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/>.
27 uint8_t PopCnt16[1 << 16];
28 uint8_t SquareDistance[SQUARE_NB][SQUARE_NB];
30 Bitboard SquareBB[SQUARE_NB];
31 Bitboard LineBB[SQUARE_NB][SQUARE_NB];
32 Bitboard PseudoAttacks[PIECE_TYPE_NB][SQUARE_NB];
33 Bitboard PawnAttacks[COLOR_NB][SQUARE_NB];
35 Magic RookMagics[SQUARE_NB];
36 Magic BishopMagics[SQUARE_NB];
40 Bitboard RookTable[0x19000]; // To store rook attacks
41 Bitboard BishopTable[0x1480]; // To store bishop attacks
43 void init_magics(Bitboard table[], Magic magics[], Direction directions[]);
47 /// Bitboards::pretty() returns an ASCII representation of a bitboard suitable
48 /// to be printed to standard output. Useful for debugging.
50 const std::string Bitboards::pretty(Bitboard b) {
52 std::string s = "+---+---+---+---+---+---+---+---+\n";
54 for (Rank r = RANK_8; r >= RANK_1; --r)
56 for (File f = FILE_A; f <= FILE_H; ++f)
57 s += b & make_square(f, r) ? "| X " : "| ";
59 s += "|\n+---+---+---+---+---+---+---+---+\n";
66 /// Bitboards::init() initializes various bitboard tables. It is called at
67 /// startup and relies on global objects to be already zero-initialized.
69 void Bitboards::init() {
71 for (unsigned i = 0; i < (1 << 16); ++i)
72 PopCnt16[i] = std::bitset<16>(i).count();
74 for (Square s = SQ_A1; s <= SQ_H8; ++s)
75 SquareBB[s] = (1ULL << s);
77 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
78 for (Square s2 = SQ_A1; s2 <= SQ_H8; ++s2)
79 SquareDistance[s1][s2] = std::max(distance<File>(s1, s2), distance<Rank>(s1, s2));
81 Direction RookDirections[] = { NORTH, EAST, SOUTH, WEST };
82 Direction BishopDirections[] = { NORTH_EAST, SOUTH_EAST, SOUTH_WEST, NORTH_WEST };
84 init_magics(RookTable, RookMagics, RookDirections);
85 init_magics(BishopTable, BishopMagics, BishopDirections);
87 // Helper returning the target bitboard of a step from a square
88 auto landing_square_bb = [&](Square s, int step)
90 Square to = Square(s + step);
91 return is_ok(to) && distance(s, to) <= 2 ? square_bb(to) : Bitboard(0);
94 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
96 PawnAttacks[WHITE][s1] = pawn_attacks_bb<WHITE>(square_bb(s1));
97 PawnAttacks[BLACK][s1] = pawn_attacks_bb<BLACK>(square_bb(s1));
99 for (int step : {-9, -8, -7, -1, 1, 7, 8, 9} )
100 PseudoAttacks[KING][s1] |= landing_square_bb(s1, step);
102 for (int step : {-17, -15, -10, -6, 6, 10, 15, 17} )
103 PseudoAttacks[KNIGHT][s1] |= landing_square_bb(s1, step);
105 PseudoAttacks[QUEEN][s1] = PseudoAttacks[BISHOP][s1] = attacks_bb<BISHOP>(s1, 0);
106 PseudoAttacks[QUEEN][s1] |= PseudoAttacks[ ROOK][s1] = attacks_bb< ROOK>(s1, 0);
108 for (PieceType pt : { BISHOP, ROOK })
109 for (Square s2 = SQ_A1; s2 <= SQ_H8; ++s2)
110 if (PseudoAttacks[pt][s1] & s2)
111 LineBB[s1][s2] = (attacks_bb(pt, s1, 0) & attacks_bb(pt, s2, 0)) | s1 | s2;
118 Bitboard sliding_attack(Direction directions[], Square sq, Bitboard occupied) {
122 for (int i = 0; i < 4; ++i)
123 for (Square s = sq + directions[i];
124 is_ok(s) && distance(s, s - directions[i]) == 1;
137 // init_magics() computes all rook and bishop attacks at startup. Magic
138 // bitboards are used to look up attacks of sliding pieces. As a reference see
139 // www.chessprogramming.org/Magic_Bitboards. In particular, here we use the so
140 // called "fancy" approach.
142 void init_magics(Bitboard table[], Magic magics[], Direction directions[]) {
144 // Optimal PRNG seeds to pick the correct magics in the shortest time
145 int seeds[][RANK_NB] = { { 8977, 44560, 54343, 38998, 5731, 95205, 104912, 17020 },
146 { 728, 10316, 55013, 32803, 12281, 15100, 16645, 255 } };
148 Bitboard occupancy[4096], reference[4096], edges, b;
149 int epoch[4096] = {}, cnt = 0, size = 0;
151 for (Square s = SQ_A1; s <= SQ_H8; ++s)
153 // Board edges are not considered in the relevant occupancies
154 edges = ((Rank1BB | Rank8BB) & ~rank_bb(s)) | ((FileABB | FileHBB) & ~file_bb(s));
156 // Given a square 's', the mask is the bitboard of sliding attacks from
157 // 's' computed on an empty board. The index must be big enough to contain
158 // all the attacks for each possible subset of the mask and so is 2 power
159 // the number of 1s of the mask. Hence we deduce the size of the shift to
160 // apply to the 64 or 32 bits word to get the index.
161 Magic& m = magics[s];
162 m.mask = sliding_attack(directions, s, 0) & ~edges;
163 m.shift = (Is64Bit ? 64 : 32) - popcount(m.mask);
165 // Set the offset for the attacks table of the square. We have individual
166 // table sizes for each square with "Fancy Magic Bitboards".
167 m.attacks = s == SQ_A1 ? table : magics[s - 1].attacks + size;
169 // Use Carry-Rippler trick to enumerate all subsets of masks[s] and
170 // store the corresponding sliding attack bitboard in reference[].
174 reference[size] = sliding_attack(directions, s, b);
177 m.attacks[pext(b, m.mask)] = reference[size];
180 b = (b - m.mask) & m.mask;
186 PRNG rng(seeds[Is64Bit][rank_of(s)]);
188 // Find a magic for square 's' picking up an (almost) random number
189 // until we find the one that passes the verification test.
190 for (int i = 0; i < size; )
192 for (m.magic = 0; popcount((m.magic * m.mask) >> 56) < 6; )
193 m.magic = rng.sparse_rand<Bitboard>();
195 // A good magic must map every possible occupancy to an index that
196 // looks up the correct sliding attack in the attacks[s] database.
197 // Note that we build up the database for square 's' as a side
198 // effect of verifying the magic. Keep track of the attempt count
199 // and save it in epoch[], little speed-up trick to avoid resetting
200 // m.attacks[] after every failed attempt.
201 for (++cnt, i = 0; i < size; ++i)
203 unsigned idx = m.index(occupancy[i]);
205 if (epoch[idx] < cnt)
208 m.attacks[idx] = reference[i];
210 else if (m.attacks[idx] != reference[i])