bitcount.cpp

template <typename T>
    constexpr unsigned BitsRequired(T min, T max)
    {
        template <unsigned x> struct PopCount
        {
            enum {
                a = x - ((x >> 1) & 0x55555555),
                b = (((a >> 2) & 0x33333333) + (a & 0x33333333)),
                c = (((b >> 4) + b) & 0x0f0f0f0f),
                d = c + (c >> 8),
                e = d + (d >> 16),
                result = e & 0x0000003f
            };
        };

        template <unsigned x> struct Log2
        {
            enum {
                a = x | (x >> 1),
                b = a | (a >> 2),
                c = b | (b >> 4),
                d = c | (c >> 8),
                e = d | (d >> 16),
                f = e >> 1,
                result = PopCount<f>::result
            };
        };

        return (min == max) ? 0 : (Log2<uint32_t(max - min)>::result + 1);
    }