int main() {
// Sieve all primes under 1 million
const int MAX = 1000000;
std::vector<bool> const * const IS_PRIME = PrimeSieve(MAX);
// Make an array of just the primes
std::vector<int> primes;
for (int i = 0; i < MAX; ++i) {
if (IS_PRIME->at(i)) {
primes.push_back(i);
}
}
const int N_PRIMES = static_cast<int>(primes.size());
// Put a limit on the maximum number of primes we can sum together without
// go over MAX. We start at the lowest prime because this will give us the
// most primes, and hence the more stringent limit.
int max_length = 0;
int total = 0;
for (int i = 0; i < N_PRIMES; ++i) {
total += primes[i];
if (total < MAX) {
max_length++;
}
}
// We now sum numbers from our primes list and test to see if the result is
// also prime
int64_t prime_from_sum = 0;
int longest_length = 0;
for (int test_length = max_length; test_length >= 2; test_length--) {
if (test_length < longest_length) {
break;
}
for (int start = 0; start < N_PRIMES - test_length; start++) {
int64_t test_sum = 0;
for (int i = start; i < start + test_length; i++) {
test_sum += primes[i];
}
if (test_sum > MAX) {
break;
} else if (IS_PRIME->at(test_sum) && test_length > longest_length) {
prime_from_sum = test_sum;
longest_length = test_length;
}
}
}
std::cout << prime_from_sum << " is the sum of " << longest_length << " primes." << std::endl;
return 0;
}
void TenThousandFirstPrime()
{
auto number = PrimeSieve(10001, 125000);
std::cout << number << std::endl;
}
