2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2016 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (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, GOOD_CAPTURES, KILLERS, QUIET, BAD_CAPTURES,
30 EVASION, ALL_EVASIONS,
31 QSEARCH_WITH_CHECKS, QCAPTURES_1, CHECKS,
32 QSEARCH_WITHOUT_CHECKS, QCAPTURES_2,
33 PROBCUT, PROBCUT_CAPTURES,
34 RECAPTURE, RECAPTURES,
38 // Our insertion sort, which is guaranteed to be stable, as it should be
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 // pick_best() finds the best move in the range (begin, end) and moves it to
53 // the front. It's faster than sorting all the moves in advance when there
54 // are few moves, e.g., the possible captures.
55 Move pick_best(ExtMove* begin, ExtMove* end)
57 std::swap(*begin, *std::max_element(begin, end));
64 /// Constructors of the MovePicker class. As arguments we pass information
65 /// to help it to return the (presumably) good moves first, to decide which
66 /// moves to return (in the quiescence search, for instance, we only want to
67 /// search captures, promotions, and some checks) and how important good move
68 /// ordering is at the current node.
70 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, Search::Stack* s)
71 : pos(p), ss(s), depth(d) {
73 assert(d > DEPTH_ZERO);
75 Square prevSq = to_sq((ss-1)->currentMove);
76 countermove = pos.this_thread()->counterMoves[pos.piece_on(prevSq)][prevSq];
78 stage = pos.checkers() ? EVASION : MAIN_SEARCH;
79 ttMove = ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE;
80 endMoves += (ttMove != MOVE_NONE);
83 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, Square s)
86 assert(d <= DEPTH_ZERO);
91 else if (d > DEPTH_QS_NO_CHECKS)
92 stage = QSEARCH_WITH_CHECKS;
94 else if (d > DEPTH_QS_RECAPTURES)
95 stage = QSEARCH_WITHOUT_CHECKS;
104 ttMove = ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE;
105 endMoves += (ttMove != MOVE_NONE);
108 MovePicker::MovePicker(const Position& p, Move ttm, Value th)
109 : pos(p), threshold(th) {
111 assert(!pos.checkers());
115 // In ProbCut we generate captures with SEE higher than the given threshold
117 && pos.pseudo_legal(ttm)
119 && pos.see(ttm) > threshold ? ttm : MOVE_NONE;
121 endMoves += (ttMove != MOVE_NONE);
125 /// score() assigns a numerical value to each move in a move list. The moves with
126 /// highest values will be picked first.
128 void MovePicker::score<CAPTURES>() {
129 // Winning and equal captures in the main search are ordered by MVV, preferring
130 // captures near our home rank. Surprisingly, this appears to perform slightly
131 // better than SEE-based move ordering: exchanging big pieces before capturing
132 // a hanging piece probably helps to reduce the subtree size.
133 // In the main search we want to push captures with negative SEE values to the
134 // badCaptures[] array, but instead of doing it now we delay until the move
135 // has been picked up, saving some SEE calls in case we get a cutoff.
136 for (auto& m : *this)
137 m.value = PieceValue[MG][pos.piece_on(to_sq(m))]
138 - Value(200 * relative_rank(pos.side_to_move(), to_sq(m)));
142 void MovePicker::score<QUIETS>() {
144 const HistoryStats& history = pos.this_thread()->history;
145 const FromToStats& fromTo = pos.this_thread()->fromTo;
147 const CounterMoveStats* cm = (ss-1)->counterMoves;
148 const CounterMoveStats* fm = (ss-2)->counterMoves;
149 const CounterMoveStats* f2 = (ss-4)->counterMoves;
151 Color c = pos.side_to_move();
153 for (auto& m : *this)
154 m.value = history[pos.moved_piece(m)][to_sq(m)]
155 + (cm ? (*cm)[pos.moved_piece(m)][to_sq(m)] : VALUE_ZERO)
156 + (fm ? (*fm)[pos.moved_piece(m)][to_sq(m)] : VALUE_ZERO)
157 + (f2 ? (*f2)[pos.moved_piece(m)][to_sq(m)] : VALUE_ZERO)
162 void MovePicker::score<EVASIONS>() {
163 // Try winning and equal captures ordered by MVV/LVA, then non-captures ordered
164 // by history value, then bad captures and quiet moves with a negative SEE ordered
166 const HistoryStats& history = pos.this_thread()->history;
167 const FromToStats& fromTo = pos.this_thread()->fromTo;
168 Color c = pos.side_to_move();
171 for (auto& m : *this)
172 if ((see = pos.see_sign(m)) < VALUE_ZERO)
173 m.value = see - HistoryStats::Max; // At the bottom
175 else if (pos.capture(m))
176 m.value = PieceValue[MG][pos.piece_on(to_sq(m))]
177 - Value(type_of(pos.moved_piece(m))) + HistoryStats::Max;
179 m.value = history[pos.moved_piece(m)][to_sq(m)] + fromTo.get(c, m);
183 /// generate_next_stage() generates, scores, and sorts the next bunch of moves
184 /// when there are no more moves to try for the current stage.
186 void MovePicker::generate_next_stage() {
188 assert(stage != STOP);
194 case GOOD_CAPTURES: case QCAPTURES_1: case QCAPTURES_2:
195 case PROBCUT_CAPTURES: case RECAPTURES:
196 endMoves = generate<CAPTURES>(pos, moves);
201 killers[0] = ss->killers[0];
202 killers[1] = ss->killers[1];
203 killers[2] = countermove;
205 endMoves = cur + 2 + (countermove != killers[0] && countermove != killers[1]);
209 endMoves = generate<QUIETS>(pos, moves);
211 if (depth < 3 * ONE_PLY)
213 ExtMove* goodQuiet = std::partition(cur, endMoves, [](const ExtMove& m)
214 { return m.value > VALUE_ZERO; });
215 insertion_sort(cur, goodQuiet);
217 insertion_sort(cur, endMoves);
221 // Just pick them in reverse order to get correct ordering
222 cur = moves + MAX_MOVES - 1;
223 endMoves = endBadCaptures;
227 endMoves = generate<EVASIONS>(pos, moves);
228 if (endMoves - moves > 1)
233 endMoves = generate<QUIET_CHECKS>(pos, moves);
236 case EVASION: case QSEARCH_WITH_CHECKS: case QSEARCH_WITHOUT_CHECKS:
237 case PROBCUT: case RECAPTURE: case STOP:
247 /// next_move() is the most important method of the MovePicker class. It returns
248 /// a new pseudo legal move every time it is called, until there are no more moves
249 /// left. It picks the move with the biggest value from a list of generated moves
250 /// taking care not to return the ttMove if it has already been searched.
252 Move MovePicker::next_move() {
258 while (cur == endMoves && stage != STOP)
259 generate_next_stage();
263 case MAIN_SEARCH: case EVASION: case QSEARCH_WITH_CHECKS:
264 case QSEARCH_WITHOUT_CHECKS: case PROBCUT:
269 move = pick_best(cur++, endMoves);
272 if (pos.see_sign(move) >= VALUE_ZERO)
275 // Losing capture, move it to the tail of the array
276 *endBadCaptures-- = move;
282 if ( move != MOVE_NONE
284 && pos.pseudo_legal(move)
285 && !pos.capture(move))
292 && move != killers[0]
293 && move != killers[1]
294 && move != killers[2])
301 case ALL_EVASIONS: case QCAPTURES_1: case QCAPTURES_2:
302 move = pick_best(cur++, endMoves);
307 case PROBCUT_CAPTURES:
308 move = pick_best(cur++, endMoves);
309 if (move != ttMove && pos.see(move) > threshold)
314 move = pick_best(cur++, endMoves);
315 if (to_sq(move) == recaptureSquare)