num of bytes

num_of_bytes.cpp

#include <iostream>
#include <cstdint>

std::uint8_t num_of_bytes(std::uint64_t v) {
    std::uint8_t c = 0;

    c |= (!!(v & 0xFF00ull));
    c |= (!!(v & 0xFF0000ull)) << 1;
    c |= (!!(v & 0xFF000000ull)) << 2;
    c |= (!!(v & 0xFF00000000ull)) << 3;
    c |= (!!(v & 0xFF0000000000ull)) << 4;
    c |= (!!(v & 0xFF000000000000ull)) << 5;
    c |= (!!(v & 0xFF00000000000000ull)) << 6;

    static const std::uint8_t sizes[] = {
         1
        ,2
        ,3,3
        ,4,4,4,4
        ,5,5,5,5,5,5,5,5
        ,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6
        ,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7
        ,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8
    };

    return sizes[c];
}

int main() {
    std::cout << "1=" << static_cast<std::size_t>(num_of_bytes(0ull)) << std::endl;
    std::cout << "1=" << static_cast<std::size_t>(num_of_bytes(1ull<<7)) << std::endl;

    std::cout << "2=" << static_cast<std::size_t>(num_of_bytes(1ull<<8)) << std::endl;
    std::cout << "2=" << static_cast<std::size_t>(num_of_bytes(1ull<<15)) << std::endl;

    std::cout << "3=" << static_cast<std::size_t>(num_of_bytes(1ull<<16)) << std::endl;
    std::cout << "3=" << static_cast<std::size_t>(num_of_bytes(1ull<<17)) << std::endl;

    std::cout << "4=" << static_cast<std::size_t>(num_of_bytes(1ull<<31)) << std::endl;
    std::cout << "5=" << static_cast<std::size_t>(num_of_bytes(1ull<<32)) << std::endl;

    std::cout << "6=" << static_cast<std::size_t>(num_of_bytes(1ull<<47)) << std::endl;

    std::cout << "7=" << static_cast<std::size_t>(num_of_bytes(1ull<<48)) << std::endl;

    std::cout << "8=" << static_cast<std::size_t>(num_of_bytes(1ull<<56)) << std::endl;
}