nth_prime: work in progress

cpp/nth-prime/nth_prime.cpp

@@ -0,0 +1,59 @@
+#include "nth_prime.h"
+
+#include <cstddef>
+#include <cstring>
+#include <iostream>
+#include <stdexcept>
+#include <vector>
+
+
+namespace prime {
+
+using namespace std;
+
+number_t nth(size_t n)
+{
+    if (n < 1)
+        throw std::domain_error("invalid prime index");
+
+    vector<number_t> primes = {2, 3, 5, 7, 11, 13};
+    primes.reserve(n - 1);
+
+    static const size_t sieve_size = 32*1024;
+
+    bool sieve[sieve_size] = {0};
+    sieve[0] = sieve[1] = true;
+
+    size_t page = 0;
+
+    while (primes.size() < n) {
+
+        for (auto p : primes) {
+
+            size_t start = page*sieve_size % p;
+
+            for (size_t i = start; i < sieve_size; i += p) {
+
+                sieve[i] = true;
+            }
+        }
+
+        for (auto i : sieve) {
+
+            if (!sieve[i]) {
+
+                number_t new_prime = page*sieve_size + i;
+                //cout <<  page << " " << i << " " << new_prime << endl;
+                primes.push_back(new_prime);
+                break;
+            }
+        }
+
+        //if (
+        //memset(sieve, 0, sieve_size);
+    }
+    
+    return primes[n - 1];
+}
+
+}