// init_magics() computes all rook and bishop attacks at startup. Magic
// bitboards are used to look up attacks of sliding pieces. As a reference see
// chessprogramming.wikispaces.com/Magic+Bitboards. In particular, here we
// use the so called "fancy" approach.
public static void init_magics(PieceType pt, Bitboard[][] attacks, Bitboard[] magics,
Bitboard[] masks, uint[] shifts, Square[] deltas, Fn index)
{
int[][] MagicBoosters = new int[2][] {
new int[] { 969, 1976, 2850, 542, 2069, 2852, 1708, 164 },
new int[] { 3101, 552, 3555, 926, 834, 26, 2131, 1117 }
};
RKISS rk = new RKISS();
Bitboard[] occupancy = new UInt64[4096], reference = new UInt64[4096];
Bitboard edges, b;
int i, size, booster;
for (Square s = SquareS.SQ_A1; s <= SquareS.SQ_H8; s++)
{
// Board edges are not considered in the relevant occupancies
edges = ((BitBoard.Rank1BB | BitBoard.Rank8BB) & ~BitBoard.rank_bb_square(s)) | ((BitBoard.FileABB | BitBoard.FileHBB) & ~BitBoard.file_bb_square(s));
// Given a square 's', the mask is the bitboard of sliding attacks from
// 's' computed on an empty board. The index must be big enough to contain
// all the attacks for each possible subset of the mask and so is 2 power
// the number of 1s of the mask. Hence we deduce the size of the shift to
// apply to the 64 or 32 bits word to get the index.
masks[s] = sliding_attack(deltas, s, 0) & ~edges;
shifts[s] = 32 - (uint)Bitcount.popcount_Max15(masks[s]);
// Use Carry-Rippler trick to enumerate all subsets of masks[s] and
// store the corresponding sliding attack bitboard in reference[].
b = 0;
size = 0;
do
{
occupancy[size] = b;
reference[size] = sliding_attack(deltas, s, b);
size++;
b = (b - masks[s]) & masks[s];
} while (b != 0);
// Set the offset for the table of the next square. We have individual
// table sizes for each square with "Fancy Magic Bitboards".
attacks[s] = new Bitboard[size];
booster = MagicBoosters[0][Types.rank_of(s)];
// Find a magic for square 's' picking up an (almost) random number
// until we find the one that passes the verification test.
do
{
do
{
magics[s] = rk.magic_rand(booster);
}while (Bitcount.popcount_Max15((magics[s] * masks[s]) >> 56) < 6);
Array.Clear(attacks[s], 0, size);
// A good magic must map every possible occupancy to an index that
// looks up the correct sliding attack in the attacks[s] database.
// Note that we build up the database for square 's' as a side
// effect of verifying the magic.
for (i = 0; i < size; i++)
{
Bitboard attack = attacks[s][index(s, occupancy[i], pt)];
if (attack != 0 && attack != reference[i])
{
break;
}
Debug.Assert(reference[i] != 0);
//attack = reference[i];
attacks[s][index(s, occupancy[i], pt)] = reference[i];
}
} while (i != size);
}
}
public PolyglotBook()
{
RKiss = new RKISS((int)(Time.now() % 1000));
}
