prime_generator.cpp

#include <prime_generator.h>
#include <iostream>

unsigned long PrimeGenerator::next()
{
    unsigned long factor;
    unsigned long next_composite;

    if (next_candidate >= max_value) {
        throw "Integer overfow!\n";
    }
    
    if ( !lazy_sieve.count(next_candidate) ) {
        lazy_sieve[next_candidate * next_candidate] = next_candidate;
        return next_candidate++;
    } else {
        factor = lazy_sieve.at(next_candidate);
        next_composite = next_candidate + factor;
        while ( lazy_sieve.count(next_composite) ) {
            next_composite += factor;
        }
        lazy_sieve[next_composite] = factor;
        lazy_sieve.erase(next_candidate);
        ++next_candidate;
        next();
    }
}

prime_generator.h

#ifndef PRIME_GENERATOR_H
#define PRIME_GENERATOR_H

#include <limits.h>
#include <unordered_map>

using namespace std;

class PrimeGenerator
{
public:
    PrimeGenerator(unsigned long max = ULONG_MAX) : max_value(max) 
    {
        next_candidate = 2;
    }
    unsigned long next();
private:
    unsigned long next_candidate;
    std::unordered_map<unsigned long, unsigned long> lazy_sieve;
    const unsigned long max_value;
};

#endif /* end of include guard: PRIME_GENERATOR_H */