binary exponentiation modulo m

long long binpow(long long a, long long b, long long m) {
    a %= m;
    long long res = 1;
    while (b > 0) {
        if (b & 1)
            res = res * a % m;
        a = a * a % m;
        b >>= 1;
    }
    return res;
}

// Iterative C++ program to compute modular power  
#include <iostream> 
using namespace std; 
  
/* Iterative Function to calculate (x^y)%p in O(log y) */
int power(int x, unsigned int y, int p)  
{  
    int res = 1;     // Initialize result  
    x = x % p; // Update x if it is more than or  
                // equal to p  
    if (x == 0) 
      return 0; // In case x is divisible by p; 
  
    while (y > 0)  
    {  
        // If y is odd, multiply x with result  
        if (y & 1)  
            res = (res*x) % p;  
        // y must be even now  
        y = y>>1; // y = y/2  
        x = (x*x) % p;  
    }  
    return res;  
}  
  
// Driver code  
int main()  
{  
    int x = 2;  
    int y = 5;  
    int p = 13;  
    cout << "Power is " << power(x, y, p);  
    return 0;  
}  
  
// This code is contributed by shubhamsingh10