end);
264 if (depth >= 3 * ONE_PLY)
265 insertion_sort(cur, end);
268 case BAD_CAPTURES_S1:
269 // Just pick them in reverse order to get MVV/LVA ordering
270 cur = moves + MAX_MOVES - 1;
271 end = endBadCaptures;
275 end = generate<EVASIONS>(pos, moves);
280 case QUIET_CHECKS_S3:
281 end = generate<QUIET_CHECKS>(pos, moves);
284 case EVASION: case QSEARCH_0: case QSEARCH_1: case PROBCUT: case RECAPTURE:
287 end = cur + 1; // Avoid another next_phase() call
296 /// next_move() is the most important method of the MovePicker class. It returns
297 /// a new pseudo legal move every time it is called, until there are no more moves
298 /// left. It picks the move with the biggest value from a list of generated moves
299 /// taking care not to return the ttMove if it has already been searched.
301 Move MovePicker::next_move<false>() {
308 generate_next_stage();
312 case MAIN_SEARCH: case EVASION: case QSEARCH_0: case QSEARCH_1: case PROBCUT:
317 move = pick_best(cur++, end)->move;
320 if (pos.see_sign(move) >= VALUE_ZERO)
323 // Losing capture, move it to the tail of the array
324 (endBadCaptures--)->move = move;
329 move = (cur++)->move;
330 if ( move != MOVE_NONE
331 && pos.pseudo_legal(move)
333 && !pos.capture(move))
337 case QUIETS_1_S1: case QUIETS_2_S1:
338 move = (cur++)->move;
340 && move != killers[0].move
341 && move != killers[1].move
342 && move != killers[2].move
343 && move != killers[3].move
344 && move != killers[4].move
345 && move != killers[5].move)
349 case BAD_CAPTURES_S1:
350 return (cur--)->move;
352 case EVASIONS_S2: case CAPTURES_S3: case CAPTURES_S4:
353 move = pick_best(cur++, end)->move;
359 move = pick_best(cur++, end)->move;
360 if (move != ttMove && pos.see(move) > captureThreshold)
365 move = pick_best(cur++, end)->move;
366 if (to_sq(move) == recaptureSquare)
370 case QUIET_CHECKS_S3:
371 move = (cur++)->move;
386 /// Version of next_move() to use at split point nodes where the move is grabbed
387 /// from the split point's shared MovePicker object. This function is not thread
388 /// safe so must be lock protected by the caller.
390 Move MovePicker::next_move<true>() { return ss->splitPoint->movePicker->next_move<false>(); }