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) 2015-2020 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 // Used to drive the king towards the edge of the board
30 // in KX vs K and KQ vs KR endgames.
31 inline int push_to_edge(Square s) {
32 int rd = edge_distance(rank_of(s)), fd = edge_distance(file_of(s));
33 return 90 - (7 * fd * fd / 2 + 7 * rd * rd / 2);
36 // Used to drive the king towards A1H8 corners in KBN vs K endgames.
37 inline int push_to_corner(Square s) {
38 return abs(7 - rank_of(s) - file_of(s));
41 // Drive a piece close to or away from another piece
42 inline int push_close(Square s1, Square s2) { return 140 - 20 * distance(s1, s2); }
43 inline int push_away(Square s1, Square s2) { return 120 - push_close(s1, s2); }
46 bool verify_material(const Position& pos, Color c, Value npm, int pawnsCnt) {
47 return pos.non_pawn_material(c) == npm && pos.count<PAWN>(c) == pawnsCnt;
51 // Map the square as if strongSide is white and strongSide's only pawn
52 // is on the left half of the board.
53 Square normalize(const Position& pos, Color strongSide, Square sq) {
55 assert(pos.count<PAWN>(strongSide) == 1);
57 if (file_of(pos.square<PAWN>(strongSide)) >= FILE_E)
60 return strongSide == WHITE ? sq : flip_rank(sq);
68 std::pair<Map<Value>, Map<ScaleFactor>> maps;
86 add<KBPPKB>("KBPPKB");
87 add<KRPPKRP>("KRPPKRP");
92 /// Mate with KX vs K. This function is used to evaluate positions with
93 /// king and plenty of material vs a lone king. It simply gives the
94 /// attacking side a bonus for driving the defending king towards the edge
95 /// of the board, and for keeping the distance between the two kings small.
97 Value Endgame<KXK>::operator()(const Position& pos) const {
99 assert(verify_material(pos, weakSide, VALUE_ZERO, 0));
100 assert(!pos.checkers()); // Eval is never called when in check
102 // Stalemate detection with lone king
103 if (pos.side_to_move() == weakSide && !MoveList<LEGAL>(pos).size())
106 Square winnerKSq = pos.square<KING>(strongSide);
107 Square loserKSq = pos.square<KING>(weakSide);
109 Value result = pos.non_pawn_material(strongSide)
110 + pos.count<PAWN>(strongSide) * PawnValueEg
111 + push_to_edge(loserKSq)
112 + push_close(winnerKSq, loserKSq);
114 if ( pos.count<QUEEN>(strongSide)
115 || pos.count<ROOK>(strongSide)
116 ||(pos.count<BISHOP>(strongSide) && pos.count<KNIGHT>(strongSide))
117 || ( (pos.pieces(strongSide, BISHOP) & ~DarkSquares)
118 && (pos.pieces(strongSide, BISHOP) & DarkSquares)))
119 result = std::min(result + VALUE_KNOWN_WIN, VALUE_TB_WIN_IN_MAX_PLY - 1);
121 return strongSide == pos.side_to_move() ? result : -result;
125 /// Mate with KBN vs K. This is similar to KX vs K, but we have to drive the
126 /// defending king towards a corner square that our bishop attacks.
128 Value Endgame<KBNK>::operator()(const Position& pos) const {
130 assert(verify_material(pos, strongSide, KnightValueMg + BishopValueMg, 0));
131 assert(verify_material(pos, weakSide, VALUE_ZERO, 0));
133 Square winnerKSq = pos.square<KING>(strongSide);
134 Square loserKSq = pos.square<KING>(weakSide);
135 Square bishopSq = pos.square<BISHOP>(strongSide);
137 // If our bishop does not attack A1/H8, we flip the enemy king square
138 // to drive to opposite corners (A8/H1).
140 Value result = (VALUE_KNOWN_WIN + 3520)
141 + push_close(winnerKSq, loserKSq)
142 + 420 * push_to_corner(opposite_colors(bishopSq, SQ_A1) ? flip_file(loserKSq) : loserKSq);
144 assert(abs(result) < VALUE_TB_WIN_IN_MAX_PLY);
145 return strongSide == pos.side_to_move() ? result : -result;
149 /// KP vs K. This endgame is evaluated with the help of a bitbase
151 Value Endgame<KPK>::operator()(const Position& pos) const {
153 assert(verify_material(pos, strongSide, VALUE_ZERO, 1));
154 assert(verify_material(pos, weakSide, VALUE_ZERO, 0));
156 // Assume strongSide is white and the pawn is on files A-D
157 Square wksq = normalize(pos, strongSide, pos.square<KING>(strongSide));
158 Square bksq = normalize(pos, strongSide, pos.square<KING>(weakSide));
159 Square psq = normalize(pos, strongSide, pos.square<PAWN>(strongSide));
161 Color us = strongSide == pos.side_to_move() ? WHITE : BLACK;
163 if (!Bitbases::probe(wksq, psq, bksq, us))
166 Value result = VALUE_KNOWN_WIN + PawnValueEg + Value(rank_of(psq));
168 return strongSide == pos.side_to_move() ? result : -result;
172 /// KR vs KP. This is a somewhat tricky endgame to evaluate precisely without
173 /// a bitbase. The function below returns drawish scores when the pawn is
174 /// far advanced with support of the king, while the attacking king is far
177 Value Endgame<KRKP>::operator()(const Position& pos) const {
179 assert(verify_material(pos, strongSide, RookValueMg, 0));
180 assert(verify_material(pos, weakSide, VALUE_ZERO, 1));
182 Square wksq = relative_square(strongSide, pos.square<KING>(strongSide));
183 Square bksq = relative_square(strongSide, pos.square<KING>(weakSide));
184 Square rsq = relative_square(strongSide, pos.square<ROOK>(strongSide));
185 Square psq = relative_square(strongSide, pos.square<PAWN>(weakSide));
187 Square queeningSq = make_square(file_of(psq), RANK_1);
190 // If the stronger side's king is in front of the pawn, it's a win
191 if (forward_file_bb(WHITE, wksq) & psq)
192 result = RookValueEg - distance(wksq, psq);
194 // If the weaker side's king is too far from the pawn and the rook,
196 else if ( distance(bksq, psq) >= 3 + (pos.side_to_move() == weakSide)
197 && distance(bksq, rsq) >= 3)
198 result = RookValueEg - distance(wksq, psq);
200 // If the pawn is far advanced and supported by the defending king,
201 // the position is drawish
202 else if ( rank_of(bksq) <= RANK_3
203 && distance(bksq, psq) == 1
204 && rank_of(wksq) >= RANK_4
205 && distance(wksq, psq) > 2 + (pos.side_to_move() == strongSide))
206 result = Value(80) - 8 * distance(wksq, psq);
209 result = Value(200) - 8 * ( distance(wksq, psq + SOUTH)
210 - distance(bksq, psq + SOUTH)
211 - distance(psq, queeningSq));
213 return strongSide == pos.side_to_move() ? result : -result;
217 /// KR vs KB. This is very simple, and always returns drawish scores. The
218 /// score is slightly bigger when the defending king is close to the edge.
220 Value Endgame<KRKB>::operator()(const Position& pos) const {
222 assert(verify_material(pos, strongSide, RookValueMg, 0));
223 assert(verify_material(pos, weakSide, BishopValueMg, 0));
225 Value result = Value(push_to_edge(pos.square<KING>(weakSide)));
226 return strongSide == pos.side_to_move() ? result : -result;
230 /// KR vs KN. The attacking side has slightly better winning chances than
231 /// in KR vs KB, particularly if the king and the knight are far apart.
233 Value Endgame<KRKN>::operator()(const Position& pos) const {
235 assert(verify_material(pos, strongSide, RookValueMg, 0));
236 assert(verify_material(pos, weakSide, KnightValueMg, 0));
238 Square bksq = pos.square<KING>(weakSide);
239 Square bnsq = pos.square<KNIGHT>(weakSide);
240 Value result = Value(push_to_edge(bksq) + push_away(bksq, bnsq));
241 return strongSide == pos.side_to_move() ? result : -result;
245 /// KQ vs KP. In general, this is a win for the stronger side, but there are a
246 /// few important exceptions. A pawn on 7th rank and on the A,C,F or H files
247 /// with a king positioned next to it can be a draw, so in that case, we only
248 /// use the distance between the kings.
250 Value Endgame<KQKP>::operator()(const Position& pos) const {
252 assert(verify_material(pos, strongSide, QueenValueMg, 0));
253 assert(verify_material(pos, weakSide, VALUE_ZERO, 1));
255 Square winnerKSq = pos.square<KING>(strongSide);
256 Square loserKSq = pos.square<KING>(weakSide);
257 Square pawnSq = pos.square<PAWN>(weakSide);
259 Value result = Value(push_close(winnerKSq, loserKSq));
261 if ( relative_rank(weakSide, pawnSq) != RANK_7
262 || distance(loserKSq, pawnSq) != 1
263 || ((FileBBB | FileDBB | FileEBB | FileGBB) & pawnSq))
264 result += QueenValueEg - PawnValueEg;
266 return strongSide == pos.side_to_move() ? result : -result;
270 /// KQ vs KR. This is almost identical to KX vs K: We give the attacking
271 /// king a bonus for having the kings close together, and for forcing the
272 /// defending king towards the edge. If we also take care to avoid null move for
273 /// the defending side in the search, this is usually sufficient to win KQ vs KR.
275 Value Endgame<KQKR>::operator()(const Position& pos) const {
277 assert(verify_material(pos, strongSide, QueenValueMg, 0));
278 assert(verify_material(pos, weakSide, RookValueMg, 0));
280 Square winnerKSq = pos.square<KING>(strongSide);
281 Square loserKSq = pos.square<KING>(weakSide);
283 Value result = QueenValueEg
285 + push_to_edge(loserKSq)
286 + push_close(winnerKSq, loserKSq);
288 return strongSide == pos.side_to_move() ? result : -result;
292 /// KNN vs KP. Very drawish, but there are some mate opportunities if we can
293 // press the weakSide King to a corner before the pawn advances too much.
295 Value Endgame<KNNKP>::operator()(const Position& pos) const {
297 assert(verify_material(pos, strongSide, 2 * KnightValueMg, 0));
298 assert(verify_material(pos, weakSide, VALUE_ZERO, 1));
300 Value result = PawnValueEg
301 + 2 * push_to_edge(pos.square<KING>(weakSide))
302 - 10 * relative_rank(weakSide, pos.square<PAWN>(weakSide));
304 return strongSide == pos.side_to_move() ? result : -result;
308 /// Some cases of trivial draws
309 template<> Value Endgame<KNNK>::operator()(const Position&) const { return VALUE_DRAW; }
312 /// KB and one or more pawns vs K. It checks for draws with rook pawns and
313 /// a bishop of the wrong color. If such a draw is detected, SCALE_FACTOR_DRAW
314 /// is returned. If not, the return value is SCALE_FACTOR_NONE, i.e. no scaling
317 ScaleFactor Endgame<KBPsK>::operator()(const Position& pos) const {
319 assert(pos.non_pawn_material(strongSide) == BishopValueMg);
320 assert(pos.count<PAWN>(strongSide) >= 1);
322 // No assertions about the material of weakSide, because we want draws to
323 // be detected even when the weaker side has some pawns.
325 Bitboard strongPawns = pos.pieces(strongSide, PAWN);
326 Bitboard allPawns = pos.pieces(PAWN);
328 // All strongSide pawns are on a single rook file?
329 if (!(strongPawns & ~FileABB) || !(strongPawns & ~FileHBB))
331 Square bishopSq = pos.square<BISHOP>(strongSide);
332 Square queeningSq = relative_square(strongSide, make_square(file_of(lsb(strongPawns)), RANK_8));
333 Square weakKingSq = pos.square<KING>(weakSide);
335 if ( opposite_colors(queeningSq, bishopSq)
336 && distance(queeningSq, weakKingSq) <= 1)
337 return SCALE_FACTOR_DRAW;
340 // If all the pawns are on the same B or G file, then it's potentially a draw
341 if ((!(allPawns & ~FileBBB) || !(allPawns & ~FileGBB))
342 && pos.non_pawn_material(weakSide) == 0
343 && pos.count<PAWN>(weakSide) >= 1)
345 // Get the least advanced weakSide pawn
346 Square weakPawnSq = frontmost_sq(strongSide, pos.pieces(weakSide, PAWN));
348 Square strongKingSq = pos.square<KING>(strongSide);
349 Square weakKingSq = pos.square<KING>(weakSide);
350 Square bishopSq = pos.square<BISHOP>(strongSide);
352 // There's potential for a draw if our pawn is blocked on the 7th rank,
353 // the bishop cannot attack it or they only have one pawn left
354 if ( relative_rank(strongSide, weakPawnSq) == RANK_7
355 && (strongPawns & (weakPawnSq + pawn_push(weakSide)))
356 && (opposite_colors(bishopSq, weakPawnSq) || !more_than_one(strongPawns)))
358 int strongKingDist = distance(weakPawnSq, strongKingSq);
359 int weakKingDist = distance(weakPawnSq, weakKingSq);
361 // It's a draw if the weak king is on its back two ranks, within 2
362 // squares of the blocking pawn and the strong king is not
363 // closer. (I think this rule only fails in practically
364 // unreachable positions such as 5k1K/6p1/6P1/8/8/3B4/8/8 w
365 // and positions where qsearch will immediately correct the
366 // problem such as 8/4k1p1/6P1/1K6/3B4/8/8/8 w)
367 if ( relative_rank(strongSide, weakKingSq) >= RANK_7
369 && weakKingDist <= strongKingDist)
370 return SCALE_FACTOR_DRAW;
374 return SCALE_FACTOR_NONE;
378 /// KQ vs KR and one or more pawns. It tests for fortress draws with a rook on
379 /// the third rank defended by a pawn.
381 ScaleFactor Endgame<KQKRPs>::operator()(const Position& pos) const {
383 assert(verify_material(pos, strongSide, QueenValueMg, 0));
384 assert(pos.count<ROOK>(weakSide) == 1);
385 assert(pos.count<PAWN>(weakSide) >= 1);
387 Square kingSq = pos.square<KING>(weakSide);
388 Square rsq = pos.square<ROOK>(weakSide);
390 if ( relative_rank(weakSide, kingSq) <= RANK_2
391 && relative_rank(weakSide, pos.square<KING>(strongSide)) >= RANK_4
392 && relative_rank(weakSide, rsq) == RANK_3
393 && ( pos.pieces(weakSide, PAWN)
394 & attacks_bb<KING>(kingSq)
395 & pawn_attacks_bb(strongSide, rsq)))
396 return SCALE_FACTOR_DRAW;
398 return SCALE_FACTOR_NONE;
402 /// KRP vs KR. This function knows a handful of the most important classes of
403 /// drawn positions, but is far from perfect. It would probably be a good idea
404 /// to add more knowledge in the future.
406 /// It would also be nice to rewrite the actual code for this function,
407 /// which is mostly copied from Glaurung 1.x, and isn't very pretty.
409 ScaleFactor Endgame<KRPKR>::operator()(const Position& pos) const {
411 assert(verify_material(pos, strongSide, RookValueMg, 1));
412 assert(verify_material(pos, weakSide, RookValueMg, 0));
414 // Assume strongSide is white and the pawn is on files A-D
415 Square wksq = normalize(pos, strongSide, pos.square<KING>(strongSide));
416 Square bksq = normalize(pos, strongSide, pos.square<KING>(weakSide));
417 Square wrsq = normalize(pos, strongSide, pos.square<ROOK>(strongSide));
418 Square wpsq = normalize(pos, strongSide, pos.square<PAWN>(strongSide));
419 Square brsq = normalize(pos, strongSide, pos.square<ROOK>(weakSide));
421 File f = file_of(wpsq);
422 Rank r = rank_of(wpsq);
423 Square queeningSq = make_square(f, RANK_8);
424 int tempo = (pos.side_to_move() == strongSide);
426 // If the pawn is not too far advanced and the defending king defends the
427 // queening square, use the third-rank defence.
429 && distance(bksq, queeningSq) <= 1
431 && (rank_of(brsq) == RANK_6 || (r <= RANK_3 && rank_of(wrsq) != RANK_6)))
432 return SCALE_FACTOR_DRAW;
434 // The defending side saves a draw by checking from behind in case the pawn
435 // has advanced to the 6th rank with the king behind.
437 && distance(bksq, queeningSq) <= 1
438 && rank_of(wksq) + tempo <= RANK_6
439 && (rank_of(brsq) == RANK_1 || (!tempo && distance<File>(brsq, wpsq) >= 3)))
440 return SCALE_FACTOR_DRAW;
443 && bksq == queeningSq
444 && rank_of(brsq) == RANK_1
445 && (!tempo || distance(wksq, wpsq) >= 2))
446 return SCALE_FACTOR_DRAW;
448 // White pawn on a7 and rook on a8 is a draw if black's king is on g7 or h7
449 // and the black rook is behind the pawn.
452 && (bksq == SQ_H7 || bksq == SQ_G7)
453 && file_of(brsq) == FILE_A
454 && (rank_of(brsq) <= RANK_3 || file_of(wksq) >= FILE_D || rank_of(wksq) <= RANK_5))
455 return SCALE_FACTOR_DRAW;
457 // If the defending king blocks the pawn and the attacking king is too far
458 // away, it's a draw.
460 && bksq == wpsq + NORTH
461 && distance(wksq, wpsq) - tempo >= 2
462 && distance(wksq, brsq) - tempo >= 2)
463 return SCALE_FACTOR_DRAW;
465 // Pawn on the 7th rank supported by the rook from behind usually wins if the
466 // attacking king is closer to the queening square than the defending king,
467 // and the defending king cannot gain tempi by threatening the attacking rook.
470 && file_of(wrsq) == f
471 && wrsq != queeningSq
472 && (distance(wksq, queeningSq) < distance(bksq, queeningSq) - 2 + tempo)
473 && (distance(wksq, queeningSq) < distance(bksq, wrsq) + tempo))
474 return ScaleFactor(SCALE_FACTOR_MAX - 2 * distance(wksq, queeningSq));
476 // Similar to the above, but with the pawn further back
478 && file_of(wrsq) == f
480 && (distance(wksq, queeningSq) < distance(bksq, queeningSq) - 2 + tempo)
481 && (distance(wksq, wpsq + NORTH) < distance(bksq, wpsq + NORTH) - 2 + tempo)
482 && ( distance(bksq, wrsq) + tempo >= 3
483 || ( distance(wksq, queeningSq) < distance(bksq, wrsq) + tempo
484 && (distance(wksq, wpsq + NORTH) < distance(bksq, wrsq) + tempo))))
485 return ScaleFactor( SCALE_FACTOR_MAX
486 - 8 * distance(wpsq, queeningSq)
487 - 2 * distance(wksq, queeningSq));
489 // If the pawn is not far advanced and the defending king is somewhere in
490 // the pawn's path, it's probably a draw.
491 if (r <= RANK_4 && bksq > wpsq)
493 if (file_of(bksq) == file_of(wpsq))
494 return ScaleFactor(10);
495 if ( distance<File>(bksq, wpsq) == 1
496 && distance(wksq, bksq) > 2)
497 return ScaleFactor(24 - 2 * distance(wksq, bksq));
499 return SCALE_FACTOR_NONE;
503 ScaleFactor Endgame<KRPKB>::operator()(const Position& pos) const {
505 assert(verify_material(pos, strongSide, RookValueMg, 1));
506 assert(verify_material(pos, weakSide, BishopValueMg, 0));
508 // Test for a rook pawn
509 if (pos.pieces(PAWN) & (FileABB | FileHBB))
511 Square ksq = pos.square<KING>(weakSide);
512 Square bsq = pos.square<BISHOP>(weakSide);
513 Square psq = pos.square<PAWN>(strongSide);
514 Rank rk = relative_rank(strongSide, psq);
515 Direction push = pawn_push(strongSide);
517 // If the pawn is on the 5th rank and the pawn (currently) is on
518 // the same color square as the bishop then there is a chance of
519 // a fortress. Depending on the king position give a moderate
520 // reduction or a stronger one if the defending king is near the
521 // corner but not trapped there.
522 if (rk == RANK_5 && !opposite_colors(bsq, psq))
524 int d = distance(psq + 3 * push, ksq);
526 if (d <= 2 && !(d == 0 && ksq == pos.square<KING>(strongSide) + 2 * push))
527 return ScaleFactor(24);
529 return ScaleFactor(48);
532 // When the pawn has moved to the 6th rank we can be fairly sure
533 // it's drawn if the bishop attacks the square in front of the
534 // pawn from a reasonable distance and the defending king is near
537 && distance(psq + 2 * push, ksq) <= 1
538 && (attacks_bb<BISHOP>(bsq) & (psq + push))
539 && distance<File>(bsq, psq) >= 2)
540 return ScaleFactor(8);
543 return SCALE_FACTOR_NONE;
546 /// KRPP vs KRP. There is just a single rule: if the stronger side has no passed
547 /// pawns and the defending king is actively placed, the position is drawish.
549 ScaleFactor Endgame<KRPPKRP>::operator()(const Position& pos) const {
551 assert(verify_material(pos, strongSide, RookValueMg, 2));
552 assert(verify_material(pos, weakSide, RookValueMg, 1));
554 Square wpsq1 = pos.squares<PAWN>(strongSide)[0];
555 Square wpsq2 = pos.squares<PAWN>(strongSide)[1];
556 Square bksq = pos.square<KING>(weakSide);
558 // Does the stronger side have a passed pawn?
559 if (pos.pawn_passed(strongSide, wpsq1) || pos.pawn_passed(strongSide, wpsq2))
560 return SCALE_FACTOR_NONE;
562 Rank r = std::max(relative_rank(strongSide, wpsq1), relative_rank(strongSide, wpsq2));
564 if ( distance<File>(bksq, wpsq1) <= 1
565 && distance<File>(bksq, wpsq2) <= 1
566 && relative_rank(strongSide, bksq) > r)
568 assert(r > RANK_1 && r < RANK_7);
569 return ScaleFactor(7 * r);
571 return SCALE_FACTOR_NONE;
575 /// K and two or more pawns vs K. There is just a single rule here: If all pawns
576 /// are on the same rook file and are blocked by the defending king, it's a draw.
578 ScaleFactor Endgame<KPsK>::operator()(const Position& pos) const {
580 assert(pos.non_pawn_material(strongSide) == VALUE_ZERO);
581 assert(pos.count<PAWN>(strongSide) >= 2);
582 assert(verify_material(pos, weakSide, VALUE_ZERO, 0));
584 Square ksq = pos.square<KING>(weakSide);
585 Bitboard pawns = pos.pieces(strongSide, PAWN);
587 // If all pawns are ahead of the king on a single rook file, it's a draw.
588 if (!((pawns & ~FileABB) || (pawns & ~FileHBB)) &&
589 !(pawns & ~passed_pawn_span(weakSide, ksq)))
590 return SCALE_FACTOR_DRAW;
592 return SCALE_FACTOR_NONE;
596 /// KBP vs KB. There are two rules: if the defending king is somewhere along the
597 /// path of the pawn, and the square of the king is not of the same color as the
598 /// stronger side's bishop, it's a draw. If the two bishops have opposite color,
599 /// it's almost always a draw.
601 ScaleFactor Endgame<KBPKB>::operator()(const Position& pos) const {
603 assert(verify_material(pos, strongSide, BishopValueMg, 1));
604 assert(verify_material(pos, weakSide, BishopValueMg, 0));
606 Square pawnSq = pos.square<PAWN>(strongSide);
607 Square strongBishopSq = pos.square<BISHOP>(strongSide);
608 Square weakBishopSq = pos.square<BISHOP>(weakSide);
609 Square weakKingSq = pos.square<KING>(weakSide);
611 // Case 1: Defending king blocks the pawn, and cannot be driven away
612 if ( (forward_file_bb(strongSide, pawnSq) & weakKingSq)
613 && ( opposite_colors(weakKingSq, strongBishopSq)
614 || relative_rank(strongSide, weakKingSq) <= RANK_6))
615 return SCALE_FACTOR_DRAW;
617 // Case 2: Opposite colored bishops
618 if (opposite_colors(strongBishopSq, weakBishopSq))
619 return SCALE_FACTOR_DRAW;
621 return SCALE_FACTOR_NONE;
625 /// KBPP vs KB. It detects a few basic draws with opposite-colored bishops
627 ScaleFactor Endgame<KBPPKB>::operator()(const Position& pos) const {
629 assert(verify_material(pos, strongSide, BishopValueMg, 2));
630 assert(verify_material(pos, weakSide, BishopValueMg, 0));
632 Square wbsq = pos.square<BISHOP>(strongSide);
633 Square bbsq = pos.square<BISHOP>(weakSide);
635 if (!opposite_colors(wbsq, bbsq))
636 return SCALE_FACTOR_NONE;
638 Square ksq = pos.square<KING>(weakSide);
639 Square psq1 = pos.squares<PAWN>(strongSide)[0];
640 Square psq2 = pos.squares<PAWN>(strongSide)[1];
641 Square blockSq1, blockSq2;
643 if (relative_rank(strongSide, psq1) > relative_rank(strongSide, psq2))
645 blockSq1 = psq1 + pawn_push(strongSide);
646 blockSq2 = make_square(file_of(psq2), rank_of(psq1));
650 blockSq1 = psq2 + pawn_push(strongSide);
651 blockSq2 = make_square(file_of(psq1), rank_of(psq2));
654 switch (distance<File>(psq1, psq2))
657 // Both pawns are on the same file. It's an easy draw if the defender firmly
658 // controls some square in the frontmost pawn's path.
659 if ( file_of(ksq) == file_of(blockSq1)
660 && relative_rank(strongSide, ksq) >= relative_rank(strongSide, blockSq1)
661 && opposite_colors(ksq, wbsq))
662 return SCALE_FACTOR_DRAW;
664 return SCALE_FACTOR_NONE;
667 // Pawns on adjacent files. It's a draw if the defender firmly controls the
668 // square in front of the frontmost pawn's path, and the square diagonally
669 // behind this square on the file of the other pawn.
671 && opposite_colors(ksq, wbsq)
672 && ( bbsq == blockSq2
673 || (attacks_bb<BISHOP>(blockSq2, pos.pieces()) & pos.pieces(weakSide, BISHOP))
674 || distance<Rank>(psq1, psq2) >= 2))
675 return SCALE_FACTOR_DRAW;
677 else if ( ksq == blockSq2
678 && opposite_colors(ksq, wbsq)
679 && ( bbsq == blockSq1
680 || (attacks_bb<BISHOP>(blockSq1, pos.pieces()) & pos.pieces(weakSide, BISHOP))))
681 return SCALE_FACTOR_DRAW;
683 return SCALE_FACTOR_NONE;
686 // The pawns are not on the same file or adjacent files. No scaling.
687 return SCALE_FACTOR_NONE;
692 /// KBP vs KN. There is a single rule: If the defending king is somewhere along
693 /// the path of the pawn, and the square of the king is not of the same color as
694 /// the stronger side's bishop, it's a draw.
696 ScaleFactor Endgame<KBPKN>::operator()(const Position& pos) const {
698 assert(verify_material(pos, strongSide, BishopValueMg, 1));
699 assert(verify_material(pos, weakSide, KnightValueMg, 0));
701 Square pawnSq = pos.square<PAWN>(strongSide);
702 Square strongBishopSq = pos.square<BISHOP>(strongSide);
703 Square weakKingSq = pos.square<KING>(weakSide);
705 if ( file_of(weakKingSq) == file_of(pawnSq)
706 && relative_rank(strongSide, pawnSq) < relative_rank(strongSide, weakKingSq)
707 && ( opposite_colors(weakKingSq, strongBishopSq)
708 || relative_rank(strongSide, weakKingSq) <= RANK_6))
709 return SCALE_FACTOR_DRAW;
711 return SCALE_FACTOR_NONE;
715 /// KP vs KP. This is done by removing the weakest side's pawn and probing the
716 /// KP vs K bitbase: If the weakest side has a draw without the pawn, it probably
717 /// has at least a draw with the pawn as well. The exception is when the stronger
718 /// side's pawn is far advanced and not on a rook file; in this case it is often
719 /// possible to win (e.g. 8/4k3/3p4/3P4/6K1/8/8/8 w - - 0 1).
721 ScaleFactor Endgame<KPKP>::operator()(const Position& pos) const {
723 assert(verify_material(pos, strongSide, VALUE_ZERO, 1));
724 assert(verify_material(pos, weakSide, VALUE_ZERO, 1));
726 // Assume strongSide is white and the pawn is on files A-D
727 Square wksq = normalize(pos, strongSide, pos.square<KING>(strongSide));
728 Square bksq = normalize(pos, strongSide, pos.square<KING>(weakSide));
729 Square psq = normalize(pos, strongSide, pos.square<PAWN>(strongSide));
731 Color us = strongSide == pos.side_to_move() ? WHITE : BLACK;
733 // If the pawn has advanced to the fifth rank or further, and is not a
734 // rook pawn, it's too dangerous to assume that it's at least a draw.
735 if (rank_of(psq) >= RANK_5 && file_of(psq) != FILE_A)
736 return SCALE_FACTOR_NONE;
738 // Probe the KPK bitbase with the weakest side's pawn removed. If it's a draw,
739 // it's probably at least a draw even with the pawn.
740 return Bitbases::probe(wksq, psq, bksq, us) ? SCALE_FACTOR_NONE : SCALE_FACTOR_DRAW;