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_MOVES, // Transposition table move and mate killer
32 PH_GOOD_CAPTURES, // Queen promotions and captures with SEE values >= 0
33 PH_KILLERS, // Killer moves from the current ply
34 PH_NONCAPTURES, // Non-captures and underpromotions
35 PH_BAD_CAPTURES, // Queen promotions and captures with SEE values < 0
36 PH_EVASIONS, // Check evasions
37 PH_QCAPTURES, // Captures in quiescence search
38 PH_QCHECKS, // Non-capture checks in quiescence search
43 const uint8_t MainSearchTable[] = { PH_TT_MOVES, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP };
44 const uint8_t EvasionTable[] = { PH_TT_MOVES, PH_EVASIONS, PH_STOP };
45 const uint8_t QsearchWithChecksTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_QCHECKS, PH_STOP };
46 const uint8_t QsearchWithoutChecksTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_STOP };
49 bool MovePicker::isBadCapture() const { return phase == PH_BAD_CAPTURES; }
51 /// Constructor for the MovePicker class. Apart from the position for which
52 /// it is asked to pick legal moves, MovePicker also wants some 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) {
61 ttMoves[0].move = ttm;
62 badCaptureThreshold = 0;
63 badCaptures = moves + MAX_MOVES;
65 assert(d > DEPTH_ZERO);
69 ttMoves[1].move = killers[0].move = killers[1].move = MOVE_NONE;
70 phasePtr = EvasionTable;
74 ttMoves[1].move = (ss->mateKiller == ttm) ? MOVE_NONE : ss->mateKiller;
75 searchTT |= ttMoves[1].move;
76 killers[0].move = ss->killers[0];
77 killers[1].move = ss->killers[1];
79 // Consider sligtly negative captures as good if at low
80 // depth and far from beta.
81 if (ss && ss->eval < beta - PawnValueMidgame && d < 3 * ONE_PLY)
82 badCaptureThreshold = -PawnValueMidgame;
84 phasePtr = MainSearchTable;
87 phasePtr += int(!searchTT) - 1;
91 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h)
94 ttMoves[0].move = ttm;
95 ttMoves[1].move = MOVE_NONE;
97 assert(d <= DEPTH_ZERO);
100 phasePtr = EvasionTable;
101 else if (d >= DEPTH_QS_CHECKS)
102 phasePtr = QsearchWithChecksTable;
105 phasePtr = QsearchWithoutChecksTable;
107 // Skip TT move if is not a capture or a promotion, this avoids
108 // qsearch tree explosion due to a possible perpetual check or
109 // similar rare cases when TT table is full.
110 if (ttm != MOVE_NONE && !pos.move_is_capture(ttm) && !move_is_promotion(ttm))
111 searchTT = ttMoves[0].move = MOVE_NONE;
114 phasePtr += int(!searchTT) - 1;
119 /// MovePicker::go_next_phase() generates, scores and sorts the next bunch
120 /// of moves when there are no more moves to try for the current phase.
122 void MovePicker::go_next_phase() {
125 phase = *(++phasePtr);
130 lastMove = curMove + 2;
133 case PH_GOOD_CAPTURES:
134 lastMove = generate<MV_CAPTURE>(pos, moves);
140 lastMove = curMove + 2;
144 lastMove = generate<MV_NON_CAPTURE>(pos, moves);
146 sort_moves(moves, lastMove, &lastGoodNonCapture);
149 case PH_BAD_CAPTURES:
150 // Bad captures SEE value is already calculated so just pick
151 // them in order to get SEE move ordering.
152 curMove = badCaptures;
153 lastMove = moves + MAX_MOVES;
157 assert(pos.in_check());
158 lastMove = generate<MV_EVASION>(pos, moves);
163 lastMove = generate<MV_CAPTURE>(pos, moves);
168 lastMove = generate<MV_NON_CAPTURE_CHECK>(pos, moves);
172 lastMove = curMove + 1; // Avoid another go_next_phase() call
182 /// MovePicker::score_captures(), MovePicker::score_noncaptures() and
183 /// MovePicker::score_evasions() assign a numerical move ordering score
184 /// to each move in a move list. The moves with highest scores will be
185 /// picked first by get_next_move().
187 void MovePicker::score_captures() {
188 // Winning and equal captures in the main search are ordered by MVV/LVA.
189 // Suprisingly, this appears to perform slightly better than SEE based
190 // move ordering. The reason is probably that in a position with a winning
191 // capture, capturing a more valuable (but sufficiently defended) piece
192 // first usually doesn't hurt. The opponent will have to recapture, and
193 // the hanging piece will still be hanging (except in the unusual cases
194 // where it is possible to recapture with the hanging piece). Exchanging
195 // big pieces before capturing a hanging piece probably helps to reduce
197 // In main search we want to push captures with negative SEE values to
198 // badCaptures[] array, but instead of doing it now we delay till when
199 // the move has been picked up in pick_move_from_list(), this way we save
200 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
203 // Use MVV/LVA ordering
204 for (MoveStack* cur = moves; cur != lastMove; cur++)
207 if (move_is_promotion(m))
208 cur->score = QueenValueMidgame;
210 cur->score = pos.midgame_value_of_piece_on(move_to(m))
211 - pos.type_of_piece_on(move_from(m));
215 void MovePicker::score_noncaptures() {
220 for (MoveStack* cur = moves; cur != lastMove; cur++)
224 cur->score = H.value(pos.piece_on(from), move_to(m));
228 void MovePicker::score_evasions() {
229 // Try good captures ordered by MVV/LVA, then non-captures if
230 // destination square is not under attack, ordered by history
231 // value, and at the end bad-captures and non-captures with a
232 // negative SEE. This last group is ordered by the SEE score.
236 // Skip if we don't have at least two moves to order
237 if (lastMove < moves + 2)
240 for (MoveStack* cur = moves; cur != lastMove; cur++)
243 if ((seeScore = pos.see_sign(m)) < 0)
244 cur->score = seeScore - History::MaxValue; // Be sure we are at the bottom
245 else if (pos.move_is_capture(m))
246 cur->score = pos.midgame_value_of_piece_on(move_to(m))
247 - pos.type_of_piece_on(move_from(m)) + History::MaxValue;
249 cur->score = H.value(pos.piece_on(move_from(m)), move_to(m));
253 /// MovePicker::get_next_move() is the most important method of the MovePicker
254 /// class. It returns a new legal move every time it is called, until there
255 /// are no more moves left. It picks the move with the biggest score from a list
256 /// of generated moves taking care not to return the tt move if has already been
257 /// searched previously. Note that this function is not thread safe so should be
258 /// lock protected by caller when accessed through a shared MovePicker object.
260 Move MovePicker::get_next_move() {
266 while (curMove == lastMove)
272 move = (curMove++)->move;
273 if ( move != MOVE_NONE
274 && pos.move_is_pl(move))
278 case PH_GOOD_CAPTURES:
279 move = pick_best(curMove++, lastMove).move;
280 if ( move != ttMoves[0].move
281 && move != ttMoves[1].move)
283 // Check for a non negative SEE now
284 int seeValue = pos.see_sign(move);
285 if (seeValue >= badCaptureThreshold)
288 // Losing capture, move it to the tail of the array, note
289 // that move has now been already checked for legality.
290 (--badCaptures)->move = move;
291 badCaptures->score = seeValue;
296 move = (curMove++)->move;
297 if ( move != MOVE_NONE
298 && pos.move_is_pl(move)
299 && move != ttMoves[0].move
300 && move != ttMoves[1].move
301 && !pos.move_is_capture(move))
306 // Sort negative scored moves only when we get there
307 if (curMove == lastGoodNonCapture)
308 insertion_sort<MoveStack>(lastGoodNonCapture, lastMove);
310 move = (curMove++)->move;
311 if ( move != ttMoves[0].move
312 && move != ttMoves[1].move
313 && move != killers[0].move
314 && move != killers[1].move)
318 case PH_BAD_CAPTURES:
319 move = pick_best(curMove++, lastMove).move;
324 move = pick_best(curMove++, lastMove).move;
325 if (move != ttMoves[0].move)
330 move = (curMove++)->move;
331 if ( move != ttMoves[0].move)