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main.cpp

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+#include <cstdint>
+#include <climits>
+#include <cmath>
+#include <iostream>
+#include <fstream>
+#include <list>
+
+#define PRIMES_FILENAME "primes.txt"
+#define PRIME_TYPE uint64_t
+#define PRIME_LIMIT ULLONG_MAX // > 18e18
+
+// SIGINT catcher
+// Credit: https://stackoverflow.com/questions/13010531/leaving-an-infinite-while-loop-in-c
+//#include <unistd.h>
+#include <csignal>
+volatile sig_atomic_t stop;
+void inthand(int signum) {
+    stop = 1;
+}
+
+std::list<PRIME_TYPE> primes;
+std::fstream primesFile;
+
+void addPrime(PRIME_TYPE prime) {
+    if (primes.size() % 100000 == 0) std::cout << primes.size() << "th prime: " << prime << "\n";
+    primesFile << prime << "\n";
+    primes.push_back(prime);
+}
+
+int main() {
+    std::cout << "Started!\n";
+    PRIME_TYPE n = 0;
+
+    // Open primes file for reading
+    std::cout << "Opening " << PRIMES_FILENAME << "...\n";
+    primesFile.open(PRIMES_FILENAME, std::ios::in | std::ios::out | std::ios::app);
+
+    // Read in the current list of primes
+    if (primesFile.is_open()) {
+        while (true) {
+            primesFile >> n;
+            if (primesFile.eof()) break;
+            primes.push_back(n);
+        }
+    } else {
+        primesFile.open(PRIMES_FILENAME, std::ios::out);
+        addPrime(2);
+        addPrime(3);
+        n = 3;
+    }
+    std::cout << "Primes loaded.\n";
+    primesFile.close();
+
+    // Open primes file for writing
+    primesFile.open(PRIMES_FILENAME, std::ios::out | std::ios::app);
+
+    // Loop to generate more primes, until ctrl-C (SIGINT) is given
+    std::cout << "Searching for primes... (do ctrl-C to stop)\n";
+    signal(SIGINT, inthand);
+    while (!stop && n < PRIME_LIMIT) {
+        n += 2;
+        bool n_is_prime = true;
+
+        for (PRIME_TYPE prime : primes) {
+            if (prime > sqrt(n)) break;
+
+            if (n % prime == 0) { // n is divisible by prime
+                n_is_prime = false;
+                break;
+            }
+        }
+
+        if (n_is_prime)
+            addPrime(n);
+    }
+
+    std::cout << "n = " << n << "\n";
+    std::cout << "Last prime found: " << primes.back() << "\n";
+    primesFile.close();
+    return 0;
+}