Fill by Subtraction
Home * Board Representation * Bitboards * Sliding Piece Attacks * Fill by Subtraction
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); }