2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2021 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/>.
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 BetweenBB[SQUARE_NB][SQUARE_NB];
33 Bitboard PseudoAttacks[PIECE_TYPE_NB][SQUARE_NB];
34 Bitboard PawnAttacks[COLOR_NB][SQUARE_NB];
36 Magic RookMagics[SQUARE_NB];
37 Magic BishopMagics[SQUARE_NB];
41 Bitboard RookTable[0x19000]; // To store rook attacks
42 Bitboard BishopTable[0x1480]; // To store bishop attacks
44 void init_magics(PieceType pt, Bitboard table[], Magic magics[]);
48 /// safe_destination() returns the bitboard of target square for the given step
49 /// from the given square. If the step is off the board, returns empty bitboard.
51 inline Bitboard safe_destination(Square s, int step) {
52 Square to = Square(s + step);
53 return is_ok(to) && distance(s, to) <= 2 ? square_bb(to) : Bitboard(0);
57 /// Bitboards::pretty() returns an ASCII representation of a bitboard suitable
58 /// to be printed to standard output. Useful for debugging.
60 std::string Bitboards::pretty(Bitboard b) {
62 std::string s = "+---+---+---+---+---+---+---+---+\n";
64 for (Rank r = RANK_8; r >= RANK_1; --r)
66 for (File f = FILE_A; f <= FILE_H; ++f)
67 s += b & make_square(f, r) ? "| X " : "| ";
69 s += "| " + std::to_string(1 + r) + "\n+---+---+---+---+---+---+---+---+\n";
71 s += " a b c d e f g h\n";
77 /// Bitboards::init() initializes various bitboard tables. It is called at
78 /// startup and relies on global objects to be already zero-initialized.
80 void Bitboards::init() {
82 for (unsigned i = 0; i < (1 << 16); ++i)
83 PopCnt16[i] = uint8_t(std::bitset<16>(i).count());
85 for (Square s = SQ_A1; s <= SQ_H8; ++s)
86 SquareBB[s] = (1ULL << s);
88 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
89 for (Square s2 = SQ_A1; s2 <= SQ_H8; ++s2)
90 SquareDistance[s1][s2] = std::max(distance<File>(s1, s2), distance<Rank>(s1, s2));
92 init_magics(ROOK, RookTable, RookMagics);
93 init_magics(BISHOP, BishopTable, BishopMagics);
95 for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
97 PawnAttacks[WHITE][s1] = pawn_attacks_bb<WHITE>(square_bb(s1));
98 PawnAttacks[BLACK][s1] = pawn_attacks_bb<BLACK>(square_bb(s1));
100 for (int step : {-9, -8, -7, -1, 1, 7, 8, 9} )
101 PseudoAttacks[KING][s1] |= safe_destination(s1, step);
103 for (int step : {-17, -15, -10, -6, 6, 10, 15, 17} )
104 PseudoAttacks[KNIGHT][s1] |= safe_destination(s1, step);
106 PseudoAttacks[QUEEN][s1] = PseudoAttacks[BISHOP][s1] = attacks_bb<BISHOP>(s1, 0);
107 PseudoAttacks[QUEEN][s1] |= PseudoAttacks[ ROOK][s1] = attacks_bb< ROOK>(s1, 0);
109 for (PieceType pt : { BISHOP, ROOK })
110 for (Square s2 = SQ_A1; s2 <= SQ_H8; ++s2)
112 if (PseudoAttacks[pt][s1] & s2)
114 LineBB[s1][s2] = (attacks_bb(pt, s1, 0) & attacks_bb(pt, s2, 0)) | s1 | s2;
115 BetweenBB[s1][s2] = (attacks_bb(pt, s1, square_bb(s2)) & attacks_bb(pt, s2, square_bb(s1)));
117 BetweenBB[s1][s2] |= s2;
124 Bitboard sliding_attack(PieceType pt, Square sq, Bitboard occupied) {
126 Bitboard attacks = 0;
127 Direction RookDirections[4] = {NORTH, SOUTH, EAST, WEST};
128 Direction BishopDirections[4] = {NORTH_EAST, SOUTH_EAST, SOUTH_WEST, NORTH_WEST};
130 for (Direction d : (pt == ROOK ? RookDirections : BishopDirections))
133 while (safe_destination(s, d) && !(occupied & s))
141 // init_magics() computes all rook and bishop attacks at startup. Magic
142 // bitboards are used to look up attacks of sliding pieces. As a reference see
143 // www.chessprogramming.org/Magic_Bitboards. In particular, here we use the so
144 // called "fancy" approach.
146 void init_magics(PieceType pt, Bitboard table[], Magic magics[]) {
148 // Optimal PRNG seeds to pick the correct magics in the shortest time
149 int seeds[][RANK_NB] = { { 8977, 44560, 54343, 38998, 5731, 95205, 104912, 17020 },
150 { 728, 10316, 55013, 32803, 12281, 15100, 16645, 255 } };
152 Bitboard occupancy[4096], reference[4096], edges, b;
153 int epoch[4096] = {}, cnt = 0, size = 0;
155 for (Square s = SQ_A1; s <= SQ_H8; ++s)
157 // Board edges are not considered in the relevant occupancies
158 edges = ((Rank1BB | Rank8BB) & ~rank_bb(s)) | ((FileABB | FileHBB) & ~file_bb(s));
160 // Given a square 's', the mask is the bitboard of sliding attacks from
161 // 's' computed on an empty board. The index must be big enough to contain
162 // all the attacks for each possible subset of the mask and so is 2 power
163 // the number of 1s of the mask. Hence we deduce the size of the shift to
164 // apply to the 64 or 32 bits word to get the index.
165 Magic& m = magics[s];
166 m.mask = sliding_attack(pt, s, 0) & ~edges;
167 m.shift = (Is64Bit ? 64 : 32) - popcount(m.mask);
169 // Set the offset for the attacks table of the square. We have individual
170 // table sizes for each square with "Fancy Magic Bitboards".
171 m.attacks = s == SQ_A1 ? table : magics[s - 1].attacks + size;
173 // Use Carry-Rippler trick to enumerate all subsets of masks[s] and
174 // store the corresponding sliding attack bitboard in reference[].
178 reference[size] = sliding_attack(pt, s, b);
181 m.attacks[pext(b, m.mask)] = reference[size];
184 b = (b - m.mask) & m.mask;
190 PRNG rng(seeds[Is64Bit][rank_of(s)]);
192 // Find a magic for square 's' picking up an (almost) random number
193 // until we find the one that passes the verification test.
194 for (int i = 0; i < size; )
196 for (m.magic = 0; popcount((m.magic * m.mask) >> 56) < 6; )
197 m.magic = rng.sparse_rand<Bitboard>();
199 // A good magic must map every possible occupancy to an index that
200 // looks up the correct sliding attack in the attacks[s] database.
201 // Note that we build up the database for square 's' as a side
202 // effect of verifying the magic. Keep track of the attempt count
203 // and save it in epoch[], little speed-up trick to avoid resetting
204 // m.attacks[] after every failed attempt.
205 for (++cnt, i = 0; i < size; ++i)
207 unsigned idx = m.index(occupancy[i]);
209 if (epoch[idx] < cnt)
212 m.attacks[idx] = reference[i];
214 else if (m.attacks[idx] != reference[i])
222 } // namespace Stockfish