Решение задач ProjectEuler №3 и № 4.

#include <iostream>
#include <cstdlib>
#include <chrono>
#include <vector>
#include <cassert>
 
bool is_prime(unsigned long long value)
{
    for (unsigned long long i = 3; i < value / 2; i += 2)
        if (value % i == 0)
            return false;
    return true;
}
 
unsigned long long  solve(unsigned long long  value)
{
    unsigned long long num = 3;
    unsigned long long answer = 2;//минимально возможный простой делитель у числа
    while (value % 2 == 0)
        value /= 2;
    while (value != 1)
        for (long i = num; i <= value; i += 2)
            if (value % i == 0)
            {
        value /= i;
        if (is_prime(i)) answer = i;
        num = i;
        break;
            }
    return answer;
}
 
int main()
{
    assert(solve(10) == 5);
    auto  now = std::chrono::high_resolution_clock::now();
    auto answer = solve(600851475143);
    auto elapsed = std::chrono::duration_cast<std::chrono::nanoseconds>(std::chrono::high_resolution_clock::now() - now);
    std::cout << "Answer = " << answer << "\n";
    std::cout << "Time : " << elapsed.count() << "ns.\n";
    return 0;
}

for (unsigned long long i = 3; i < value / 2; i += 2)

for (unsigned long long i = 3; i * i <= value; i += 2)

unsigned long long solve(unsigned long long value)
{
    unsigned long long answer = 0;
    while (value % 2 == 0) {
        value /= 2; answer = 2;
    }
    for (unsigned long i = 3; i * i <= value; i += 2) {
        while (value % i == 0) {
            value /= i; answer = i;
        }
    }
    return answer > value ? answer : value;
}

unsigned long long solve(unsigned long long value)
{
    unsigned long long answer = 0;
    while (value % 2 == 0) {
        value /= 2; answer = 2;
    }
    while (value % 3 == 0) {
        value /= 3; answer = 3;
    }
    for (unsigned long i = 5, delta = 2; i * i <= value; i += delta, delta ^= 6) {
        while (value % i == 0) {
            value /= i; answer = i;
        }
    }
    return answer > value ? answer : value;
}

unsigned long long solve(unsigned long long value)
{
    unsigned long long answer = 0;
    if (!(value % 2)) {
        answer = 2; do value /= 2; while (!(value % 2));
    }
    if (!(value % 3)) {
        answer = 3; do value /= 3; while (!(value % 3));
    }
    for (unsigned long i = 5, delta = 2; i * i <= value; i += delta, delta ^= 6) {
        if (!(value % i)) {
            answer = i; do value /= i; while (!(value % i)); 
        }
    }
    return answer > value ? answer : value;
}