2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2010 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/>.
31 PH_TT_MOVE, // Transposition table move
32 PH_GOOD_CAPTURES, // Queen promotions and captures with SEE values >= captureThreshold (captureThreshold <= 0)
33 PH_GOOD_PROBCUT, // Queen promotions and captures with SEE values > captureThreshold (captureThreshold >= 0)
34 PH_KILLERS, // Killer moves from the current ply
35 PH_NONCAPTURES_1, // Non-captures and underpromotions with positive score
36 PH_NONCAPTURES_2, // Non-captures and underpromotions with non-positive score
37 PH_BAD_CAPTURES, // Queen promotions and captures with SEE values < captureThreshold (captureThreshold <= 0)
38 PH_EVASIONS, // Check evasions
39 PH_QCAPTURES, // Captures in quiescence search
40 PH_QCHECKS, // Non-capture checks in quiescence search
45 const uint8_t MainSearchTable[] = { PH_TT_MOVE, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES_1, PH_NONCAPTURES_2, PH_BAD_CAPTURES, PH_STOP };
46 const uint8_t EvasionTable[] = { PH_TT_MOVE, PH_EVASIONS, PH_STOP };
47 const uint8_t QsearchWithChecksTable[] = { PH_TT_MOVE, PH_QCAPTURES, PH_QCHECKS, PH_STOP };
48 const uint8_t QsearchWithoutChecksTable[] = { PH_TT_MOVE, PH_QCAPTURES, PH_STOP };
49 const uint8_t ProbCutTable[] = { PH_TT_MOVE, PH_GOOD_PROBCUT, PH_STOP };
52 /// Constructors for the MovePicker class. As arguments we pass information
53 /// to help it to return the presumably good moves first, to decide which
54 /// moves to return (in the quiescence search, for instance, we only want to
55 /// search captures, promotions and some checks) and about how important good
56 /// move ordering is at the current node.
58 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h,
59 SearchStack* ss, Value beta) : pos(p), H(h), depth(d) {
61 badCaptures = moves + MAX_MOVES;
63 assert(d > DEPTH_ZERO);
67 killers[0].move = killers[1].move = MOVE_NONE;
68 phasePtr = EvasionTable;
72 killers[0].move = ss->killers[0];
73 killers[1].move = ss->killers[1];
75 // Consider sligtly negative captures as good if at low
76 // depth and far from beta.
77 if (ss && ss->eval < beta - PawnValueMidgame && d < 3 * ONE_PLY)
78 captureThreshold = -PawnValueMidgame;
80 phasePtr = MainSearchTable;
83 ttMove = (ttm && pos.move_is_pl(ttm) ? ttm : MOVE_NONE);
84 phasePtr += int(ttMove == MOVE_NONE) - 1;
88 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h)
91 assert(d <= DEPTH_ZERO);
94 phasePtr = EvasionTable;
95 else if (d >= DEPTH_QS_CHECKS)
96 phasePtr = QsearchWithChecksTable;
99 phasePtr = QsearchWithoutChecksTable;
101 // Skip TT move if is not a capture or a promotion, this avoids
102 // qsearch tree explosion due to a possible perpetual check or
103 // similar rare cases when TT table is full.
104 if (ttm != MOVE_NONE && !pos.move_is_capture(ttm) && !move_is_promotion(ttm))
108 ttMove = (ttm && pos.move_is_pl(ttm) ? ttm : MOVE_NONE);
109 phasePtr += int(ttMove == MOVE_NONE) - 1;
113 MovePicker::MovePicker(const Position& p, Move ttm, const History& h, int parentCapture)
116 assert (!pos.in_check());
118 // In ProbCut we consider only captures better than parent's move
119 captureThreshold = parentCapture;
120 phasePtr = ProbCutTable;
122 if ( ttm != MOVE_NONE
123 && (!pos.move_is_capture(ttm) || pos.see(ttm) <= captureThreshold))
126 ttMove = (ttm && pos.move_is_pl(ttm) ? ttm : MOVE_NONE);
127 phasePtr += int(ttMove == MOVE_NONE) - 1;
132 /// MovePicker::go_next_phase() generates, scores and sorts the next bunch
133 /// of moves when there are no more moves to try for the current phase.
135 void MovePicker::go_next_phase() {
138 phase = *(++phasePtr);
142 lastMove = curMove + 1;
145 case PH_GOOD_CAPTURES:
146 case PH_GOOD_PROBCUT:
147 lastMove = generate<MV_CAPTURE>(pos, moves);
153 lastMove = curMove + 2;
156 case PH_NONCAPTURES_1:
157 lastNonCapture = lastMove = generate<MV_NON_CAPTURE>(pos, moves);
159 sort_moves(moves, lastNonCapture, &lastMove);
162 case PH_NONCAPTURES_2:
164 lastMove = lastNonCapture;
165 if (depth >= 3 * ONE_PLY)
166 insertion_sort<MoveStack>(curMove, lastMove);
169 case PH_BAD_CAPTURES:
170 // Bad captures SEE value is already calculated so just pick
171 // them in order to get SEE move ordering.
172 curMove = badCaptures;
173 lastMove = moves + MAX_MOVES;
177 assert(pos.in_check());
178 lastMove = generate<MV_EVASION>(pos, moves);
183 lastMove = generate<MV_CAPTURE>(pos, moves);
188 lastMove = generate<MV_NON_CAPTURE_CHECK>(pos, moves);
192 lastMove = curMove + 1; // Avoid another go_next_phase() call
202 /// MovePicker::score_captures(), MovePicker::score_noncaptures() and
203 /// MovePicker::score_evasions() assign a numerical move ordering score
204 /// to each move in a move list. The moves with highest scores will be
205 /// picked first by get_next_move().
207 void MovePicker::score_captures() {
208 // Winning and equal captures in the main search are ordered by MVV/LVA.
209 // Suprisingly, this appears to perform slightly better than SEE based
210 // move ordering. The reason is probably that in a position with a winning
211 // capture, capturing a more valuable (but sufficiently defended) piece
212 // first usually doesn't hurt. The opponent will have to recapture, and
213 // the hanging piece will still be hanging (except in the unusual cases
214 // where it is possible to recapture with the hanging piece). Exchanging
215 // big pieces before capturing a hanging piece probably helps to reduce
217 // In main search we want to push captures with negative SEE values to
218 // badCaptures[] array, but instead of doing it now we delay till when
219 // the move has been picked up in pick_move_from_list(), this way we save
220 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
223 // Use MVV/LVA ordering
224 for (MoveStack* cur = moves; cur != lastMove; cur++)
227 cur->score = pos.midgame_value_of_piece_on(move_to(m))
228 - pos.type_of_piece_on(move_from(m));
230 if (move_is_promotion(m))
231 cur->score += QueenValueMidgame;
235 void MovePicker::score_noncaptures() {
240 for (MoveStack* cur = moves; cur != lastMove; cur++)
244 cur->score = H.value(pos.piece_on(from), move_to(m));
248 void MovePicker::score_evasions() {
249 // Try good captures ordered by MVV/LVA, then non-captures if
250 // destination square is not under attack, ordered by history
251 // value, and at the end bad-captures and non-captures with a
252 // negative SEE. This last group is ordered by the SEE score.
256 // Skip if we don't have at least two moves to order
257 if (lastMove < moves + 2)
260 for (MoveStack* cur = moves; cur != lastMove; cur++)
263 if ((seeScore = pos.see_sign(m)) < 0)
264 cur->score = seeScore - History::MaxValue; // Be sure we are at the bottom
265 else if (pos.move_is_capture(m))
266 cur->score = pos.midgame_value_of_piece_on(move_to(m))
267 - pos.type_of_piece_on(move_from(m)) + History::MaxValue;
269 cur->score = H.value(pos.piece_on(move_from(m)), move_to(m));
273 /// MovePicker::get_next_move() is the most important method of the MovePicker
274 /// class. It returns a new pseudo legal move every time it is called, until there
275 /// are no more moves left. It picks the move with the biggest score from a list
276 /// of generated moves taking care not to return the tt move if has already been
277 /// searched previously. Note that this function is not thread safe so should be
278 /// lock protected by caller when accessed through a shared MovePicker object.
280 Move MovePicker::get_next_move() {
286 while (curMove == lastMove)
296 case PH_GOOD_CAPTURES:
297 move = pick_best(curMove++, lastMove).move;
300 assert(captureThreshold <= 0); // Otherwise we must use see instead of see_sign
302 // Check for a non negative SEE now
303 int seeValue = pos.see_sign(move);
304 if (seeValue >= captureThreshold)
307 // Losing capture, move it to the tail of the array, note
308 // that move has now been already checked for pseudo legality.
309 (--badCaptures)->move = move;
310 badCaptures->score = seeValue;
314 case PH_GOOD_PROBCUT:
315 move = pick_best(curMove++, lastMove).move;
317 && pos.see(move) > captureThreshold)
322 move = (curMove++)->move;
323 if ( move != MOVE_NONE
324 && pos.move_is_pl(move)
326 && !pos.move_is_capture(move))
330 case PH_NONCAPTURES_1:
331 case PH_NONCAPTURES_2:
332 move = (curMove++)->move;
334 && move != killers[0].move
335 && move != killers[1].move)
339 case PH_BAD_CAPTURES:
340 move = pick_best(curMove++, lastMove).move;
345 move = pick_best(curMove++, lastMove).move;
351 move = (curMove++)->move;