gistfile1.cs

    public static class BitOperations
    {
        public static int PopCountIn(ulong value)
        {
            unchecked
            {
                const ulong mask0 = 0x0101010101010101;
                const ulong mask1 = ~0ul / 3 << 1;
                const ulong mask2 = ~0ul / 5;
                const ulong mask3 = ~0ul / 17;
                value -= (mask1 & value) >> 1;
                value = (value & mask2) + ((value >> 2) & mask2);
                value += value >> 4;
                value &= mask3;
                return (int)(((value * mask0) >> 56));
            }
        }

        public static int HighestBitPosition(ulong value)
        {
            if (value == 0) return -1;

            int position = 0;
            if ((value & 0xFFFFFFFF00000000) != 0) position += 32;
            if ((value & 0xFFFF0000FFFF0000) != 0) position += 16;
            if ((value & 0xFF00FF00FF00FF00) != 0) position += 8;
            if ((value & 0xF0F0F0F0F0F0F0F0) != 0) position += 4;
            if ((value & 0xCCCCCCCCCCCCCCCC) != 0) position += 2;
            if ((value & 0xAAAAAAAAAAAAAAAA) != 0) position += 1;

            return position;
        }
    }