Shift right would overflow

[sengerts]

I am getting the following errors in the bitboard module while compiling the source code:

The code affected by these errors is the following:

// Compute the attack's index using the 'magic bitboards' approach
fn index_bishop(s: Square, occupied: Bitboard) -> usize {
    unsafe {
        u64::wrapping_mul((occupied & BISHOP_MAGICS.masks[s.0 as usize]).0,
            BISHOP_MAGICS.magics[s.0 as usize]) as usize >> (64-9)
    }
}

fn index_rook(s: Square, occupied: Bitboard) -> usize {
    unsafe {
        u64::wrapping_mul((occupied & ROOK_MAGICS.masks[s.0 as usize]).0,
            ROOK_MAGICS.magics[s.0 as usize]) as usize >> (64-12)
    }
}

Do you know why this error occurs or what I can do to fix the overflow?

[sengerts]

Can I jsut use a wrapping_shr on the usize value or will this differ from the intended semantics?

[niklasf]

Looks like usize has less than 64 bits on your platform. The intention is to do the shift first in u64, then cast to usize:

(u64::wrapping_mul(
    (occupied & ROOK_MAGICS.masks[s.0 as usize]).0,
    ROOK_MAGICS.magics[s.0 as usize]
) >> (64-12)) as usize

[syzygy1]

Yes, this looks like a bug, and @niklasf 's fix seems correct.

[sengerts]

Thank you both! I have forked the repo and created a pull request.