calculate integer binary log star on compile time

log_star.cpp

#include <climits>

namespace {

typedef unsigned long log_number_t;

// count needed bits for a number
template<log_number_t N> struct num_bits_t {
    static const log_number_t value = num_bits_t<(N >> 1)>::value + 1;
};
template<> struct num_bits_t<0> {
    static const log_number_t value = 0;
};

// floor(lb(n)), lb(0) = 0
template<log_number_t N> struct lb_floor_t {
    static const log_number_t value = N == 0 ? 0 : num_bits_t<N>::value - 1;
};

// ceil(lb(n)), lb(0) = 0
template<log_number_t N> struct lb_ceil_t {
    static const log_number_t value = N <= 1 ? 0 : lb_floor_t<N - 1>::value + 1;
};

// 0-2: 0, 3-4: 1, 5-16: 2, 17-65536: 3, 65536-: 4
template<log_number_t N> struct log_star_t {
    static const log_number_t value =
        N <= 2 ? 0 : log_star_t<lb_ceil_t<N>::value>::value + 1;
};
template<> struct log_star_t<0> {
    static const log_number_t value = 0;
};

}

int main()
{
    return log_star_t<sizeof(unsigned short) * CHAR_BIT>::value;
}