factor.cpp

#include <iostream>
#include <sstream>
#include <vector>
#include <numeric>

using std::vector;

int main(int argc, char **argv) {
    size_t value = 0;
    if(argc < 2) {
        std::cout << "Usage: factor <number to factorize>" << std::endl;   
        return 1;
    }
    std::stringstream in(argv[1]);
    in >> value;
    if(!in) {
        std::cout << "Incorrect input" << std::endl;
        return 2;
    }
    
    if(!value) {
        std::cout << "Oh well." << std::endl;
        return 3;
    }
    
    std::cout << value << ": ";
    
    const size_t limit = std::min<size_t>(1e8, value/2);
    
    vector<bool> num(limit, true);
    vector<size_t> factor;

    auto check = [&](size_t candidate) mutable -> bool {
        while(value % candidate == 0) {
            factor.push_back(candidate);
            value /= candidate;
            if(value == 1) {
                return true;   
            }
        }
        return false;
    };

    if(!check(2)) {    
        for(size_t i=3; i<limit; i+=2) {
            if(num[i]) {
                for(size_t k=2*i; k<limit; k+=i) {
                    num[k] = false;
                }            
                if(check(i)) break;
            }
        }
    }
    
    auto fold = [](size_t a, size_t b) {
        std::cout << b << " ";
        return a*b;  
    };
    
    size_t multiplication = std::accumulate(factor.begin(), factor.end(), (size_t)1, fold);
    
    std::cout << "-> " << multiplication << std::endl;
    
    return 0;
}