modExpo (Java)

// x^y (mod p)
int modExpo(int x, 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) != 0)
        res = (res * x) % p;

      // y must be even now
      y = y >> 1; // y = y/2
      x = (x * x) % p;
    }
    return res;
  }