Fill by Subtraction
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Performing the o^(o-2r) - trick in a SWAR-wise fashion with bytes (ranks), saves some instructions compared to Kogge-Stone.
| Code samples and bitboard diagrams rely on Little endian file and rank mapping. |
Unfortunately it only works in positive horizontal direction, so the possible savings are marginal, despite even less generalization of handling one out of eight cases with a different code pattern.
Source Code
U64 eastAttacks(U64 rooks, U64 empty) {
const U64 H = 0x8080808080808080;
U64 occInclRook = rooks | ~empty | H;
U64 occExclRook = (rooks &= ~H) ^ occInclRook;
U64 rookAttacks = (occExclRook - rooks) ^ occInclRook;
return rookAttacks;
}
Even more with vectors of two bitboards and SSE2-instructions.
Comparison with Kogge-Stone
For comparison the Kogge-Stone attack-getter. It gives an idea how to implement a Kogge-Stone Adder with bitwise operations only.
U64 eastAttacks(U64 rooks, U64 empty) {
empty &= notAFile;
rooks |= empty & (rooks << 1);
empty &= (empty << 1);
rooks |= empty & (rooks << 2);
empty &= (empty << 2);
rooks |= empty & (rooks << 4);
return notAFile& (rooks << 1);
}
