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/>.
31 // There are 24 possible pawn squares: files A to D and ranks from 2 to 7.
32 // Positions with the pawn on files E to H will be mirrored before probing.
33 constexpr unsigned MAX_INDEX = 2*24*64*64; // stm * psq * wksq * bksq = 196608
35 std::bitset<MAX_INDEX> KPKBitbase;
37 // A KPK bitbase index is an integer in [0, IndexMax] range
39 // Information is mapped in a way that minimizes the number of iterations:
41 // bit 0- 5: white king square (from SQ_A1 to SQ_H8)
42 // bit 6-11: black king square (from SQ_A1 to SQ_H8)
43 // bit 12: side to move (WHITE or BLACK)
44 // bit 13-14: white pawn file (from FILE_A to FILE_D)
45 // bit 15-17: white pawn RANK_7 - rank (from RANK_7 - RANK_7 to RANK_7 - RANK_2)
46 unsigned index(Color stm, Square bksq, Square wksq, Square psq) {
47 return int(wksq) | (bksq << 6) | (stm << 12) | (file_of(psq) << 13) | ((RANK_7 - rank_of(psq)) << 15);
57 Result& operator|=(Result& r, Result v) { return r = Result(r | v); }
60 KPKPosition() = default;
61 explicit KPKPosition(unsigned idx);
62 operator Result() const { return result; }
63 Result classify(const std::vector<KPKPosition>& db);
66 Square ksq[COLOR_NB], psq;
73 bool Bitbases::probe(Square wksq, Square wpsq, Square bksq, Color stm) {
75 assert(file_of(wpsq) <= FILE_D);
77 return KPKBitbase[index(stm, bksq, wksq, wpsq)];
81 void Bitbases::init() {
83 std::vector<KPKPosition> db(MAX_INDEX);
84 unsigned idx, repeat = 1;
86 // Initialize db with known win / draw positions
87 for (idx = 0; idx < MAX_INDEX; ++idx)
88 db[idx] = KPKPosition(idx);
90 // Iterate through the positions until none of the unknown positions can be
91 // changed to either wins or draws (15 cycles needed).
93 for (repeat = idx = 0; idx < MAX_INDEX; ++idx)
94 repeat |= (db[idx] == UNKNOWN && db[idx].classify(db) != UNKNOWN);
96 // Fill the bitbase with the decisive results
97 for (idx = 0; idx < MAX_INDEX; ++idx)
105 KPKPosition::KPKPosition(unsigned idx) {
107 ksq[WHITE] = Square((idx >> 0) & 0x3F);
108 ksq[BLACK] = Square((idx >> 6) & 0x3F);
109 stm = Color ((idx >> 12) & 0x01);
110 psq = make_square(File((idx >> 13) & 0x3), Rank(RANK_7 - ((idx >> 15) & 0x7)));
112 // Check if two pieces are on the same square or if a king can be captured
113 if ( distance(ksq[WHITE], ksq[BLACK]) <= 1
116 || (stm == WHITE && (PawnAttacks[WHITE][psq] & ksq[BLACK])))
119 // Immediate win if a pawn can be promoted without getting captured
120 else if ( stm == WHITE
121 && rank_of(psq) == RANK_7
122 && ksq[stm] != psq + NORTH
123 && ( distance(ksq[~stm], psq + NORTH) > 1
124 || (PseudoAttacks[KING][ksq[stm]] & (psq + NORTH))))
127 // Immediate draw if it is a stalemate or a king captures undefended pawn
128 else if ( stm == BLACK
129 && ( !(PseudoAttacks[KING][ksq[stm]] & ~(PseudoAttacks[KING][ksq[~stm]] | PawnAttacks[~stm][psq]))
130 || (PseudoAttacks[KING][ksq[stm]] & psq & ~PseudoAttacks[KING][ksq[~stm]])))
133 // Position will be classified later
138 Result KPKPosition::classify(const std::vector<KPKPosition>& db) {
140 // White to move: If one move leads to a position classified as WIN, the result
141 // of the current position is WIN. If all moves lead to positions classified
142 // as DRAW, the current position is classified as DRAW, otherwise the current
143 // position is classified as UNKNOWN.
145 // Black to move: If one move leads to a position classified as DRAW, the result
146 // of the current position is DRAW. If all moves lead to positions classified
147 // as WIN, the position is classified as WIN, otherwise the current position is
148 // classified as UNKNOWN.
149 const Result Good = (stm == WHITE ? WIN : DRAW);
150 const Result Bad = (stm == WHITE ? DRAW : WIN);
153 Bitboard b = PseudoAttacks[KING][ksq[stm]];
156 r |= stm == WHITE ? db[index(BLACK, ksq[BLACK] , pop_lsb(&b), psq)]
157 : db[index(WHITE, pop_lsb(&b), ksq[WHITE], psq)];
161 if (rank_of(psq) < RANK_7) // Single push
162 r |= db[index(BLACK, ksq[BLACK], ksq[WHITE], psq + NORTH)];
164 if ( rank_of(psq) == RANK_2 // Double push
165 && psq + NORTH != ksq[WHITE]
166 && psq + NORTH != ksq[BLACK])
167 r |= db[index(BLACK, ksq[BLACK], ksq[WHITE], psq + NORTH + NORTH)];
170 return result = r & Good ? Good : r & UNKNOWN ? UNKNOWN : Bad;