223 lines
7.7 KiB
C++
223 lines
7.7 KiB
C++
/*
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Stockfish, a UCI chess playing engine derived from Glaurung 2.1
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Copyright (C) 2004-2021 The Stockfish developers (see AUTHORS file)
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Stockfish is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
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(at your option) any later version.
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Stockfish is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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#include <algorithm>
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#include <bitset>
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#include "bitboard.h"
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#include "misc.h"
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namespace Stockfish {
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uint8_t PopCnt16[1 << 16];
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uint8_t SquareDistance[SQUARE_NB][SQUARE_NB];
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Bitboard SquareBB[SQUARE_NB];
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Bitboard LineBB[SQUARE_NB][SQUARE_NB];
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Bitboard BetweenBB[SQUARE_NB][SQUARE_NB];
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Bitboard PseudoAttacks[PIECE_TYPE_NB][SQUARE_NB];
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Bitboard PawnAttacks[COLOR_NB][SQUARE_NB];
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Magic RookMagics[SQUARE_NB];
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Magic BishopMagics[SQUARE_NB];
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namespace {
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Bitboard RookTable[0x19000]; // To store rook attacks
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Bitboard BishopTable[0x1480]; // To store bishop attacks
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void init_magics(PieceType pt, Bitboard table[], Magic magics[]);
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}
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/// safe_destination() returns the bitboard of target square for the given step
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/// from the given square. If the step is off the board, returns empty bitboard.
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inline Bitboard safe_destination(Square s, int step) {
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Square to = Square(s + step);
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return is_ok(to) && distance(s, to) <= 2 ? square_bb(to) : Bitboard(0);
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}
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/// Bitboards::pretty() returns an ASCII representation of a bitboard suitable
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/// to be printed to standard output. Useful for debugging.
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std::string Bitboards::pretty(Bitboard b) {
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std::string s = "+---+---+---+---+---+---+---+---+\n";
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for (Rank r = RANK_8; r >= RANK_1; --r)
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{
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for (File f = FILE_A; f <= FILE_H; ++f)
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s += b & make_square(f, r) ? "| X " : "| ";
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s += "| " + std::to_string(1 + r) + "\n+---+---+---+---+---+---+---+---+\n";
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}
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s += " a b c d e f g h\n";
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return s;
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}
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/// Bitboards::init() initializes various bitboard tables. It is called at
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/// startup and relies on global objects to be already zero-initialized.
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void Bitboards::init() {
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for (unsigned i = 0; i < (1 << 16); ++i)
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PopCnt16[i] = uint8_t(std::bitset<16>(i).count());
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for (Square s = SQ_A1; s <= SQ_H8; ++s)
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SquareBB[s] = (1ULL << s);
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for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
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for (Square s2 = SQ_A1; s2 <= SQ_H8; ++s2)
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SquareDistance[s1][s2] = std::max(distance<File>(s1, s2), distance<Rank>(s1, s2));
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init_magics(ROOK, RookTable, RookMagics);
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init_magics(BISHOP, BishopTable, BishopMagics);
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for (Square s1 = SQ_A1; s1 <= SQ_H8; ++s1)
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{
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PawnAttacks[WHITE][s1] = pawn_attacks_bb<WHITE>(square_bb(s1));
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PawnAttacks[BLACK][s1] = pawn_attacks_bb<BLACK>(square_bb(s1));
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for (int step : {-9, -8, -7, -1, 1, 7, 8, 9} )
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PseudoAttacks[KING][s1] |= safe_destination(s1, step);
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for (int step : {-17, -15, -10, -6, 6, 10, 15, 17} )
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PseudoAttacks[KNIGHT][s1] |= safe_destination(s1, step);
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PseudoAttacks[QUEEN][s1] = PseudoAttacks[BISHOP][s1] = attacks_bb<BISHOP>(s1, 0);
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PseudoAttacks[QUEEN][s1] |= PseudoAttacks[ ROOK][s1] = attacks_bb< ROOK>(s1, 0);
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for (PieceType pt : { BISHOP, ROOK })
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for (Square s2 = SQ_A1; s2 <= SQ_H8; ++s2)
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{
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if (PseudoAttacks[pt][s1] & s2)
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{
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LineBB[s1][s2] = (attacks_bb(pt, s1, 0) & attacks_bb(pt, s2, 0)) | s1 | s2;
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BetweenBB[s1][s2] = (attacks_bb(pt, s1, square_bb(s2)) & attacks_bb(pt, s2, square_bb(s1)));
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}
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BetweenBB[s1][s2] |= s2;
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}
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}
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}
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namespace {
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Bitboard sliding_attack(PieceType pt, Square sq, Bitboard occupied) {
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Bitboard attacks = 0;
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Direction RookDirections[4] = {NORTH, SOUTH, EAST, WEST};
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Direction BishopDirections[4] = {NORTH_EAST, SOUTH_EAST, SOUTH_WEST, NORTH_WEST};
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for (Direction d : (pt == ROOK ? RookDirections : BishopDirections))
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{
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Square s = sq;
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while (safe_destination(s, d) && !(occupied & s))
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attacks |= (s += d);
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}
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return attacks;
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}
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// init_magics() computes all rook and bishop attacks at startup. Magic
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// bitboards are used to look up attacks of sliding pieces. As a reference see
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// www.chessprogramming.org/Magic_Bitboards. In particular, here we use the so
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// called "fancy" approach.
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void init_magics(PieceType pt, Bitboard table[], Magic magics[]) {
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// Optimal PRNG seeds to pick the correct magics in the shortest time
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int seeds[][RANK_NB] = { { 8977, 44560, 54343, 38998, 5731, 95205, 104912, 17020 },
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{ 728, 10316, 55013, 32803, 12281, 15100, 16645, 255 } };
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Bitboard occupancy[4096], reference[4096], edges, b;
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int epoch[4096] = {}, cnt = 0, size = 0;
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for (Square s = SQ_A1; s <= SQ_H8; ++s)
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{
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// Board edges are not considered in the relevant occupancies
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edges = ((Rank1BB | Rank8BB) & ~rank_bb(s)) | ((FileABB | FileHBB) & ~file_bb(s));
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// Given a square 's', the mask is the bitboard of sliding attacks from
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// 's' computed on an empty board. The index must be big enough to contain
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// all the attacks for each possible subset of the mask and so is 2 power
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// the number of 1s of the mask. Hence we deduce the size of the shift to
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// apply to the 64 or 32 bits word to get the index.
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Magic& m = magics[s];
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m.mask = sliding_attack(pt, s, 0) & ~edges;
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m.shift = (Is64Bit ? 64 : 32) - popcount(m.mask);
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// Set the offset for the attacks table of the square. We have individual
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// table sizes for each square with "Fancy Magic Bitboards".
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m.attacks = s == SQ_A1 ? table : magics[s - 1].attacks + size;
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// Use Carry-Rippler trick to enumerate all subsets of masks[s] and
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// store the corresponding sliding attack bitboard in reference[].
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b = size = 0;
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do {
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occupancy[size] = b;
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reference[size] = sliding_attack(pt, s, b);
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if (HasPext)
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m.attacks[pext(b, m.mask)] = reference[size];
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size++;
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b = (b - m.mask) & m.mask;
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} while (b);
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if (HasPext)
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continue;
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PRNG rng(seeds[Is64Bit][rank_of(s)]);
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// Find a magic for square 's' picking up an (almost) random number
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// until we find the one that passes the verification test.
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for (int i = 0; i < size; )
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{
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for (m.magic = 0; popcount((m.magic * m.mask) >> 56) < 6; )
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m.magic = rng.sparse_rand<Bitboard>();
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// A good magic must map every possible occupancy to an index that
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// looks up the correct sliding attack in the attacks[s] database.
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// Note that we build up the database for square 's' as a side
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// effect of verifying the magic. Keep track of the attempt count
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// and save it in epoch[], little speed-up trick to avoid resetting
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// m.attacks[] after every failed attempt.
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for (++cnt, i = 0; i < size; ++i)
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{
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unsigned idx = m.index(occupancy[i]);
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if (epoch[idx] < cnt)
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{
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epoch[idx] = cnt;
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m.attacks[idx] = reference[i];
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}
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else if (m.attacks[idx] != reference[i])
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break;
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}
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}
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}
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}
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}
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} // namespace Stockfish
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