2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2023 The Stockfish developers (see AUTHORS file)
5 Stockfish is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation, either version 3 of the License, or
8 (at your option) any later version.
10 Stockfish is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>.
30 // There are 24 possible pawn squares: files A to D and ranks from 2 to 7.
31 // Positions with the pawn on files E to H will be mirrored before probing.
32 constexpr unsigned MAX_INDEX = 2*24*64*64; // stm * psq * wksq * bksq = 196608
34 std::bitset<MAX_INDEX> KPKBitbase;
36 // A KPK bitbase index is an integer in [0, IndexMax] range
38 // Information is mapped in a way that minimizes the number of iterations:
40 // bit 0- 5: white king square (from SQ_A1 to SQ_H8)
41 // bit 6-11: black king square (from SQ_A1 to SQ_H8)
42 // bit 12: side to move (WHITE or BLACK)
43 // bit 13-14: white pawn file (from FILE_A to FILE_D)
44 // bit 15-17: white pawn RANK_7 - rank (from RANK_7 - RANK_7 to RANK_7 - RANK_2)
45 unsigned index(Color stm, Square bksq, Square wksq, Square psq) {
46 return int(wksq) | (bksq << 6) | (stm << 12) | (file_of(psq) << 13) | ((RANK_7 - rank_of(psq)) << 15);
56 Result& operator|=(Result& r, Result v) { return r = Result(r | v); }
59 KPKPosition() = default;
60 explicit KPKPosition(unsigned idx);
61 operator Result() const { return result; }
62 Result classify(const std::vector<KPKPosition>& db);
65 Square ksq[COLOR_NB], psq;
71 bool Bitbases::probe(Square wksq, Square wpsq, Square bksq, Color stm) {
73 assert(file_of(wpsq) <= FILE_D);
75 return KPKBitbase[index(stm, bksq, wksq, wpsq)];
79 void Bitbases::init() {
81 std::vector<KPKPosition> db(MAX_INDEX);
82 unsigned idx, repeat = 1;
84 // Initialize db with known win / draw positions
85 for (idx = 0; idx < MAX_INDEX; ++idx)
86 db[idx] = KPKPosition(idx);
88 // Iterate through the positions until none of the unknown positions can be
89 // changed to either wins or draws (15 cycles needed).
91 for (repeat = idx = 0; idx < MAX_INDEX; ++idx)
92 repeat |= (db[idx] == UNKNOWN && db[idx].classify(db) != UNKNOWN);
94 // Fill the bitbase with the decisive results
95 for (idx = 0; idx < MAX_INDEX; ++idx)
102 KPKPosition::KPKPosition(unsigned idx) {
104 ksq[WHITE] = Square((idx >> 0) & 0x3F);
105 ksq[BLACK] = Square((idx >> 6) & 0x3F);
106 stm = Color ((idx >> 12) & 0x01);
107 psq = make_square(File((idx >> 13) & 0x3), Rank(RANK_7 - ((idx >> 15) & 0x7)));
109 // Invalid if two pieces are on the same square or if a king can be captured
110 if ( distance(ksq[WHITE], ksq[BLACK]) <= 1
113 || (stm == WHITE && (pawn_attacks_bb(WHITE, psq) & ksq[BLACK])))
116 // Win if the pawn can be promoted without getting captured
117 else if ( stm == WHITE
118 && rank_of(psq) == RANK_7
119 && ksq[WHITE] != psq + NORTH
120 && ( distance(ksq[BLACK], psq + NORTH) > 1
121 || (distance(ksq[WHITE], psq + NORTH) == 1)))
124 // Draw if it is stalemate or the black king can capture the pawn
125 else if ( stm == BLACK
126 && ( !(attacks_bb<KING>(ksq[BLACK]) & ~(attacks_bb<KING>(ksq[WHITE]) | pawn_attacks_bb(WHITE, psq)))
127 || (attacks_bb<KING>(ksq[BLACK]) & ~attacks_bb<KING>(ksq[WHITE]) & psq)))
130 // Position will be classified later
135 Result KPKPosition::classify(const std::vector<KPKPosition>& db) {
137 // White to move: If one move leads to a position classified as WIN, the result
138 // of the current position is WIN. If all moves lead to positions classified
139 // as DRAW, the current position is classified as DRAW, otherwise the current
140 // position is classified as UNKNOWN.
142 // Black to move: If one move leads to a position classified as DRAW, the result
143 // of the current position is DRAW. If all moves lead to positions classified
144 // as WIN, the position is classified as WIN, otherwise the current position is
145 // classified as UNKNOWN.
146 const Result Good = (stm == WHITE ? WIN : DRAW);
147 const Result Bad = (stm == WHITE ? DRAW : WIN);
150 Bitboard b = attacks_bb<KING>(ksq[stm]);
153 r |= stm == WHITE ? db[index(BLACK, ksq[BLACK], pop_lsb(b), psq)]
154 : db[index(WHITE, pop_lsb(b), ksq[WHITE], psq)];
158 if (rank_of(psq) < RANK_7) // Single push
159 r |= db[index(BLACK, ksq[BLACK], ksq[WHITE], psq + NORTH)];
161 if ( rank_of(psq) == RANK_2 // Double push
162 && psq + NORTH != ksq[WHITE]
163 && psq + NORTH != ksq[BLACK])
164 r |= db[index(BLACK, ksq[BLACK], ksq[WHITE], psq + NORTH + NORTH)];
167 return result = r & Good ? Good : r & UNKNOWN ? UNKNOWN : Bad;
172 } // namespace Stockfish