Get an array of the bit positions within a 64-bit integer

Question

OK, it may sound a bit complicated, but this is what I\'m trying to do :

• Take e.g. 10101010101
• And return { 0, 2, 4, 6, 8, 10 }

Answer 1

I actually think the fastest, simplest method is simply to loop around, but if we pass in a vector instead of later making a copy, it should be a little faster.

void DQBitboard::bits(U64 bitboard, vector &res)
{
    res.clear();   // Make sure vector is empty. Not necessary if caller does this!
    int bit = 0;
    while (bitboard)
    {
        if (bitboard & 1) 
            res.push_back(bit);
        bit++;
        bitboard >>= 1;
    }

    return res;
}

The multiply in the findfirst will cost a bit, so I doubt it's really worth it.