C++20で素数を求めるコードその2

my_primes_2.cpp

// コンパイルオプション：g++ -std=c++20 -Wall -O3 -march=native -mtune=native my_primes_2.cpp
#include <chrono>    // for std::chrono
#include <cmath>     // for std::log
#include <cstdint>   // for std::int64_t
#include <format>    // for std::format
#include <iostream>  // for std::iostream
#include <vector>    // for std::vector

namespace {
static auto constexpr MAX = 10000000000LL;
void seek_prime();
}  // namespace

int main()
{
    std::cout << std::format("max = {}\n", MAX);
    seek_prime();

    return 0;
}

namespace {
void seek_prime()
{
    using namespace std::chrono;

    auto const start = system_clock::now();

    auto const max  = MAX + 1;
    auto const pmax = static_cast<std::int64_t>(static_cast<double>(max) / std::log(static_cast<double>(max)) * 1.2);

    std::vector<char>         sieve(max / 2 + 16);
    std::vector<std::int64_t> prime(pmax);

    auto * const p = reinterpret_cast<std::int64_t *>(sieve.data());
    for (auto i = 0; i <= max / 16; i += 3) {
        p[i + 0] = 0x0001010001010001;  //0x01010001;
        p[i + 1] = 0x0101000101000101;  //0x00010100;
        p[i + 2] = 0x0100010100010100;  //0x01000101;
    }
    sieve[1] = 1;

    for (auto i = 5LL; i * i < max; i += 2LL) {
        if (sieve[i / 2]) {
            auto j   = i * i / 2;
            sieve[j] = 0;
            j += i * (i % 3 - 1);
            for (; j < max / 2 - i * 2; j += i) {
                sieve[j] = 0;
                j += i * 2;
                sieve[j] = 0;
            }
            if (j < max / 2) {
                sieve[j] = 0;
            }
        }
    }

    prime[0]  = 2;
    auto pnum = 1LL;

    for (auto i = 1LL; i < max / 2; i++) {
        prime[pnum] = i * 2 + 1;
        pnum += sieve[i];
    }

    auto const end     = system_clock::now();
    auto const elapsed = std::chrono::duration<double>(end - start).count();

    std::cout << std::format("{:.3f} (sec)\n素数が{}個見つかりました\n", elapsed, pnum);
}
}  // namespace