De Bruijn algorithm binary digit count 64bits C#

Question

Im using the \"De Bruijn\" Algorithm to discover the number of digits in binary that a big number (up to 64bits) has.

For example:

• 1022 has 10 digits i

Answer 1

After perusing various bit-twiddling info, this is how I'd do it... don't know how this stacks up next to DeBruijn, but should be considerably faster than using powers.

ulong NumBits64(ulong x)
{
    return (Ones64(Msb64(x) - 1ul) + 1ul);
}

ulong Msb64(ulong x)
{  
    //http://aggregate.org/MAGIC/
    x |= (x >> 1);
    x |= (x >> 2);
    x |= (x >> 4);
    x |= (x >> 8);
    x |= (x >> 16);
    x |= (x >> 32);
    return(x & ~(x >> 1));
}

ulong Ones64(ulong x)
{
    //https://chessprogramming.wikispaces.com/SIMD+and+SWAR+Techniques
    const ulong k1 = 0x5555555555555555ul;
    const ulong k2 = 0x3333333333333333ul;
    const ulong k4 = 0x0f0f0f0f0f0f0f0ful;
    x = x - ((x >> 1) & k1);
    x = (x & k2) + ((x >> 2) & k2);
    x = (x + (x >> 4)) & k4;
    x = (x * 0x0101010101010101ul) >> 56;
    return x;
}