add missing segmented sieve

theory/prime_and_factorisation/segmented_sieve.cpp

@@ -0,0 +1,71 @@
+#include <iostream>
+#include <vector>
+
+#define N 100000
+
+typedef long long ll;
+
+int sieveArr[N+1]={0};
+std::vector<int> primes;
+
+
+void sieve(){
+
+    for(ll i = 2; i <= N; ++i)
+        if(sieveArr[i] == 0) {
+            primes.push_back(i);
+            for(ll j = i*i; j<= N; j+=i)
+                sieveArr[j] = 1;
+        }
+
+}
+
+
+int main() {
+
+    // Precompute the sieve
+    sieve();
+
+    int t;
+    std::cin >> t;
+
+    while(t--) {
+        int a,b;
+        std::cin >> a >> b;
+
+        std::vector<int> segmented(b-a+1, 0);
+
+        for(auto p : primes) {
+            // stop when p^2 > n
+            if(p*p > b) 
+                break;
+
+            // finding the nearest starting point
+            int start = (a/p)*p;
+
+            // special case
+            if(p>=a and p<=b)
+                start = 2*p;
+
+
+            for(int j = start; j <= b; j+=p) {
+                if(j < a)
+                    continue;
+                // not prime    
+                segmented[j-a] = 1;
+            }
+        }
+
+        // primes stored as 0 in the segment
+        for(int i = a; i <= b; ++i)
+            if(segmented[i-a] == 0 && i != 1)
+                std::cout << i << std::endl;
+    }
+
+    std::cout << std::endl;
+
+    return EXIT_SUCCESS;
+}
+
+
+