C++ Program to Perform Fermat Primality Test

Fermat Primality test performs to check a given number is prime or not. Here is a C++ code of this algorithm.

Algorithm

Begin
   modulo(base, e, mod)
   a = 1
   b = base
   while (e > 0)
      if (e mod 2 == 1)
         a = (a * b) % mod
         b = (b * b) % mod
         e = e / 2
      return a % mod
End

Begin
   Fermat(ll m, int iterations)
   if (m == 1)
      return false
   done
   for (int i = 0; i < iterations; i++)
      ll x = rand() mod (m - 1) + 1
      if (modulo(x, m - 1, m) != 1)
         return false
      done
   return true
End

Example Code

#include <cstring>
#include <iostream>
#include <cstdlib>
#define ll long long
using namespace std;
ll modulo(ll base, ll e, ll mod) {
   ll a = 1;
   ll b = base;
   while (e > 0) {
      if (e % 2 == 1)
         a = (a * b) % mod;
         b = (b * b) % mod;
         e = e / 2;
   }
   return a % mod;
}
bool Fermat(ll m, int iterations) {
   if (m == 1) {
      return false;
   }
   for (int i = 0; i < iterations; i++) {
      ll x = rand() % (m - 1) + 1;
      if (modulo(x, m - 1, m) != 1) {
         return false;
      }
   }
   return true;
}
int main() {
   int iteration = 70;
   ll num;
   cout<<"Enter integer to test primality: ";
   cin>>num;
   if (Fermat(num, iteration))
      cout<<num<<" is prime"<<endl;
   else
      cout<<num<<" is not prime"<<endl;
   return 0;
}

Output

Enter integer to test primality: 13 is prime